Electric Motorcycles

Here's my notes on building an electric motorcycle.

I think I've decided by now that it won't be practical for me to build one right now, but it was fun finding out more about them. I hope these notes will be useful to others ...

Initial Parameters

From my initial enquiry to the EV list ...
So far I'm thinking:

  • Electric motor with 10-20hp peak and maybe 1-2hp continuous.
  • Simple chain drive to rear wheel.
  • About 80kg of sealed batteries in case it falls over -- maybe Optima Yellow Top? Either a 48V or a 60V system.
  • Some kind of twistgrip controller. Are regenerative ones available in this power range?
  • Some kind of onboard charger, so the bike can just be plugged in to the 240V mains and left overnight / at work / etc.
  • All shoehorned into a rolling chassis from a streamlined 250-400cc commuter would be about right ... kerb weight about 160kg before, less than 200kg after I hope.

Does this sound reasonable?

I've gone through the bikes in the photo album and going by the stated ranges and battery configs, the efficiency seems to vary enormously ... between 50 Wh/km and 400 Wh/km! Anyone know a good rule of thumb for a two-wheeled electric beastie?

I'd need to get a 'regular' range of maybe 40km (25mi) and a 'regular' speed of 60km/h (38mi/h) out of it to make it a practical bike, and maybe occasionally a 60km (38 mi) range or a 80km/h (50 mi/h) top speed.

Is this practical with the available tech, or will I just have to wait for fuel cells or unobtainium batteries or something?


-----Nick 'sharks' Moore

Some investigation


I found a whole heap of data on electric bikes on the Electric Motorcycles Photo Album, and tried to use this to work out what kind of Wh/km to expect. As you can see from the graph and table below, there's quite a lot of variation:

BatteryRangeSpeedDrainNameBatteriesMotorFinal Drive
4865312032-517776EMB LectraOptima D750SEtekChain3.375
48552460488055E-ScramlerPowerSonic 55AhAdvanced DCChain2.53
48401920408148250XLSLA 40AhEtekChain
48502400308680XinetaHawker 12V 50AhCupex (perm)Chain
4844211224-408866ReCycle22AH SLA (paired)EtekChain
3611541403288129ElectroCycletrojan 12V TMHPrestoliteChain6
724129526-13100+307BEMOptima YT D51Advanced DCChain
72100720027104265PiggyTrojan 100AHAdvanced DCChain4.6
9642403224-32128+144REVisionHawker GenesisAdvanced DCShaft~3

Of course, this is total battery Wh, not actual Wh used to travel that range. There's a huge variation in claimed efficiency though ... perhaps different depth-of-discharge (DoD) strategies, different motors, different gearing, aerodynamics all add up.

Choosing Batteries / Optimizing Optimas

I'll probably be stuck with the lower lines on the graph, eg: Optima YTs or Federal Gel-Tech, because the deep-cycle types don't offer enough peak amps. Such is the way of engineering tradeoffs! Found some good numbers of Optima YTs discharge cycle, and it seems like about 40% DoD should give the best lifetime in km.


Okay, let's assume for the sake of the argument I can get 100Wh(cap)/km ... assuming a 50% DoD, that's really a consumption of 50Wh/km, at say 50km/h that's 2500W, which sounds about right.

So for a ~35km range I'd need 3.5kWh off battery capacity ...

#TypeCapacity (total)Weight
Price Retail
VAh(20hr)Wh(cap)CCAPeak kWkgmmAUDWh / $
4Optima YT D314875360090043.2108325*165*238$18401.9
4Federal Gel-Tech 8G244873.6353041019.797276*171*251$12002.9
5Optima YT D346055330075045.097.5254*178*198$19251.7
6Federal Gel-Tech 8G22NF7250360024517.6103238*140*235$14342.5
14Thunder Sky LiIon TS-LMP8050.4804032?200??10?42220*145*61?$2000??2?
14Thunder Sky LiIon TS-LMP20050.420010080?200??10?95.2285*182*100?$5000??2?

... fitting them into a bike-sized vehicle won't be easy. Cardboard models seem the way to go. I've added in LiIon battery options because they might be a good option ... they seem to be about the same $/Wh but half the kg/Wh. On the down-side, they require a per-cell Battery Management System. See Victor's CRX under 'Links:' below ...

