# Gear ratio?

Discussion in 'Transmission / Drivetrain' started by ghost681, Oct 14, 2013.

1. ### ghost681New Member

Hello Im new here and was wondering if anyone could help me. I just bought a 33cc chainsaw engine and planned on mounting it onto my bike and using the current gears on the bike to drive it. I have a mountain bike with 8 gears but a derailleur made for 7 gears so i have one thats useless, and its the largest gear aswell. My question was with 26" tires, 6 teeth on the drive sprocket, and 32 teeth on the sprocket on my bike, how fast would i be able to go? Would I have more torque or horsepower, and would this even be able to push me like this? Ive been reading articles and I just cant figure out how the ratio works, I even downloaded a program someone here made a gear ratio calculator and it tells me Id be moving 43mph at 3000rpm which I just cant see to be correct. Everything I was reading was saying 20:1 or 30:1 and mine is a 5.33:1 so I dont know the difference, could anyone explain? Thanks you!

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2. ### MotorBicycleRacingWell-Known Member

The gear ratio calculator is correct. 43.5mph at 3000 rpm

Your bike would be way over geared so you need more reduction.

what is the redline on that 33 cc engine?

3. ### 5-7HEAVENActive Member

If your gear ratio is 5.33:1, which is totally useless for your 9000rpm engine.
You will definitely burn your clutch, while your bike will not move from a standing start.
To put things in perspective, my first gear is 46.36:1, which I use to climb very steep hills and ramps.
Normally, I use 2nd gear for starters, which is 38.18:1.
Eighth gear is 15:1, but I use 7th on the flats, which is 17.73:1.
Chainsaws like very low gearing (high numerically).
You'll need gears like mine to fly.
If your engine runs counterclockwise, you cannot chain it directly to your cassette gears.
In fact, your engine sprocket will be facing the other side of the bicycle.
Then you'd have to build one, maybe two jackshafts to get the required gear ratio
to twist that chainsaw engine. Given your level of expertise, I would advise you to
abandon the idea of connecting to the cassette. The easiest way would be to fab
a friction drive to connect your engine to the rear tire.

Good luck.

4. ### ghost681New Member

Im not entirely sure on the redline of the engine, i only bought it a few days ago. So does this mean I would need to pedal the bike up first and then it would push me at higher speeds? Or would I have more torque and less top speed? Im pretty sure the engine spins clockwise which seemed great at first but now doesn't seem so hot if I need to build a jack shaft. Friction drive is really out of the option I don't want to waste my time on that, i hear it doesn't grab all the time well, will slip if wet outside, and wears the tires down. If I did make a jack shaft, how many teeth would I need on either sprocket in the jackshaft to get a desired gear ratio with my current sprockets? What I want to do is be able to take the bike on trails or on the streets and pedal along with it in a gear with a good amount of torque so its not just the engine pushing, Id be helping it. And is there no way that the engine will help me with the current setup I have? Thanks!

5. ### zippinaroundActive Member

about 89tooth to 120 tooth rear spocket if your using a 6t drive sprocket should do the trick.

6. ### bigoilbobMember

Kiddin' right? Nothing's impossible, but that 120 tooth sprocket is one manly sprocket. FUR not kiddin', would U pix 1 such sprocket? Not dusputin', just never seen 1.

8. ### bigoilbobMember

Thx Z, my brain was anchored in the concept of larger teeth. But F the smaller ones can pull hard enough (especially thru the smaller sprocket), why not?

9. ### 5-7HEAVENActive Member

bigolbob, think of starting off in your Toyota Corolla in 5th gear from a stop sign. That's what improper gearing would feel like.

10. ### bigoilbobMember

Thanks 5-7HEAVEN, but I get it and got it long ago. My Bikee E2 tandem recumbent with a Nuvinci Developer's kit has a low gear engine/wheel rotational velocity ratio of ~78, and the powered wheel is 20". Auto clutch engages fully at less that 3 mph, and my wife and I + groceries can climb 18-20% grades, with Honda GX35 power only. So, I'm hip to "improper gearing".

Now, I'm revved out at 19 mph, so I certainly admit to the "other" problem. That's why any insight into my question in the "jackshafts" subforum would be appreciated.....

11. ### ghost681New Member

ok so how fast would i have to pedal the bike before the engine would start to do anything? and no one answered how many teeth id need on sprockets for a jack shaft. If I wanted to make one what two sprockets would I put on either end? Thanks for all the help btw.

12. ### zippinaroundActive Member

just use the ratio calculator to work it out!

13. ### 5-7HEAVENActive Member

I would say 30mph downhill.
In other words, no matter how fast you pedal, the engine will not help you.

Using Murphy's Law, the smallest engine sprocket you'll find is a T8F 11-tooth one.
Match that to a 54t on the jackshaft to get 4.91. Use another 11t on the other jackshaft end and a 72t chainring.
The inside chainring is 24t, which chains to the 8-speed cassette. First gear is 34t, with a 45.52:1 gear ratio.
This allows you to climb very steep hills and ramps. Second gear is 28t with a ratio of 37.49'
which you can easily drive away from a stop. Seventh gear is 17.4:1 and highway 8th gear is 14.73:1.

With these gears, your engine will scream up to 40mph.