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Such as with electric assistance. I'd like to install a gasoline engine kit (because electric ones are wildly expensive) on my bicycle, without the sprocket: I want to attach it to the existing gears, because I would then be able to shift through the gears to increase/decrease speed as I wouldn't with a 48-tooth/32-tooth sprocket. I have not been able to find a kit that will place the chain on the right of the bike, where the gears are. Is there a kit that can do this, or is there a way to fadangle a normal kit to run through the existing speeds?

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  • I'm new to this particular community, and have looked in the what's on-topic page. Please retag if necessary. – Ollie Apr 11 at 19:34
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    You might be more on topic at mechanics.stackexchange.com because you are basically building a motorbike. – Vladimir F Apr 11 at 19:44
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    It's unclear what you're asking. How can you have a chain without sprockets? – Daniel R Hicks Apr 12 at 0:09
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    @DanielRHicks I suspect its a handedness issue, All the refit kits I've seen put an additional chain on the left-hand side of the bike, and need to be pedalled up to speed before the rider engages the liquid engine because they can't pull away from stopped. I'm guessing OP wants to find a motor kit that simply powers the bike's main chain and the gears/derailleur directly. – Criggie Apr 12 at 11:32
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    @Criggie - But as it stands the question is very unclear. – Daniel R Hicks Apr 12 at 11:37
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Generally speaking, no.

A liquid-fuel engine wants to turn a lot faster than a cyclist would turn the cranks. That's why a retrofit engine often has a small drive cog and an enormous cog on the rear wheel, to decrease the revs and increase the effective torque.

The optimal speed of a small capacity liquid engine is somewhere between 1000 and 2500 RPM. A cyclist would do 60-90 RPM on the crank, with 120 being possible-but-abnormally high.

If you want a motor bike, its often cheaper to buy a 50cc motor bike and leave your normal bicycle unmolested for riding.

Also, if you remove the ability to pedal, then your ride becomes a motorbike no matter what displacement limits you're under.


Additionally - these liquid engines cannot generate enough torque to pull away from stopped. So the rider has to pedal up to a minimim of 10-20 km/h and then can release the clutch. Trying to start a small displacement motorbike from stopped is impossible without stalling.

You need the pedals working to get them moving.

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    Sounds like a novel technique for high cadence training. 120rpm?? That’s for losers, 2500 is where it’s really at. – MaplePanda Apr 12 at 0:01
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    @MaplePanda go back to the 90s an a certain Texan used his high crank RPM as a "new technique" to explain the mysterious power he could provide. Since then, only fixed gear track riders tend to pedal up that fast. – Criggie Apr 12 at 0:21
  • What if the bicycle was a free-wheeler, spinning without pedaling? Would it be possible then? – Ollie Apr 12 at 0:38
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    @Ollie the mechanicals of liquid engines are outside the scope of bicycles. But if your bike has no working pedals, then its not a bike, and needs to be licensed/registered/warranted/insured as a motorbike. Pedals have to work for it to be a bicycle. Look at the chain size on a motorbike, and compare it with a normal bike chain - there's a huge difference even on a 50cc scooter – Criggie Apr 12 at 2:53

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