Motor and Drivetrain

This is a bit out of date, as the Etek M7 has been replaced with a couple of new contenders: from B&S, the Etek RT and Mars Brushless, and from Perm the PMG132. So the following is a bit out of date. Needless to say, the technology has only gotten better.

The simplest setup is likely to be the permanent-magnet Etek M7 from Briggs & Stratton. They're pretty widely used in the electric motorcycle world. There's quite a bit of info on the Etek at robotcombat.com and powerped.com.au (scroll down) and it looks like it'd be easy enough to mount in a motorcycle application.

The Etek will spin at 72RPM/volt, so at 48V that's a top speed of 3456RPM. A motorcycle wheel&tyre is about (hand wave) 2m in circumference. Front sprockets go down to about 13 teeth, and rears up to about 57, so we'd have a max speed of 3456 RPM * 13/57 * 2 m/R = 26 m/s, or about 94 kmh flat chat. Sounds about right, perhaps a little high, but to get a higher final drive ratio would mean adding an idler shaft or similar, and I'd rather not. On the other hand, if you were going to add an idler shaft, you could add it coaxial to the swingarm pivot, to keep the chain/belt lengths constant.

Motorcycle sprockets are generally splined to fit a shaft, but it's probably easier to machine a sleeve keyed on the inside to fit the Etek output shaft and stepped on the outside to make it easy to braze/weld the front sprocket to the sleeve.


One of the disappointing things about a permanent magnet setup is that it makes it hard to do regenerative braking. Since motor voltage is proportional to motor RPM, the only time you'll get a voltage high enough to recharge the batteries is revving the motor beyond its normal maximum RPM ... and on a direct-drive machine, that'd only be when rolling down a hill quicker than the bikes top speed ... not likely to be a common occurance ...

One way around this is to use a "Contactor controller", or series-parallel switch. Put simply: split the pack in half, and wire these halves up so that they can be connected in series or parallel. This provides two 'gears' ... in parallel, maximum current is increased for more torque, and in series maximum voltage is increased for more top speed.

Serial vs. Parallel

Pretty obviously, fuses would be needed to prevent the batteries exploding if a contactor stuck or was closed at the wrong time.

And, of course, dropping down into Parallel reduces the battery voltage, so if you've sped up to top speed in Series and then shift into Parallel, there's sufficient voltage to regeneratively charge the batteries. It's not exactly sophisticated, but it's a start. The idea can be extended:
Num Batteries Num Gears. Gears
( Nseries * Nparallel )
Top Speeds
With top gear top speed = 100
2 2 1 * 2 2 * 1 50 100
4 3 1 * 4 2 * 2 4 * 1 25 50 100
6 4 1 * 6 2 * 3 3 * 2 6 * 1 17 33 50 100
8 4 1 * 8 2 * 4 4 * 2 8 * 1 12 25 50 100
The complexity gets out of hand pretty rapidly. Six is probably the largest useful number ...

Just as the selection of series vs. parallel is a bit like the selection of gears in a gearbox, the two likely strategies are manual or automatic switching. For manual switching, a control would select which contactors to close, giving a selection "Parallel-Neutral-Series" just like 1-N-2 on a motorcycle gearbox. The throttle setting would provide PWM control over power and regen. For automatic switching, a controller would monitor throttle position, battery voltage and motor voltage, and select the appropriate 'gear' and PWM setting.

Someone on the list pointed out that this would be a Bad Idea with most controllers: the input stage capactitors need 'precharging', generally done by a resistor in parallel with the contactor. Switching voltages would be messy. Instead, if you wanted to do S/P switching, you'd need a controller designed for the job, with input stage capacitors _before_ the switch. Since the S/P switching is mostly there to do regeneration, and since the controllers people are talking about don't do regen anyway, it's not a big deal -- I'd need to build my own controller anyway!

Old Shopping List

Kawasaki GPX250 or similar rolling chassis
inc. minor repairs
trading post?$500
Chain & Sprockets (motorcycle)bike shop$250
KIT: Etek M7, Alltrax controller, charger, throttlecurrietech.com.au$2400
4 * Optima YT D31?$2000
Motor mount plate & front sprocket carrier & battery cagescustom$300
Sundries: (lights, paint, shrinkage) $500

... which, sadly, probably puts this project out of my league. Compared to buying a decent GPX250 for $3000 and $3000 worth of fuel, it'd take perhaps 40,000km to break even ... and given the range (40km or so), that's 1000 discharge/recharge cycles (perhaps 3 years) and by then the batteries are probably half shagged. Oh well. It'd be a nice toy :-).

