Electrically Assisted Pedal Cycles.  Converting an existing bike using the Bafang BBS01 Mid-Drive kit
Installation, gear-ratio optimisation, and review.
by David Knight
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The terms "Electrically Assisted Pedal Cycle (EAPC)", "Pedelec" and "e-bike", can have various meanings; but the most common, and indeed the most useful, definition is:  'A bike that provides the rider with a degree of electrical assistance, but is regarded in law as a pedal cycle.' This means that the e-bike of common usage can be ridden without registration plates, road tax, MOT certificate and insurance (at time of writing), and the rider does not need a driving license or a type-approved helmet. In the UK (excluding Northern Ireland), the basic requirements for such an exemption are that it must be possible to propel the vehicle using pedals, the electrical assistance must stop when the speed reaches 15.6 mph (25 km/h), and the motor power must not exceed 250 W.  In the UK also, there is a minimum age requirement of 14 years.
     People from all walks of life seem to like the idea of owning an e-bike; but there is a particular attraction for those who would like to do a lot more cycling but must realistically admit that they are never going to achieve the necessary level of fitness. Such is the case for the author, who underwent an operation for a replacement aortic valve in 2012, has some permanent heart muscle damage as a consequence of the valve failure, and is separated from his local town (Ottery St Mary) by a 1.2 mile (1.9 km) long downhill stretch with a height difference of about 90 m.  Thus the road I most need to use has an average gradient of around 1/20 (i.e., 5%).  Going down to Ottery is therefore no problem, but coming back again without stopping to rest several times is a potentially life-threatening experience.  I notice also, that my perfectly able-bodied neighbours don't like cycling up the hill either, and show a general preference for 4×4s.
     There are two routes to owning an e-bike.  Such a vehicle can be bought outright for the price of a fairly good second-hand car; or for those who might question the sense of paying thousands of UK£ for a push-bike with a battery and a small electric motor, it is possible to convert an existing bike. In the matter of conversions also, there are two principal choices; that of fitting a motorised wheel, or that of motorising the pedal-drive system; the latter being known as a Mid-Drive or M-Drive conversion. The motorised wheel option seems to have little to recommend it for people who live in hilly terrain, because it lacks any method for changing the gearing ratio.  The most straightforward approach also involves the front wheel, and so will make the steering heavy. So it seems that, for the fiscally prudent hill dweller with an engineering bent, the M-Drive is the obvious solution.
     The huge advantage of an M-Drive system is that it provides electrical assistance directly to the pedals. This means that it operates through the existing rear gear-changing system and so, in principle, can provide high torque when configured with front and rear sprockets having suitable numbers of teeth. Thus diverse mobility requirements can be accommodated, but that does not necessarily mean that you will be supplied with the right ratios at the outset. It is also the case that an M-drive kit will require removal of the front dérailleur, if provided, generally eliminating the small chainwheels with high mechanical advantage.
     At this point, assuming you decide not to break the law (advisable, see legal info below); the  remaining choice is that of  the Bafang 8Fun BBS01 or clones thereof.  

Some initial thoughts on gearing and choice of bike.
A straightforward approach to carrying out an e-bike conversion is to start with a modern mid-priced bike that uses a cassette-type rear dérailleur system. This allows cogs to be changed easily and inexpensively during maintenance and to optimise gear ratios. Another possibility would be to put together an e-bike that uses rear-wheel hub gears, but obtaining a wide range of ratios is then going to be expensive. Adding a motor unit and battery will add about 8 kg to the overall weight, so the chosen bike should be a tough workhorse not a lightweight thoroughbred.
     I own a Saracen Tufftrax Comp Disc 29 mountain bike, which, as the name implies, has 29" wheels  This came with an 11-32 tooth Shimano 9-speed rear dérailleur, and a triple front chainwheel with a choice of 44, 32 or 22 teeth (i.e., a 3×9 drivetrain). The 44T chainwheel is for going fast on good roads, and has proved to be of little use in the cramped streets of Ottery and the pot-holed single-track lanes through which my house communicates. Also, it has to be said that, since heart surgery, I haven't got the strength to make the bike go fast in top gear; and the fitting of a mechanical aortic valve requires that I take Warfarin to keep my blood-clotting time between 2 and 3 times that of a normal person. I therefore cycle extremely defensively, and generally pull to the side and stop when I meet other vehicles in the lanes. Given the dire consequences of falling off, I also don't go fast downhill. It follows that I would barely notice if I didn't have the 44T chainwheel, and this is of concern because the fitting of a Bafang BBS01 drive normally involves discarding the front dérailleur and fitting a single front chainwheel of either 44, 46 or 48 teeth.  That would put the bike permanently into my top front gear or even higher; and I find it hard to get going from standstill in 44/32 (1.375:1).  Changing to a 12-36T MtB (mountain bike) cassette would offer 44/36 = 1.22:1 with the smallest Bafang chainwheel, which is somewhat better, and of course, the whole point is that the motor supplements the rider's strength; but the bike essentially becomes unusable once the power is off.
     As an alternative to fitting the supplied chainwheel, people also manufacture spider adapters that can be used to mount non-Bafang chainwheels. These are available for bolt circle diameters (BCDs) of 104, 110 and 130 mm. The problem here however is that the standard Bafang chainwheel is dished to bring the chainline towards the body of the motor. The use of a smaller chainwheel therefore might not allow such a good chainline; introducing the possibility of dérailleur adjustment issues and wear. On the other hand, a 104 mm BCD spider might allow the use of a 32T chainwheel, and 32/36 would give a very acceptable bottom ratio of 0.89:1.
     A further set of alternatives is provided by various 3rd party manufacturers who make Bafang compatible chainwheels. Some of these sprockets are flat, and some are dished to improve the chainline.  Exact details of the design can however be hard to obtain from the online information.
     The difficulty, for me, at this point was the accumulation of unanswerable questions. This eventually led me to the decision that I would not start by experimenting on my perfectly serviceable Saracen, and that I would instead get hold of another bike to use as a test-rig. A straightforward and harmless modification for the Saracen however, was to fit a 12-36T rear cassette and get a bit more mileage out of the 44T front chainwheel. This also gave a somewhat balance-challenging bottom ratio of 22/36 = 0.61:1, which is not particularly practical for ordinary roads, but is good for cycling on the steep tracks found in this part of the world. Note that when fitting a casette with larger sprockets, it will often be necessary to purchase a longer chain.
     Once I had built my first e-bike, I did eventually get around to converting the Saracen to make a second one. That was done because e-biking proved to be good fun, and it turns out that able bodied (but not particularly athletic) cyclists on ordinary bikes can't easily keep up on hilly terrain with a heart patient on a well-fettled electric bike. I therefore built the second bike so that others could come cycling with me.
     Apart from gearing considerations; while changing the rear cassette on the Saracen, and thinking about the possibility of carrying-out an e-bike conversion, I become aware of an annoying issue that might be worth considering when selecting a bike. The Saracen has bare sections in its gear shifter cables. For both shifters, a short length of Bowden-cable outer is used to get from the handlebars to a lug on the upper side of the top frame-tube. A bare steel cable then goes from this point to a lug just in front of the saddle-post. Then, for the front dérailleur, a length of cable-outer is used to get around the corner to a lug on the down-tube to the bottom bracket. For the rear dérailleur, a length of outer goes to another lug on the rear fork, and a bare cable goes to the botton of the fork and back into an outer for attachment to the shift mechanism.

