Chain driven chainsaw build

Hello,

I am a bored college student home for the summer, and I really enjoy building various things. I have always wanted a motorcycle, and I was thinking about attaching some kind of engine to my bicycle.

The bike is a Pacific XL 2 26" mountain bike (21 speed). I have looked around quite a bit, and have decided to do a chain driven install using a chainsaw engine. I don't have the engine yet, but I'll be checking craigslist for the next few days to see what comes up (hopefully something around 40cc).

I plan on welding an additional sprocket (~60t #41) on the opposite side of the factory gear system on the rear wheel. Then, I plan on welding another sprocket (~8t #41) on the drive system of the chainsaw and connecting the two with a #41 chain. This should allow me to retain use of the pedals in case there is a problem.

I have ran the calculations for the setup, and it says the max speed should be around 37mph@3600rpm, which is very reasonable. I weigh ~120lbs., so I am going for something around 20-25mph for cruising. I know that the optimum gear ratio is 10:1-12:1. Should I get a larger sprocket for the rear tire to increase the 7.5:1 ratio I already have? Why are people using jackshafts and gearboxes if they can just directly connect the engine and wheel via a chain?

Another thing I am concerned about is the clutch system. I know that chainsaws have a centrifugal clutch. Can I utilize the one that's already on there and just weld on a sprocket to the output shaft? Why would I replace it with another clutch assembly or remove it like others do? Sorry if I don't understand this concept correctly..

Thanks for all your help!

Thanks for your input. I don't want to make it too complicated by introducing multiple gears. I thought the jackshaft was to change the gear ratio (manipulate speed and torque) though. Won't a 4.4:1 gear ratio be too high for the engine to turn the wheel? Will it get any torque that low? I think the typical rpm for a chainsaw is like 10-13k, so I thought it would need to be around 10:1-12:1 to be able to climb any hills.

Thanks!

Would it be possible to utilize the sprocket that's already on the centrifugal clutch (that originally gripped the chain saw blade)? That would be a lot easier than welding on another one. I can't find anything about it being a standard size (35, 41, etc...).

Thanks for the tip, I guess I'll just have to be really careful when I weld on the clutch sprocket.
Do you think that a gear ratio of about 9:1 (8t clutch, 72t rear wheel) will provide enough torque for small hills? I live in Florida, so I do mean small haha.

I have come across people using a jackshaft to further increase the gear ratio (providing more torque and less speed). I'd really like to not have to use it, but I'm a bit concerned that it'll be really whimpy if my gear ratio is that low (but will theoretically go 60mph@7000rpm).

This guy has a gear ratio of 25:1 :O
MotoredBikes.com: Motorized Bicycle Forum - View Single Post - Chainsaws,weedwhackers,snow blowers...oh my!

What do you mean by "cent clutch"? Are you referring to the stock clutch?
I might as well try it and I can always switch to a go kart clutch if I'm having trouble with it.
I'm trying to do this build as inexpensively as possible

And as far as a friction drive setup, I don't like it for a number of reasons- inefficiency, rapid tire wear, slippage when wet, etc... A chain/belt system seems like such a better way to go even if it takes a bit more work to get the product engineered.

I'm going to try and find access to a welder so I can start building the mounting hardware for the engine and see if I can locate a good engine to use. I've decided to use 72t on the rear wheel and 8t on the clutch for a gear ratio of 9:1. This should allow decent torque and speed within the engine's powerband.

I'm going to try to attach the sprocket on the left side of the rear wheel as to not interfere with the existing gears that attach to the pedal system. Hopefully that should allow me to still operate the bike in the event that the motor breaks.