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first time poster, first time lurker. I'm aware of how this sounds, but with me being a complete noob about most mechanics, I thought I'd rather ask.

If I, hypothetically, wanted to change a 740cc/750cc engine in a chopper to electric, how should I go about choosing what voltage, engine size or power in general? I've seen some builds (or this one build, precisely) place a forklift 24V after modding it, and I'm not certain if that would be a rule for all engines like that.

I'm not looking for any specific performances or the like other than looking for it to run properly, as well as have an idea of what to look for in any next build. Thanks in advance and apologies for the bother.

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  • Welcome to Motor Vehicle Maintenance & Repair! I don't know specifically, but would assume you'd want as much torque from the engine as from your ICE, that way it will feel relatively the same in the power department. If you don't have enough torque, it runs the risk of not being able to get out of its own way. If you have too much torque, you'll be blowing the drive tire off and causing a spill. Just my thinking and not a knowledgeable source, so leaving as a comment.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 29, 2022 at 14:11
  • You might get more detailed responses on Engineering.
    – DavidRecallsMonica
    Commented Jul 29, 2022 at 16:09
  • 1
    The problem you'll have with an electric motorcycle isn't the motor, it's the batteries.
    – GdD
    Commented Jul 29, 2022 at 16:58
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    There are platforms which specialize in homebrew EV conversions, and that would be a much better place for queries like this.
    – Harper - Reinstate Monica
    Commented Aug 2, 2022 at 0:40

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First of all, there is no such thing as an electric "engine". It's electric motor.

That being said, what you might choose is entirely up to what you are trying to accomplish. Engines for vehicles are usually rated in HP while electric motors are usually rated in Watts or Kilowatts. But one system is easily converted to the other using the formula:

1 HP = 745 Watts

Let's say we have a typical 750cc motorcycle engine that makes 50 HP. I know, some make more and some make less, but let's assume you want 50 HP.

So that is going to take an electric motor rated at 50 * 745 = 37 KW

That's an odd number and most motors come in increments of 5W so we might round down to 35 KW. Now you search for some DC electric motors that are in the 35 KW range. Pick one you like.

Now you need a motor controller. Search for DC motor controllers that can handle at least 35 KW (or design your own) and get going on that.

Finally you need a battery. How big depends on how much range you want. Keep in mind that you don't need to supply 35 KW 100% of the time. What does your riding profile look like? Let's say you are going to want an average power of 5 KW. If you want 100 miles of riding at an average 30 miles/hour that's going to take:

3.33 hours * 5KW = 16.6 KWh of battery capacity.

Ok, so now you buy or build a battery pack that fits into the space you have left on the bike.

Some other things you might consider:

  1. Regenerative braking
  2. Charging controller
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  • Finding a 16 kWh battery bank that fits into the space is going to be much harder than you make it sound :-). Just to illustrate, a typical car starter battery packs about half a kWh. That's lead-acid, lithium chemistries are denser, but not that dense. A laptop battery/power bank can be up to 100 Wh, so you gotta use hundreds of them.
    – TooTea
    Commented Jul 29, 2022 at 18:45
  • I didn't say any of this would be practical. My figures are just assumptions.
    – jwh20
    Commented Jul 29, 2022 at 18:47
  • I know, I just wanted to add some context for any unsuspecting readers.
    – TooTea
    Commented Jul 29, 2022 at 18:49
  • @TooTea you are grossly underestimating the density of lithium batteries. Work with real world data, not extrapolations from vaguely recollected knowledge. JWH you're overlooking that a fuel engine has only ever produced its nameplate horsepower a few times in a laboratory, whereas an electric motor regularly exceeds it by hundreds of percent for limited times, and is rated for that power continuously. So a 35KW motor would be far larger than necessary.
    – Harper - Reinstate Monica
    Commented Aug 2, 2022 at 0:46
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    Yeah, 6kWh for a 25 kilo Tesla Model S blade is spot-on. It's your sense of the physical size of the units that is a bit off. That particular unit is 66x30x7.5 cm, so 14850 cc. Only about 40% larger than a Group 24 car battery, and 6-12x the energy density depending on what you call "usable". And you're not stock with a huge flatpack, the individual 18650 cells are small and packs can be built to any shape. So this need not take any more space than engine, tranny, fuel tank, exhaust, etc. etc.
    – Harper - Reinstate Monica
    Commented Aug 2, 2022 at 22:30

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