Saracen with 12-36T and bare cable runs

     I had been aware of the stupidity of this shifter arrangement for some time: it precludes attaching anything to the top tube, and it prevents the bike from being clamped securely onto a bike stand for adjustment; but I hadn't thought to do anything about it. Apparently it reduces weight, but the net advantage of losing a few short lengths of cable-outer and then having to weld a bunch of lugs onto the frame seems marginal. It has also been said that it might reduce friction; but while having to cycle through puddles is practically unheard of in sunny England, there will eventually come a point when the ingress of muddy water will make itself apparent.
     For an e-bike conversion, any bare shifter-cable sections are best eliminated. The front down tube is needed for the battery pack, electrical cables run along the top tube, and existing attachment points are used up and you'll want to create new ones. Mercifully, the front dérailleur is removed completely, and that solves half the problem. For the rear dérailleur however, you'll need to run a single-piece Bowden-cable from the rear mechanism to the handlebars with reasonably low curvature. You'll also need the tools and parts for making-up Bowden cables, and the dérailleur will need to be readjusted afterwards. None of this is difficult, but a layer of messing around might easily be avoided if you're planning on buying a new bike

A bike for all seasons.
29ers are for tall people. I am 1.82 m tall and my son Steve is 1.98 m. Both of us can use the Saracen in comfort, but it does not suit other members of our family or anyone else under about 1.75 m.  One consideration when carrying out an electric bike conversion however is that others will want to have a go on it; and if adopters of the technology can popularise the idea by practical demonstration, it promises to change attitudes towards short and medium-range personal mobility (and make the roads safer to boot). I keep reading articles saying that e-bikes are a form of cheating; as if cycling only attains validity by being exhausting and causing the practitioner to need a shower and a change of clothes. The truth of the matter is that cycling isn't practical for the majority of people, because you don't arrive fit to work unless it was walking distance anyway; so cycle rides tend to be rare, and tend to be arranged so that they end at home. Thus most people never attain the stamina and skill required to make cycling enjoyable; and a huge barrier to uptake exists.
     With the ideas outlined above beginning to form, but not yet fully realised, my son Steve and I went to the cycle shop from which we had bought our existing mountain bikes a few years earlier. I must admit, I had been thinking of getting another 29er; but Steve made the point that none of our other family members would want to ride it, and it occurred to me that having smaller wheels results in greater mechanical advantage (5.4%) for a given drive-train.  So, after much discussion, I bought a Forme Sterndale 1.0 mountain bike with 27.5" wheels; which also conforms to what I consider to be a basic requirement, which is that of having adjustable front suspension and hydraulic disk brakes. The bike had a 9-speed 11-32T Shimano Hyperglide (HG) cassette fitted originally; but this was swapped for a 12-36T, and the slim saddle was replaced with a more testicle-friendly gel model for a modest extra cost. I also bought some spare parts, it being my intention not to do long distances on an e-bike without a spare inner tube, spare chain and quick-links, and the tools needed to fit those things. I was also given a couple of secondhand Shimano 32T 104 BCD steel middle chainwheels for some low-ratio experiments. The Sterndale came with a triple front chainwheel and dérailleur, which obviously had to be removed for the electric conversion, and it is worth noting that the middle cog from such redundant assemblies is another possible source of 104 BCD sprockets.
     A ride on the Sterndale before starting to work on it confirmed that the extra mechanical advantage (18.6%) given by the 12-36T MtB cassette and the 27.5" wheels was very noticeable in comparison to the Saracen's original 11-32T and 29" setup.  

Twist & Go.
The Bafang motor has an internal freewheel ratchet mechanism in addition to the freewheel hub at the rear wheel. This means that the motor can propel the bike while the pedals stay still, and the facility to connect a throttle control (accelerator) is provided accordingly.
     Having a hand throttle or 'Twist & Go' facility on an e-bike is both convenient and a considerable safety advantage. It permits the bike to pull-away without wobbling at busy road junctions, in crowded traffic situations, and when there are pedestrians around.  It is also particularly useful in getting going again when you have been forced to stop suddenly without time to change down.  There is however, a legal peculiarity in relation to this matter: which is that newly manufactured e-bikes require Government Type Approval if they are to have a Twist & Go control.  Conversions carried out on bikes that have previously been used on the public roads are however exempt from that requirement (see legal info).
     The upshot is that, if you intend to fit a throttle, you must convert a bike that has been used previously. If you were to buy a frame and build an e-bike from scratch, thereby sidestepping the expense of discarding a perfectly good crank-set and front dérailleur, that action would constitute manufacturing and would result in a machine that requires Type Approval. The matter might seem unprovable of course, but there is always an audit trail if someone wants to cause you legal difficulties.

Installing a Bafang M-Drive on an existing bike.
Presuming that you have a bike that you want to convert, and that you have a Mid-Drive kit waiting to be installed; the first job is to remove the front dérailleur (if present).  This is the point at which most online commentators assume that what they needed to do is universal, and some actually recommend that you destroy parts of the dérailleur shift mechanism with wire cutters in order to get the chain off.  You don't need to destroy anything, but you do need to own chain-splitting tools and Quick-Links, even if you don't require them here.
     In some cases there will be a screw on the chain shifter that can be taken out, enabling the mechanism to be opened-up and removed without splitting the chain. On the recently manufactured Shimano shifter on my Sterndale however, there was a place for the screw in the pressing, but the two parts had simply been welded together. Thus I needed to remove the chain, but I found that it had been supplied with a Quick Link already fitted. If you read about Quick Links online, people say that they are unreliable. I asked about this at the cycle shop, and was told that such links are perfectly reliable; and the practice of pushing chain pins back in with a chain splitter is the greater source of breakages. To open a Quick Link requires a set of special pliers. Some Quick Link pliers can also close the link (i.e., force the rollers apart), but this operation is also fairly easy to accomplish by assembling the link and tugging the chain. If you part the chain using a link extractor and then fit a Quick Link, note that the links come in different widths (8, 9, 10 or 11 speed) depending on the number of sprockets on the rear dérailleur cassette.
     The next operation is to remove the pedals. It's best to do this while the cranks are still on the bike, otherwise you will need to hold each crank in a vice and risk scratching it. A 15 mm open-ended spanner is usually required. The pedal on the left side of the bike (the side opposite the chain side) has a left-hand thread.
     Removal of the cranks from low-end and mid-price bikes requires a special tool known as a 'crank puller'. Before buying a crank puller, you need to take out a retaining bolt on the crank-set you intend to detach and have a look at it. The Bafang 8Fun uses a 'standard' old-fashioned square-taper fitting, but more modern bikes will often have a splined crank. Shimano Octalink® cranks (as the name implies) have an 8-lobed spline, and Isis® cranks have 10 lobes (and there may be others). The splined cranks can only be removed with a standard crank puller if there is an additional anvil piece to fit into the end of the crankshaft. I got caught-out by this, but I have a lathe, so I was able to make an adapter out of a piece of mild-steel bar in about 10 minutes. The item I made is functionally identical to the Shimano TL-FC15 (see list of tools at the end of the article).