New Shopping List

On the other hand, if the batteries and controller could be gotten cheaper, perhaps by sponsorship on the batteries and by importing parts directly from the States (I've seen Etek+controller for US$700), perhaps it'd break even ... perhaps this explains why most of the E-vehicles seem to be from the States, even though fuel is cheap over there.

The constant torque of a DC motor, as compared to the jerkiness of a ICE, probably means I could get away with a light chain. Belt drive is another option, but finding matching ratios may be a problem. Actually Cloud Electric have #40 sprockets down to 11 tooth (on a 7/8" shaft) and up to 60 teeth on a four-hole rear sprocket. So ratios up to about 5.5:1 might be possible.

Kawasaki GPX250 or similar rolling chassis
inc. minor repairs
trading post?$ 500
Chain & Sprockets (#40)Cloud Electric$ 100
KIT: Etek M7, Alltrax 300A, etcCloud Electric$1200
4 * Gel-Tech 8G24Solar Online$1200
2 * 24V 6A Smart ChargersJaycar$ 200
Motor mount plate & battery cagescustom$ 200
Sundries: (rattlecan paint) $ 100
... which makes it look a lot more tempting ... hmmmm ...

LiIon Batteries

The limiting factor on the above is the sheer weight of batteries it'd need to lug around, so I'm considering what could be done with LiIons. Thunder Sky have Lithium cells at prices competitive with a Optima YT pack. Apparently they're Cr-F-Li chemistry.
TS-LCP50AHA	3.6V	50Ah	1600g	190x116x46	US$98.
TC-LCP100AHA	3.6V	100Ah	3000g	220x145x61	US$200.

To get to a nominal 48V (for the Etek) it'd need about 14 cells in series: so that'd be 42kg / US$2800 for the 100Ah pack.

One limitation of these cells is that they become quite inefficent past 2CA discharge, eg: you shouldn't try to get more than 100A out of a 50Ah cell. This makes a series/parallel setup essential to run an Etek motor ... with two banks in parallel, you could source 200A without sagging much. Another possibility is to use UltraCapacitors as an intermediary.

The other problem is that LiIon batteries need a per-cell Battery Management System. Mostly what this seems to be is a method for making sure that the voltage across a 3.6V nominal cell never gets above about 4.3V. The easiest way to do this is to shunt current past the cell, with a circuit which is more-or-less a 'sharper' version of a Zener diode.

Simple charging system: A power supply capable of 32V 10A sits across a bank of 7 Lithium cells, each of which is clamped to 4.3V. While they're charging, the bank voltage will be <30V. When all shunts are shunting, bank voltage will have risen to 30.1V, at which point the charger can turn itself off.

These batteries apparently like to be variable current charged, so the one power supply can switch once a minute between the two banks. Now, admittedly, that's a 320VA PSU we're talking about, so it's still no lightweight. But it's not a welder either, and it doesn't have to be too sophisticated since the clampers do almost all the regulation.

They also have integrated packs with built-in charger at very good prices. Two of the 24V packs seem the obvious easy solution to my e-bike's power needs, but I'd like something which could handle regeneration ...

TS-IC36V18AH    36V	18Ah	7200g	415x97x132	US$300.
TS-IC24V50AH    24V	50Ah 	18kg 	405x122x202	US$700.
TS-IC24V90AH    24V	90Ah	24kg 	490x120x217	US$1000.
... still, it might be an effective way to get started without building too much fancy stuff. Two 24V 90Ah packs, a series/parallel contactor setup on a 'gear shift' (no clutch, so left hand has some free time) and it'd be away! About the same price as 4*Optimas + 2*chargers, slightly better range and half the weight. Sounds like a pretty good deal. It'd mean I could move to a lighter chassis, too -- now, a Gilera CX125, that'd be cool! Pity they're a little on the rare side.

LiIon Shopping List:

Light motorcycle rolling chassis
inc. minor repairs
trading post?$ 500
Chain & Sprockets (#40)Cloud Electric$ 100
KIT: Etek M7, Alltrax 300A, etcCloud Electric$1200
2 * TS-IC24V90AH 24V 90Ah batteries w/ inbuilt chargerThunderSky$2666
Motor mount plate & battery cagescustom$ 200
Sundries: (rattlecan paint) $ 100



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