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 km/h, 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 said by some commentators to be 12 kW.
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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