Octalink BB with puller
Shimano Octalink sealed crankshaft unit with crank puller and adapter.  The top-hat shaped piece fits into the end of the crankshaft, allowing the bolt in the crank puller to push against it.

     Once the cranks have been removed, it's time to remove the bottom bracket. This requires a special tool with a 20-lobed spline that fits into the retaining nuts on either side. This is the point at which you have an opportunity to ruin the nuts, destroy the tool, skin your knuckles, and write a venomous feedback article casting doubt on the tool-seller's parentage. Alternatively, you might try using the thing properly.  
     You will notice that the tool has a hexagonal part allowing it to be turned using a wrench or spanner, as well as a socket for a ½" ratchet wrench. There is a very good reason why it has both. It is usual to see explanations of how to remove a bottom bracket in which the commentator snaps the tool onto a ratchet drive and (supposedly) turns the nut. That might work, or it might not; but the second possibility is very likely. A common comment is that the nuts can be 'fairly tight'; which is like saying that being cornered by a pack of hungry wolves can be inconvenient. If the nut has been in place for any length of time, it will probably have encountered its fair share of rain and mud.
     Assuming that you do not own a pneumatic impact wrench, a reasonably stress-free way of loosening the nut is to hold the tool in place using a G-clamp, with a block of wood on the opposite side to protect the paint or the protruding crankshaft (see illustration below). An open-ended spanner or wrench can then be placed on the hexagonal part; and a sharp, but not particularly hard, tap on the spanner with a soft mallet should free the nut. What happens here is that the shock of tapping the wrench shatters the crystalline structure of any rust, dirt or adhesive in the thread. The G-clamp can then be removed, and the nut can be unscrewed using a ratchet drive.
     If using an open-ended spanner doesn't work; it is possible to insert a 2 ft breaker bar into the tool and hold it into the bottom bracket with a G-clamp as before, but with a second block of wood to protect the breaker.  It takes two people to hold this arrangement together while doing-up the clamp, but it does loosen the most belligerent of nuts.

BB removal, drive side
Holding the bottom bracket tool in place with a G-clamp while persuading the nut to turn.  The sprocket-side nut has a left-hand thread.  If the nut is very stiff, use a proper open-ended spanner (24 mm or 15/16"), not an adjustable.

     Another reason why people destroy the bottom bracket tool is that the nut on the sprocket side has a left-hand thread. It needs to be turned clockwise to undo it.  
     Note incidentally that, for the Shimano Octalink and other sealed crankshaft units, it is convenient to remove the drive-side nut first. That withdraws the complete shaft and bearing assembly. The nut on the other side is merely a sleeve to keep the shaft straight in the bottom bracket.  
     Once you have removed the crankshaft and cleared the bottom bracket without wrecking anything or injuring yourself, you are free to install the Mid-Drive motor unit. Furthermore, if the author's experience is anything to go by, the procedure is then quite closely related to the information given in the various versions of the instruction manual. That all depends however on the bottom bracket being "standard", which means that it must measure between 68 and 73 mm from one side to the other. You can estimate this approximately by waving a ruler in its general direction before the crankshaft and pedals have been removed.
     The thread around the crankshaft on the Bafang motor unit is M33×1.5.  The motor is held into the bottom bracket by a hard steel M33 castellated round-nut, which is then covered over by a splined aluminium-alloy locknut.  The outer nut has a valance to hide the castellations of the inner one.  It can be difficult to get the steel nut as tight as it needs to be to stay done up for a long time, and a good quality C-spanner (hook wrench) is needed. Also, the valanced locknut has to be unscrewed if you want to check the tightness, and this requires removal of the pedal. Early versions of the 8Fun just had two castellated nuts, this being a  less attractive but more secure arrangement. Hard steel castellated nuts can be ordered online (on ebay for example - try "round castle locknut").  Note that stainless steel is also hard and has the added benefit of corrosion resistance.

Castle nuts
Large stainless steel castle nut with the Bafang supplied nut used as locknut.

     I actually bought a BBS01 kit with a 46T chainwheel, but then bought a 44T chainwheel separately as an afterthought. Both of these sprockets came with a plastic chainguard, the 46T version having a larger diameter than the 44T version. Both guards however have the same hole circle diameter for the fastening screws, so the large one can be used with the smaller sprocket if so desired. I first commissioned the bike with a guard fitted, but later left it off on the basis that flared trousers went out of fashion over 40 years ago. It might conceivably prevent the chain from jumping off on the outside on rough terrain, but it also prevents access to the five Allen screws that hold the chainwheel to the motor shaft, making it difficult to check tightness or swap chainwheels.  Being plastic, the chainwheel cover is of little use as a bashguard; and a sprocket guard is in any case not sufficient to protect the BBS01 motor because there are three electrical cables coming out of the bottom of that. Riding without grounding the motor housing is therefore advisable unless some kind of motorschutzblech is devised. Such a guard is available for the BBSHD, but there are no obvious attachment points on the BBS01/02, which means that it would have to attach to the bike frame.
     The Bafang motor unit has square tapers on its crankshaft. The subject of fitting crank arms to such tapers has generated considerable and sometimes heated discussion in the numerous web fora; all of which can be summarised fairly succinctly by saying that no one seems to know what to do. The problem is that; while the process of doing up a bolt normally has an obvious end-point, this is not the case with bolts that draw the two parts of a taper system together. This issue is also particularly troublesome when one of the parts (the crankshaft) is made of hard steel, while the other (the pedal arm) is made of aluminium. An ordinary Allen key isn't long enough to achiveve the required tightness (NEVER us a hammer); and while a socket wrench with an Allen bit can do the job, it is impossible to decide when to stop. If the bolts are too loose; the pedal arms will creak and click, they will keep falling off, you might lose bolts on the road, and the taper fittings can be permanently damaged. If the bolts are too tight, the fittings can be permamenetly damaged, and the arms will be very difficult to remove.
     Under such circumstances, there really is no alternative but to use a torque wrench. It does not however need to be one of the expensive (and uninformative) click types; and in the author's opionon, the low-cost beam-deflection indicator type is perfect for this job. The type of Allen key that fits onto a socket wrench is incidentally known in full as a "Hex Allen key bit socket". The size required for pedal crank bolts is 8mm. A ratchet 'adaptor' between the the torque wrench and the Allen bit facilitates the job considerably; but a good quality (>500 Nm rated) adapter is likely to cost more than the wrench.

Torque wrench

     With a torque of around 40 Nm, the crank will pull slowly onto the taper. Grease doesn't make much difference. Keep going until the bolt slows down and stops. Tighten both cranks, then go for a short ride to make sure that they don't creak or click under load. If they do, tighten them a bit more. When all is well, put an ordinary 8 mm Allen key into the road toolkit, just in case, go for a longer ride, and check again. Check the tightness periodically. 

Ratio tables for various chainwheel sizes and some MtB cassettes.
The ratios given in the tables below are the number of turns of the back wheel per turn of the pedal crank. To get the distance travelled or 'metre de développement' (measure of progress), multiply the ratio by πD, where D is the effective diameter of the back wheel.

