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Electric Motorcycles:
Some thoughts on the subject of converting a motorbike to electric drive.

by Dave Knight

Apart from my mid-drive e-bikes, I also own (at time of writing) a Lexmoto ZSB 125 EFI motorcycle. I actually have a full motorcycle licence, having taken the test in 1971, when I was 19 years old, but I bought a 125 so that my son would also be able to ride it with L-plates after taking his CBT.  At 8.5 kW (11.4 bhp)  max output at 9000 rpm (top speed 100 kph, 62 mph), the 125 is definitely not powerful enough for me (I used to own a BSA B31, with a tuned-up BSA 350 Gold Star engine), but the fuel economy (120 mpg) is amazing, and the range is unlimited.
     Given my success with mid-drive conversion pedal bikes, my thoughts naturally turned to the possibility of converting a reasonably high-powered motorbike for electric drive. There are numerous websites and videos on the subject of electric conversion; but so far (Dec 2019), all of these have struck me as being profoundly unsatisfactory. The problem is that the suggestions on offer generally involve deciding whether the bike is for use in the town or on the freeway. The immediate response from someone for whom two of the roads leading to his house have 33% gradients, and who also wants to use the motorway, is "you can't be serious". I have successfully arrived at an EAPC solution that can manage 1:3 gradients and also go over 30 mph, so why can't I have a decent electric motorbike?
     The problem lies in the presumption that, since an electric bike can pull-away from standstill without a clutch, then it doesn't need a gearbox. There are strong vernacular responses to that idea; but the fact that it becomes necessary to choose sprocket ratios and then limit the use of the vehicle underlines the falsehood. So what am I to do when I come off the main A-road, make it through the back country lanes, then come down the 1:5 hill and up the 1:3 slope a few hundred metres from my house?
     The problem, of course, is that while an electric motor has a much wider power-band than a petrol engine, and it can produce useful torque at zero rpm, the power-band is still not rpm independent. To get up a 1:3 gradient, the rpm needs to be set in the peak power range; and to go fast on a motorway and get up the long hills without causing a tailback, the rpm also needs to be in the peak power range. A gearbox is definitely needed; and while the 5 or 6 speed transmission of a modern motorbike might seem a bit excessive, having plenty of available ratios will allow the rider to get optimal power output in a wide range of circumstances.
     One manufacturer that has recognised the problem is Brammo, with its Empulse R model. This is a very high performance bike, with a 6-speed transmission allowing it to handle practically any situation. It also has a clutch, which is not needed for pulling away from standstill, but which enables smooth gear-changes when the bike is in motion. The problem with the Empulse R, is that it costs about the same as a new family car, and so is perhaps not the first choice for students and pensioners.
     So, we come back to the idea of motorcycle conversion; but this time with the notion of keeping the gearbox and the clutch. This, unfortunately, would have been easier in the days when the engine and gearbox were separate items, linked by a primary chaincase that also housed the clutch. This 'pre-unit'construction is now very much associated with vintage bikes (like the author's old B31), and anyone thinking that they might use the parts from an old British bike should note that those machines had the gear-lever on the rider's right side (first gear up, higher gears down) and the rear brake-pedal on the left. Adopting the now universal 'gears left, first down' configuration is probably mandatory when seeking Government Individual Vehicle Approval (IVA) for a custom-built machine. It also gets around the horrible and dangerous business of trying to remember the foot control layout when switching from one machine to another.

So, for self-build, we come down to two broad possibilities:

1) Take a unit-construction petrol motorcycle engine, remove the crankshaft, pistons, cylinders and valve-gear, and couple an electric motor to the primary-side (clutch and gearbox input) system.
     Taking away the piston-engine components will cerainly make a lot of room, but if the original engine was a short-stroke high-revving type, the crank will be of small diameter and it may be difficult to accommodate an electric motor directly in its place. Such an approach will also demand a lot of precision engineering work, and so will require access to a well-equipped machine shop.

2) Purchase a motorcycle-type gearbox designed for use with electric motors. It is possible that more solutions of this type will become available as the need for gears becomes apparent to end users, but one option already exists. This is the Denzel Gear-Box 200; which is a unit having a 4 kW permanent magnet motor coupled to a 4-speed gearbox, designed for installation into a wide-variety of motorcycle frames. It can be configured for road use, with a top speed of about 110 kph (68.4 mph), or combined with a large back-wheel sprocket for off-road use. The claimed top speed is rather high for a 4 kW motor, but the peak power is 12 kW according to commentators on the endless-sphere forum.

     Note that, unlike an EAPC, an electric motorcycle does not require a freewheel mechanism. Regenerative engine-braking is therefore possible, depending on the design of the motor controller.

     In the process of determining an electric motor equivalent to a particular petrol engine, note that there is no convenient conversion factor to get from cylinder capacity to bhp. Piston engine power output is extremely dependent on engine rpm, compression, and gas-flow topology; but for petrol engines that stay below about 8000 rpm, a figure of 16 cc per bhp (where 1 bhp = 746 W) is a useful guide. Thus a decently performing 250 cc bike might produce about 15.6 bhp (11.7 kW after transmission losses). This is good enough to reach 70 mph on the motorway, and the bike would probably also maintain that speed on gentle uphill sections if the rider changes down a gear.


DWK
2019-Dec-11

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