Chain driven chainsaw build

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elliotsimonster

New Member
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!
 
10t on engine , 44t on rear wheel.
a jackshaft is so you can shift gears for the engine like a car.
conecting just to the rear wheel the bike will run like its in 1st gear all the time.
 
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.
 
I will comment that you won't cruise at 37mph @ 3600 rpm, the engine won't pull it. You want the rpm to be a little higher (more reduction) like around 6-7000 at 35 mph.

Your project is very do-able, with a few slight changes.
You can weld a sprocket to your present clutch, just be carefull not to warp it.

Others here with more knowledge on the DIY bike subject will weigh in here shortly.

Welcome to the forum!
 
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...).
 
elliotsimonster said:
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...).
Click to expand...

No the chainsaw centrifugal clutch will not spin a chain, if you look closely to that "sprocket" it is not a geared type sprocket like that. It will have small grooves in it only, as the chainsaw chain has small "teeth" that catch those grooves almost the opposite of a bicycle type chain.
 
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!
 
Also you will not move that bike at all with that cent clutch you will kill the motor before the rpm's are high enough to grip right(Done it). You can beef the clutch up though by adding more weigh and strong springs however it wouldn't be very cost effective as opposed to just buying one already designed for a performance go kart. All in all it is going to be a lot of work to get it working right. That by no means is meant to discourage you because the payoff is much greater if you have had to engineer the bike yourself. Then you can garuantee a fully custom ride. And a big fat smile on your own face when the questions come about who built it or where did you buy it.
 
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.
 
I used a trick peg from a bmx. bolted to the motors crankshaft. fricktion drive. this set up runs the bike at 12/19 mph with out over reving the motor. if you run full thotle you will kill the motor in a mile or two
 
I have a 65 tooth rear sprocket and a 10" wheel - then a 6 tooth front sprocket and a gear ratio of about 10.83 to 1. With a 61cc chainsaw engine this pulls up hills well with this type of clutch system (TINY) though it is a bit hard to climb a hill from a standing start. Once the engine winds up it hits powerband and climbs pretty quickly to around 12500rpm where i have tuned it.
 
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