Distance / cycle = measure of progress =  πD × Tf / Tr
Tf = front sprocket teeth
Tr = rear sprocket teeth
D = back wheel outer diameter

If you've just upgraded from a penny farthing and want this figure in "gear inches", leave out the π.
Gear inches = D × Tf / Tr
To convert from gear inches to progress per pedal rotation (in metres), multiply by 0.0254π = 0.0798

     The front chainwheel sizes listed below are examples of what can be fitted on a BBS01/02 M-drive conversion, 44 and 46T sizes being made by Bafang, a 38T being available as a Hallomotor aftermarket product, 42T and 36T being made by PreciAlps, and 32T being made by mounting an intermediate front sprocket on a 104 BCD spider adapter. Other sizes can also be made-up using aftermarket spiders; but the larger chainwheels can be made dished to give the best chainline, with 42T being the smallest size that can be dished in this way.

Shimano HG400 12-36T 9-speed cassette.
selector Rear sprocket
Front chainwheel teeth (smallest possible dished type is 42T)
46 44 42 38 36 32
1 36 1.278 1.222 1.167 1.056 1.000 0.889
2 32 1.438 1.375 1.313 1.188 1.125 1.000
3 28 1.643 1.571 1.500 1.357 1.286 1.143
4 24 1.917 1.833 1.750 1.583 1.500 1.333
5 21 2.190 2.095 2.000 1.810 1.714 1.524
6 18 2.556 2.444 2.333 2.111 2.000 1.778
7 16 2.875 2.750 2.625 2.375 2.250 2.000
8 14 3.286 3.143 3.000 2.714 2.571 2.286
9 12 3.833 3.667 3.500 3.167 3.000 2.667

     There are also 48T and 52T chainwheels made by Bafang, but these are intended for small-wheeled bikes, and give too high a top ratio for road use with bikes having a 12-36T cassette and 27.5" or 29" wheels (and have far too many teeth for mountain biking). A recent trend however, has been to increase the range of ratios available from the rear cassette with a view to eliminating the front dérailleur altogether. Since loss of the front changer is a matter of force majeure for e-bike builders, and front sprockets having less than 42T are flat rather than dished, these new systems are of considerable interest.
     The table below is for the Shimano Deore XT 11-46T cassette, which has a particular advantage over some of the more exotic possibilities in that it can be retro-fitted to an existing freewheel hub. Note how its use with the PreciAlps 42T (dished) front chainwheel practically covers the entire ratio range of the table above. The change ftom 37T to 46T is large, but pedaling cadence is not such an important issue for e-bikes, and it provides for hill climbing by giving an extra position on the end of a fairly reasonable set of road ratios.

Shimano Deore XT 11-46T 11-speed cassette.
Selector Rear sprocket
Front chainwheel teeth
52 48 46 44 42
1 46 1.130 1.043 1.000 0.957 0.913
2 37 1.405 1.297 1.243 1.189 1.135
3 32 1.625 1.500 1.438 1.375 1.313
4 28 1.857 1.714 1.643 1.571 1,500
5 24 2.167 2.000 1.917 1.833 1.750
6 21 2.476 2.286 2.190 2.095 2.000
7 19 2.737 2.526 2.421 2.316 2.211
8 17 3.059 2.826 2.706 2.588 2.471
9 15 3.467 3.200 3.067 2.933 2.800
10 13 4.000 3.692 3.538 3.384 3.231
11 11 4.727 4.364 4.182 4.000 3.818

Sunrace also make an 11-46T MtB cassette, model CSMX8 EAZ. This is Shimano compatible, but offers a more even ratio spacing, the tooth numbers being: 46, 40, 36, 32, 28, 24, 21, 18, 15, 13, 11.

     Another alternative for achieving a wide range of ratios is to build your own cassette out of individual sprockets. Inexpensive cassettes are often supplied either as a pair of sprocket stacks, or as a single stack plus one or more separate cogs, the stacks being held together by rivets. In some cases, the individual sprockets in a stack have their own splines for engagement with the freewheel hub, which means that the rivets can be removed. It is then possible to achieve a wide range by removing one of the smaller cogs in the series and adding a so called 'expander' 'or 'extension' sprocket at the large end to give an extra low ratio. Extension sprockets can have as many as 50 teeth, but the clean pick-up of a fully engineered cassette cannot necessarily be expected, which means that very large jumps are best avoided.
     The ratio steps in the region of the removed cog can be made more even by buying new sprockets, or by mixing sprockets from several cassettes. Note however, that the smallest cog is often serrated, to make it grip against the outer retaining collar; which means that it must either be retained or replaced by another similarly serrated cog. Special lockrings that allow the small sprocket to be discarded are also available, but it is important to check compatibility.
     It is also possible to mix and match sprockets from cassettes with different numbers of speeds by using spacers. A 2 mm spacer, for example, allegedly allows sprockets from 11-speed cassettes to be used in 10-speed systems (I haven't tried it). 1 mm, 1.5 mm and 1.85 mm spacers are also available, some cassettes are built using spacers, and thin shims can be made from sheet metal.

Controls and wiring.
Where the installation manual is justifiably vague is in the matter of how to lay-out the connecting cables and fit the controls. Bikes have a great deal of variability, and all that can be said with certainty is that the job must be done in such a way that cables cannot get accidentally crushed or snagged. There are however a few specific issues worthy of comment.
     I decided to fit a right-hand twist throttle, the same configuration as is used on motorcycles. I found a problem however, which is that the throttle was not compatible with the control levers for my rear dérailleur. It was simply not possible to get the shift control into a good ergonomic position relative to the hand-grip with the throttle twist-grip in between. So, it occurred to me that, since there was no-longer a front dérailleur, I should move the shifter to the left-hand side.
     My first thought on this matter was that it might be possible to buy a left-hand 9-speed shifter, at which point I made the mistake of looking for information online. According to some of the web fora, this is a huge problem for disabled people who lack the full use of the right hand.  The general view seemed to be that the issue is insoluble, although one suggested 'solution' was to dismantle a 3-speed shifter and get a machine shop to create a  left-handed 9-speed ratchet mechanism for it. I began to form the view that I should discard the right-hand throttle and get a left-hand one; but then I had a careful look at the bike and a strange heretical thought came into my mind:  'Er. . . ., why can't I just take the shifter off the right handlebar and put it onto the left one?' Five minutes work with a couple of Allen keys and I had a left-hand rear shifter. The numbers on the indicator are upside-down, and I have to change down with my forefinger and up with my thumb; but so what?  Getting used to the arrangement while out riding took a few hundred milliseconds.

Handlebar layout
Handlebar layoutwith 9-speed shifter on the left and twist throttle on the right. Notice that the ratio indicator lies above the handlebar, rather than sticking-out past it. If you want to fit a twist throttle, this is a feature worth looking for when selecting a bike or a shifter, because it gives more positioning options.

