Request for feedback on custom design for motorized bike



Hello all.

I'd like to describe my current ideas (very rough) for adding an engine to a bicycle without any kits or pre-fabbed parts.

Right now I'm planning on using an approximately 30 cc weed eater engine for the powerplant.

I've done a little research, and I've found that there are two basic types of drives: friction (direct to wheel), and chain drives. I'm more interested in chain drives.

However, all the chain drive designs I've found have a separate chain for the engine. I would prefer to put the engine on the same chain as the pedals.

Why? For a couple of reasons:
1) I don't have to modify the rear freehub/free wheel assembly

2) No extra chain to worry about and get tangled up in things, etc.

3) It would allow me to shift gears, thus changing the drive ratio, and allowing me to achieve a higher top speed on a smaller engine.

Now, there are two main disadvantages:
1) Where to put the engine? The place I have settled on that is out of the pedal-sweep area is about midway up the chain-side seat stay. I don't think putting the engine between the pedals would work for me, as I also ride "real" bikes a fair amount, and the wide spacing needed would upset my knees (it happens stationary bikes like you find in gyms).

2) The pedals need to be able to rotate independently of the chain/front cogs, otherwise you would have to pedal the same pace as the engine - not fun! This is a pretty big problem.

My current solution to #2 is to modify a free hub assembly to fit in the bottom bracket. This would allow the pedals and front cogs to spin independently. I realize this is not a simple task, and it may prove to be impractical. But as I said, the goal for me here is to learn and experiment.

So, what are your thoughts on my idea(s)?



augidog said:
these guys have a really different kind of part is, tho, they offer a freewheeling crank all ready to go at a decent price 8)

Thanks for that augidog! I ran across them later last night, after I posted this. My only concern is their pedal shaft and cranks may not be standard, forcing me to use their cranks and pedal shaft.


i guess yeah, you'd need to get the whole set from them. i bet that stuff's pretty heavy-duty, it sure looks like it.


Local time
7:17 PM
Sep 24, 2007
Free wheel crank

I bought an old Ross Eurosport 10 speed yesterday from Craigslist for $25 (rode it from sellers house to car, fine condition). It has freewheel cranks. In trying to research if this was stock, came across many for sale all over the net.

I liked your idea of one chain. Would simplify things.