     The Bafang kit is supplied with a pair of brake levers that have motor cut-off switches on them. An issue however, is that the included levers are only suitable for cable-operated brakes, they are of no use if you have hydraulic actuators, and they also cannot be used if the bike has combined brake and gear shifters. These limitations ensure that many people do not bother to install the switches (which are in any case not strictly essential). 
     I felt however that there might be an advantage in retaining the kill-switch  functionality, and so I ordered a pair of Bafang BBS magnetic brake-sensor switches. These can be retro-fitted to a wide variety of brake actuators, a small magnet being attached to the lever arm using an adhesive pad, and the switch body being attached to the fixed part. A problem I found when the new switches arrived however, was that my brake actuators were not compatible with them; and while I could see that I might be able to manufacture some sort of secondary mechanism to make them work, I certainly wasn't going to be able just to stick them on.  Riding without kill switches was therefore the practical solution in the short term.
     Subsequent riding experience suggests that there will be a worthwhile ergonomic improvement in having kill switches. This is because there is a lag of several hundred milliseconds between cessation of pedaling and the shutoff of power assistance. It is not a serious safety issue however, because the motor output is only 250 W, and the bike brakes can easily absorb the small burst of surplus energy.
     An issue to which I intend to return however; is that when several people ride close together, there is a serious need for brake lights. Such a safety feature is an obvious inclusion when a large battery is already available. When that matter is resolved, the use of double-pole switches solves the kill switch problem at the same time.

The setting-up procedure given in the manual is actually pretty straightforward and does not take long. There are however two peculiarities worthy of mention.
     One necessary aspect of setting-up is to input the bike's wheel size. It would be good if the wheel diameter could be put in accurately, say to the nearest 0.1", but the options are limited to standard sizes. In the case of a bike with 27.5" wheels, it is necessary to put in "700c", but this is not necessarily an exact definition. It is good enough, but a little annoying if you are a lover of precision.
     A wonderfully cryptic item in the advanced setup menu is "Level amount setting". This sets the number of increments that are available on the handlebar control when selecting the pedal-assistance level. The choice is from 2 to 9, with a factory default of 3. On one web forum, I saw a thread discussing the discovery of 'secret extra power levels', and how the performance is enhanced by going up from the default of 3 to the full 9. The only trouble is that there are no extra power levels.  It's just a setting for the interval size. Three levels is a good choice, and it certainly beats the tedium of having to press the (+) button eight times to get to full power.

The e-bike battery.
To power the bike, I bought a TLH 36V 15.6 Ah Li-ion battery with charger. This came without any documentation or instructions; but its operation seemed to be fairly self-explanatory. There is an LED status indicator that lights at the press of a button if the charger is plugged in and switched on, or if the battery is switched on. A mounting plate and connector fits onto the bike-frame down-tube, the battery unit slides on to it and is locked into place using a supplied key. An immediate issue I had with the Sterndale mountain bike frame however, is that there was only enough room for the battery on the front down tube, and the battery-plate bolt holes were too high-up to mate with the bottle holder attachment points. The work-around was to mount the battery upside-down. I also found that the heads of the M5 Allen screws that came with the bike were too proud to allow the battery to slide onto the plate; and the solution was to replace then with A4 stainless hex-head M5×12 screws with thin washers.
     The mounting plate system allows the battery to be taken away for charging or storage, but there is no obvious reason why it cannot also be charged in situ. I feel however that, given the high energy-density involved, charging should be done in an outbuilding, or somewhere with a smoke alarm, and certainly not in a building where people sleep. The battery charger gets quite warm in use, so is best placed on a wire trivet, or a luggage rack, or otherwise suspended so that air can flow over it. Charging the battery from half to full takes about 3 hours; and the charger is small enough to take with you if you are going somewhere with mains electricity.  Once charging is complete, the LED on the charger changes from red to green and the charger cools down (i.e., the charging current is switched off).
     My battery installation, unfortunately, developed a major problem after a few days riding. This was to do with the electrical connection between the battery housing and the mounting plate. Four flat pins on the plate mate with receptacles on the pack. I noticed during the installation that the four pins were chromium plated. This is one of the worst conceivable plating materials for a low voltage connector, because chromium has a habit of forming a tough non-conducting oxide layer on exposure to air. Still, the connectors were doubled-up, two for (+) and two for (-), and so I put my initial doubts aside and presumed that the arrangement must be OK in practice. Using the battery upside down however, it transpires that the connector is not quite as fully inserted as it would be with the assistance of gravity. The same situation might also arise if the battery were to be mounted horizontally. This no doubt contributed to the problem that emerged, for which the symptoms were: random loss of power, and random shutting down of the system. It was initially difficult to identify the cause, and the misbehaviour grew progressively worse and soon made the bike unusable. Eventually I worked-out what was happening by running the bike on a bike-stand and measuring the supply voltage at the input to the motor unit. Random shut-down was caused by large fluctuations in the on-load voltage, and the fault could be reproduced by rattling the battery pack up and down on its mounting plate. Careful inspection then showed that one of the connections had been getting hot and had started to melt the surrounding plastic. Also, since the connections are doubled-up, this indicates that its partner was barely working at all.

Overheated battery contacts
Mounting plate and battery pack, with signs of connector overheating.

   I might at that point have decided to demand a replacement battery, but that would not have solved the problem of having to use it upside down. Also, I do not think that the connector is fit for purpose, even if it is used the right way up. I therefore decided to forfeit the warranty and solve the problem by modifying the battery pack. I did that by installing a female XT60 connector, which, as the name implies, is rated for 60 A. XT60s have gold-plated solid copper pins and receptacles with a high contact area, and so have extremely low resistance. Note that carrying out modifications to a vehicle battery pack carries a risk of fire or explosion, and is therefore a job for a competent electrical technician (please also see my Disclaimer).

Akku with XT60
Battery pack with XT60 wired across the existing connector.

XT60 connector detail
XT60 connector detail.

     A small circuit board can be purchased for the XT60, enabling both the male or female types to be used as bulkhead connectors. To prevent accidental short-circuits, connectors on battery packs must, of course, be female; so I fitted a board to a female connector and mounted it on the battery case. In doing that, I made a couple of pilot holes in the bottom part of the battery pack, and fashioned an XT60-shaped hole in the ABS using a modeling knife and some small round and square rough-cut files. When mounting the connector, I placed a couple of spacers on the retaining screws, to minimise the protrusion, and sealed the connector body into the hole I had made using architectural silicone. I wired the XT60 directly across the existing connector, thereby retaining the normal functionality (for what it's worth), and also siliconed around the original connector to improve the weatherproofing. Finally, on reassembly, to minimise water ingress as far as possible, I taped the join between the upper and lower parts of the ABS housing. Note that it is possible to buy silicone rubber caps for female XT60 connectors. These will exclude dirt and help to prevent short-circuits in the event that the battery is stored or used on a bike with the standard connector.
     Having opened the battery pack, I had an opportunity to make observations that would not otherwise have been possible. I noticed that the On-Off switch is not capable of handling the current required by the Bafang motor, and this function is instead accomplished by using the switch to turn on a transistor. Presumably, the cost of a small switch and a MOSFET is less than that of a high-current switch. A somewhat worrying feature however, is that plugging-in the charger also switches-on the battery output, which is how the press-button battery-level indicator comes to work either when the battery is on, or when the charger is in. This means that charging the battery while it is installed on the bike has the effect of turning it on. The bike, of course, does not wake-up unless the handlebar power button is pressed, but it means that the the motor will not go to sleep straight away if the charger is plugged-in before the battery is turned off. Whether this is a serious issue is moot, but it is not ideal. "Off" should mean "Off".
     The appalling chromium connector seriously coloured my initial impressions of riding the bike. I came to the (very-pessimistic) conclusion that the motor makes a rather feeble contribution to the rider's efforts, and while it did (initially) enable me to get up the hill from Ottery without stopping, I still had to do quite a lot of work. I also noted that, while the twist throttle could start the bike from standstill on level ground, it could not do so on a 5% gradient.  The effect of fitting the XT60 connector, and thereby discovering the actual capabilities of the motor, was consequently astonishing.
     Although the following calculation involves some conjecture, we can easily get a rough idea of just how deleterious the original connector was. If the Bafang BBS01 were to have a peak power consumption of 250 W at 36 V, it would require a current of 6.9 A.  In reality however, if it is to deliver 250 W to the road, it needs a somewhat greater input than that, probably approaching 300 W.  This means that we should expect a typical peak current of about 8.3 A, or say 8 A in round numbers. Hence, in the state of maximum demand, the motor looks something like a 36/8 = 4.5 Ω resistance.  
     Now, if the connector were to have a resistance in the region of 1 - 2 Ω, then at times of peak power demand, it would be expected to cause a voltage drop of between about 6.5 and 11 V.  Instead of a nominal 8×36 = 288 W input to the motor, we would have between (36-6.5)2/4.5 = 193 W and (36-11)2/4.5 = 139 W. Approximately halving the motor power would indeed result in very feeble performance, and the reduced voltage is likely to cause the controller to assume the battery to be dead and switch off.
     When I built my second e-bike, I bought the same model of battery again but fitted an XT60 connector immediately.  

Bike performance with road gearing
(44T with 9sp 12-36T cassette, 27.5" wheels).
The gearing configuration I adopted initially for road use was a 44T front chainwheel with a 12-36T 9-speed rear cassette. This turned out to be a perfectly acceptable combination for the type of town and country cycling I do, giving speeds in the range of 6 to 20 mph in gears 3 - 7, the ability to reach 25 - 30 mph in gear 9, and the ability to pull away on the hand throttle on uphill gradients a little in excess of 10% in gear 1. The Bafang dished chainwheel gives a chainline exactly parallel to the frame-line in gear 5, which is ideal. Dérailleur changes are smooth, provided that the pedal or motor force is slackened-off, just as in normal cycling; and there seems to be no need for the gear-change sensing system provided with some of the more exotic versions of the M-drive.  
     In order to understand the effect of motor assistance, it is useful to be aware that a normal cyclist (i.e., someone who is not an athlete) can produce an output of 100 - 200 Watts for fairly long periods, and an output of 400 - 500 W in short bursts for going up hills or hard acceleration. An additional 250 W from the electric motor therefore makes climbing hills seem like traveling on the flat. In my case, having limited cardiac output, I can happily produce the 100 - 200 W needed for prolonged periods of level cycling, but the 400 - 500 W needed for steep hills is beyond me. Add the 250 W from the BBS01, and I get 450 W, which means that typical road hills no longer present a problem. If I pedal moderately hard, I find I can get up the 5% (average) hill from Ottery to my house at around 11 mph in gears 3 - 5, without any need to take a rest. I also found that I could get up the hill on the throttle, without any pedaling at all, at around 6 mph in gear 1. It is of course tedious to go so slowly, but it does give me the reassurance that I can still make it home even when I am completely exhausted. Incidentally, the fact that it is fun to pedal within the physical capabilities of the rider makes using the e-bike into an excellent form of exercise.
     The way in which the motor unit provides assistance to the rider is initially somewhat strange, but getting used to it is not difficult. As set-up for use on British roads (assistance stops at 15.6 mph), motor output is at its greatest when the rider pedals slowly, and decreases progressively as the speed climbs towards the assistance limit. This gives the curious effect that assistance can be increased by changing up when going uphill. The provision of a freewheel ratchet in the motor unit also permits what I call 'faux pedaling', which is where the act of turning the pedals too slowly to lock the ratchet and propel the bike, causes the motor to propel the bike instead. This gives onlookers the false impression that you are doing some work, even though you are actually getting a completely free ride.
     I have so far not managed to exhaust the battery before recharging it; but with moderate pedaling, I would estimate the range to be in the region of 40 to 60 miles.

Low ratio gearing
(32T with 9sp 12-36T cassette, 27.5" wheels).
For my first hill-climbing and off-road cycling experiments, I fitted a Shimano 32T intermediate front sprocket to a 104 mm BCD spider adapter from RisunMotor (also avalable from Hallomotor and ConhisMotor). Strictly, the spider was designed for chainwheels of 34T and greater, but being made from hard stainless steel, it was possible to grind it down (using an ordinary workshop tool grinder) until it just cleared the chain on a 32T sprocket.  

32T sprocket on 104BCD spider
After-market spiders with 104 mm BCD are available for the BBS01/02.  Here, a Shimano 32T intermediate front sprocket is attached to a Hallomotor stainless steel spider adapter.  A spacer of about 0.9 mm is required to prevent the chainwheel bolts from rubbing on the motor housing.  The spacer is easily made from a piece of 20 SWG aluminium sheet (starting by laying the spider on top and marking the holes with a scriber).

32T sprocket on 104BCD Al spider
Here a 32T Shimano sprocket is mounted on an aluminium 104BCD spider.  As is the case with the steel spider, this one is intended for sprockets of 34T and larger, and so requires some filing to make it clear the chain (do not try to shape aluminium using a workshop grinder).  A 0.9 mm spacer is required between the hub and the spider, as for the steel version.

     The 32T front sprocket fitted to a bike with a 12-36 cassette gives a lowest ratio of 32/36 = 0.89 turns of the back wheel per turn of the crank.  This is very roughly the same as changing down by a further two gears relative to the ratios available with the 44T front sprocket. It gives a bike with 27.5" wheels the ability to pull-away uphill on the throttle on a 17% gradient (1/6).
     The disadvantage of the small front chainwheel is that it lacks the dish-shape of the standard Bafang sprocket and so gives the best chain-line in gears 7 and 8. The Shimano HG sprockets however have a  tooth profile and shifting ramps that permit them to be used with a fairly severe degree of misalignment, and this allows the lowest ratio to be used reasonaly successfully.  One downside is that the chain tends to climb down the cassette if the bike is wheeled backwards in gears 1 or 2. That situation is usually avoidable however, because gear 3 is about the same as the lowest required road ratio. Fortunately, the additional freewheel hub in the Bafang motor prevents chain climb-down on backpedaling. Critical setting of the low-limit (L) and back-tension (B) dérailleur adjustments is necessary to avoid clicking and grumbling when climbing very steep gradients, and the drive sounds noisy under high loads. Fairly rapid wear is to be expected during heavy use.

32T front sprocket

ebike with 32T chainwheel

     This chainwheel is somewhat too small for use on the roads. Going downhill, it is difficult to pedal fast enough to catch-up with it, even in gear 9; so freewheeling is the order of the day. Going uphill, there is not much cause to use the lower ratios on ordinary roads, but the bike in this configuration is nevertheless serviceable if somewhat slow.

PreciAlps Narrow-Wide dished chainring
(42T with 9sp 12-36T cassette, 27.5" wheels).
The narrow-wide chainring is a wonderful invention; and once you realise what it is, it is difficult not to want one.  Since normal bike chains have alternating inner and outer links, and must have the same number of each (provided that there are no half-links); a chainwheel can have alternating wide and narrow teeth, provided that the total number of teeth is even. This gives a dramatic increase in the strength of the sprocket for a given material type, allowing hard aluminium alloys to out-perform steel. This profile cannot be used for the cogs in dérallieur changer assemblies, because the wide and narrow teeth must be mated respectively with outer and inner links before the chain will drop into place. It is however still perfectly possible to re-fit a chain on a narrow-wide sprocket in pitch darkness, because it won't go on unless it is properly positioned.
     Strength is also not the only or even the main advantage.  The chain mates perfectly with the narrow-wide structure, and this reduces the side-to side movements that cause noise and wear and can sometimes trigger the chain to jump off when going over rough ground. Hence, the N-W profile is becoming increasingly common on the front chainwheel of high-performance 1×N drivetrains.
     When fitting a narrow-wide chainwheel, note that while there should be no half-links in a system having a tension arm, it is important to be sure that there definitely aren't. Also, conversely, if you decide to use a narrow-wide chainwheel in a system having hub gears, you may need to use a tension arm in order to establish the correct chain length without using a half-link.

PreciAlps 42T

     The skirt of the PreciAlps chainwheel is deeper than that of the ones supplied by Bafang, and I when I bolted it directly on to the motor on the Sterndale I found that the teeth rubbed on the bike frame. To overcome that problem, I included one of the 0.9 mm spacers that I had made previously, as can be seen by careful inspection of the photograph above. It would be useful if manufacturers would recognise the importance of having such shims available, even if they are not always necessary. When I later fitted a PreciAlps 42T N-W to the Saracen 29er, it did not need a spacer. 0.9mm spacer
     Relative to the smallest dished Bafang chainwheel (44T), the 42T PreciAlps gives a 4.8% increase in mechanical advantage. This makes it slightly easier to get up steep hills, without sigificantly compromising top speed. What was most noticeable however, was the smoothness and lack of noise from the drivetrain (particularly in comparison to the non-dished 32T front chainwheel that I had just replaced). Such clean performance is representative of a reduction drivetrain in energy loss, and a concomitant reduction in wear. My appraisal of the 42T N-W promptly caused me to lose interest in all other front chainwheel options; and so I e-mailed Bertrand Clement-Rochiaz of PreciAlps to confirm that it would work with an 11-speed chain, and then ordered another for the Saracen 29er.

(42T with 9sp 12-36T cassette, 29" wheels).
Once I had got the Sterndale working well, I set about converting the Saracen 29er. Although I was planning to fit a wide-range 11-speed rear dérailleur eventually; I got it going with the existing 12-36T 9-speed cassette and shifter in order to avoid implementing too many changes at once. I then went out riding on it, with my son Steve on the Sterndale as my companion.  
     As mentioned previously, a bike with 27.5" wheels has 5.4% mechanical advantage over a bike with 29" wheels. The drive trains on our bikes were otherwise identical.

>> more to follow.

 DWK 2018-07-11

Legal information:

 Electric bikes, 2016 UK legal requirements (gov.uk).
EAPCs in the UK (excluding Northern Ireland) are limited to a maximum assisted speed of 15.6 mph (25 km/h), a maximum motor power of 250 W, and must have pedals that can be used for propulsion. The bike can go faster than 25 km/h, but only under power provided by the rider. Failure to comply with these rules causes the bike to be classified as a motorcycle; which requires registration, an MOT certificate, tax, insurance, an approved helmet and a driving license. Non compliance is therefore a serious offence and can lead to a heavy fine and a general driving ban.  In the event that some idiot knocks you off your bike, it might also compromise your ability to claim compensation.

Twist & Go - self conversion type approval exemption.
The DFT rules state that an EAPC with a Twist & Go facility, i.e., the ability to move under electrical power at speeds up to 15.6 mph without pedaling, requires type approval. The reason for the requirement is that the motor does not necessarily cut off when the rider stops pedaling. This seems to cause a problem for people who want to convert existing bikes, but the link above relates to a clarification. The DFT recognises that Twist & Go is an important improvement for people with limited physical abilities, and the Type Approval requirement is strictly directed at manufacturers. If an existing bike that has already been used as a pedal cycle on public roads is converted for electrical assist, the fitting of a Twist & Go control does not constitute an offence.

Future legal changes
There are pressures within the EC to require e-bike users to have mandatory 3rd party insurance. There will no doubt be much wrangling over this issue in the months and years to come; but to the author's mind this is not an entirely bad idea provided that the costs to the rider reflect the actual risks. UK rules might also diverge from EC rules at some point in the future. 

Note that an M-Drive kit does not usually include a battery; the latter being sold separately because there are choices of capacity.  The Bafang BBS01 and clones are sold with a thumb throttle, usually said to be for off-road use only, but see the Twist & Go type approval exemption info above. There are also ½-twist throttles available for those who are used to riding a motorcycle.  Some motorcycle throttles have an adjustable friction pad, to allow hand signals with power on, but an e-bike throttle should spring-back when released.

eBike sales UK - Bafang BBS01 Conversion kit with 44T chainwheel.  

MAF Scooters - Bafang BBS01 with 46T chainwheel.
Classified on the website under > Parts > Electric Motors.

Panda Bikes - Panda M-Drive 36 V 250 W kit ( BBS01 clone, 48T chainwheel). 

 Whoosh Bikes - Bafang BBS01 with 46T chainwheel.

Eclipse Bikes - Bafang BBS01 with 46T chainwheel.

Pedalease - TLH 36 V 15,6 Ah Li-ion battery.

TLH Battery.  Li-ion battery packs.

Empowering E-bikes.  Good source of information as well as parts.  Good advice on how to use the BBS01.

Dillinger UK.  BBS01 Kit, product manuals, Bafang BBS brake sensor , spider adapters allowing the use of non-Bafang chainrings (see Bafang/8Fun spares).

PreciAlps, France.  Mid drive parts and accessories, lightweight (T6-7075) chainwheels and spiders for the BBS01/02. 42T is the smallest possible front sprocket size that allows the optimum chainline (i.e., a dished chainwheel). PreciAlps front chainwheels use the narrow-wide tooth profile. The company also makes a 42T extension sprocket.
Note that chainwheels for the BBSHD (1 kW) M-drive do not fit the BBS01/02.  The sprockets look similar in photographs, but the ones for the high-power drive are smaller and have a smaller BCD for the 5 mounting holes.

RisunMotor (Shanghai). Spider adapter, etc.

Alpkit hoop handlebar. The perfect solution for adding extra accessories (smartphone / GPS holder, action camera, headlight) once the e-bike conversion has used up all of the handlebar space.

Don't forget to have a look on ebay.  There's plenty there, including batteries, Bafang-fitting chainwheels, spider adapters, bashguards, cassettes, twist throttles, brake sensors, bike tools (and lots of other stuff you probably don't want).  Note however that it may be cheaper to buy expensive items directly from the supplier.

Soanes Cycles, Colyton, Devon.  Long established Devon bike shop with a reputation for good deals and good service.   The author's preferred local cycle shop, not an e-bike supplier.

Information and review:
Manuals are available from most of the conversion kit suppliers (see above). Some are better than others. Read them.

California ebike - Installing the Bafang 8Fun BBS01. (video)

ebiketips, Dave Atkinson - Installing the Panda M-Drive.

ebiketips, Dave Atkinson - Panda M-Drive kit.
The reviewer lives in Bath, and needs to climb Lansdown Hill every day to get home from his office.  He finds that the supplied 48T chainwheel far too big when used with his 11-28 cassette and suggests that the ideal chainwheel size for his bike would be in the 34 - 42T range.  It depends on the rear dérailleur ratios and the wheel diameter of course, but commentators seem to agree that 48T gives self-powered top speeds that you just don't need (~ 50 km/h / 31 mph), and this should be traded for torque at the bottom end.

 Cycling UK (the CTC), Richard Hallett - Panda M-Drive kit.
Discussion of  powered wheel and friction drive options, plus review of the Panda M-Drive and its installation process.  Notes some derailing problems due to poor chain line. Also finds 48T to give too high a top gear (with an 11T rear), and suggests that 45T would be better (note that Bafang makes a 44T chainwheel for the 8Fun).  You can still get up a 1/10 hill fairly easily with the 48T front sprocket, but the ability to manage very steep hills will be improved with a smaller one.

battery Battery University - Charging Lithium Ion batteries.

Singletrack World: Shinano Deore XT 11-46T 11-speed casette.
Modern bikes are moving to a single front chainwheel configuration as rear dérailleur capabilities improve. Although even wider-range options exist, the Shimano 11-46T Deore XT can be retro-fitted to an existing freewheel hub. It gives a fairly evenly-spaced 11-37T 10-speed gear set, with a big jump to 46T to handle extremely steep climbs. Changing into the big bottom gear is smooth due to well-designed shift ramps.

Parts needed (All parts must be Dyna-sys II pull-ratio compatible):
  Shimano XT CS-M8000 11-46T 11-speed cassette.
  Shimano XT RD-M8000 11-speeed dérailleur, Shadow RD+ shift, SGS (long cage).
  Shimano XT SL-M8000  11-speed RH bar mount shifter with visual display.
  Shimano HG-X11 MtB 11-speed chain or compatible.
  Quick Link 11-sp

Inclinometer Inclinometer (handlebar mounted).
A measure of the gradient on which the bike can pull away under its own power is extremely useful when experimenting with sprocket ratios.  This Sun bike inclinometer consists of steel ball in a curved tube filled with viscous liquid, and has scales in gradient % and degrees. - Simple but effective.

Tools and consumables:
If you're happy to do the installation yourself, chances are you'll already have many general workshop tools at your disposal (i.e., socket set, spanners, adjustable wrench, copper or plastic mallet, G-clamp, bench grinder, Allen keys, screwdrivers, easing oil, Loctite blue, cable ties, etc.).  You'll probably also have a set of tyre levers.  In that case, the following additional tools will provide the capability to do general bike servicing. You probably won't need all of them for an M-drive conversion; but if you're going to start comprehensively dismantling bikes and messing around with sprocket ratios, it's best to be equipped for any eventuality.
     The individual items are not expensive, and you can get most of them on ebay (if you search for these items online, put "bike" into the search term).  Note that some of the tools shown below are for the amateur market, and will not necessarily survive heavy industrial use; but they should do the job if applied correctly and not subjected to excessive force.

chain wrench Multi-purpose chain wrench (sprocket wrench with hook wrench and splined-nut wrench).  
The spline wrench fits the Bafang outer aluminium M33 bottom bracket nut. The chain wrench is used for holding the rear sprocket cassette still while undoing the retaining nut.
C-spanner C-spanner (hook wrench).
19 - 51mm size is required for the Bafang M33×1.5 bottom bracket castle nut. A cheap multi-purpose  wrench is not good enough for the job.
crank puller Crank puller (for removing pedals).  
The basic tool is used for square taper spindles.  Shimano Octalink (8 spline) and Isis (10 spline) crank arms require an additional anvil piece that fits into the end of the shaft (see below).
Octalink / Isis adapter.  
For removing splined cranks when using a puller designed for square taper cranks.  Left: magnetic type, snaps onto the end of the puller; Right: Shimano TL-FC15.
bottom bracket tool Bottom bracket disassembly tool (20 spline).  The chainwheel side bottom bracket retainer nut has a left-hand thread.  To loosen the nut, hold the tool in place with a G-clamp, with a piece of wood to protect the paint on the other side of the bracket.  A sharp (but not particularly hard) tap on the wrench with a soft mallet should get the nut turning.
cone spanners Cone spanners.
Used for adjusting the end-play of tapered axle bearings.
casette tool Sprocket cassette removal  tool.  
Used for detaching the cassette hub (Shimano / SRAM type shown). Needed if you want to replace worn cogs or change the rear dérailleur ratios.  The outer nut  fits into a 24mm or 15/16" socket wrench.  
chain rivet extractor Chain rivet extractor.  
Needed for removing the front dérailleur if no quick-link is fitted.  Also a general necessity in any bike toolkit.
Quick link.  
Convenient for rejoining chains, and more reliable than the practice of pushing pins back in.  Make sure you get the right width for your rear cassette (8, 9, 10, 11 speed, etc.).
Quick link pliers.  Some quick-link pliers can only open the link (i.e., pull the rollers together).  You might as well get one that can also close it (i.e., pull rollers apart), although a sharp tug on the chain works just as well.
TL-FC21 Chainring nut wrench (peg spanner).  
For tightening the fasteners that attach the front chainring to the spider.  This one (Shimano TL-FC21) also has a crank cap remover and 9 and 10 mm wrenches.
T-handle peg spanner T-handled chainring nut wrench.
Allows more torque than the flat peg spanner, but can only be used when the chainwheel is off the bike.
spoke wrench Spoke wrench / spoke key (for 10 - 15 gauge spoke nipples).
Not needed for M-Drive conversion, but used for general wheel tuning.

Bikestand Bike stand (30 kg rated)
Once you have fitted the Bafang display to the handlebars, you can no longer turn the bike upside down for servicing.  A stand is also convenient for making electrical measurements and setting up the dérailleur (etc.).  A 30 kg rated bike stand can be had for about £25.
Torque wrench (0-300 Nm)
Satra S-T300W ½" beam deflection type (shown) is ideal for tightening crank bolts, inexpensive (£12.50), and refreshingly free from scales calibrated in furlong firkins or other non-SI gibberish.
8mm Allen key bit socket (½" square drive)
For tightening the pedal crank bolts (socket sets often include this item).

Ratchet Adaptor / Adapter.
Converts torque wrench or breaker bar into a ratchet drive. The amount of torque that can be transmitted must be stated by the manufacturer (if it isn't, you don't want it). Sealey AK737  ½" sq. drive (shown) withstands up to 512 nM.  Sealey AK7371 ¾" can withstand up to 1412 Nm.
Steel wire / bike cable cutter.

Quartz shard

Scale divisions in mm
This quartz shard, about 8.5 mm in length managed to insert itself point-first into the rear tyre of the Sterndale.  The bike was wheeled home, and the puncture in the inner tube located.  Then the tyre was carefully searched for penetrating objects in the vicinity of the puncture.  The problem here is that the tyre moves relative to the inner tube while the bike is being pushed. Hence the shard was not found immediately and caused two more punctures before it was finally located. A throrugh search of the tyre surface is evidently a good idea when fixing punctures.