Gear Ratio question

lowracer

New Member
Oct 17, 2008
1,319
2
0
Charleston, SC
Hi,
I'm getting ready to put my 2nd MB together & have a gear ratio question.
What diameter drive pulley would I need to use?
I'm spot welding a narrow rim to my mountain bike rear wheel to run a v-belt. The actual diameter is 22" (not 26").
I'd like to get about 40 mph using a 2hp, 2 stroke, 40cc without other transmission reduction bolt-ons. The engine makes 2hp @ 6000 rpm's & peak torque @ 5000 rpm's.
Thanks
-Alex-
 

ocscully

New Member
Jan 6, 2008
373
1
0
Orange County, CA
Index of /ratiocalc here is a gear ratio calulator that may help you figure out what you need. Most of the kits offered from the various vendors have total reductions in the neighbor hood of 18.75:1 - 22:1 to get 18.75:1 with your 22" driven pulley you would need a driver that measures 1.173 in. (I don't think a v-belt can bend that tight?) and also when you put those numbers into the ratio calucaltor and input 5000 rpm you only get 20mph.

ocscully
 

Dan

Staff
May 25, 2008
12,775
108
48
56
Moosylvania
I am doing a build with a Honda GX50. Could some one suggest a proper ratio. I tried the calculator (Thanks Ocscully) and found out my 50cc MB will go 4,054 MPH. It seems I am a bit lost.

I am looking at this jack shaft set to just try and figure this out; "Includes shaft, 10 tooth sprocket, 14 tooth sprocket, 2 bearing hangers, 2 standard bearings, 2 keys and collar"

I keep losing the concept after 2 sprockets. So please typ slow,

I was trying to avoid a gear box, but this ain't lookin good.
 
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ocscully

New Member
Jan 6, 2008
373
1
0
Orange County, CA
I am doing a build with a Honda GX50. Could some one suggest a proper ratio. I tried the calculator (Thanks Ocscully) and found out my 50cc MB will go 4,054 MPH. It seems I am a bit lost.

I am looking at this jack shaft set to just try and figure this out; "Includes shaft, 10 tooth sprocket, 14 tooth sprocket, 2 bearing hangers, 2 standard bearings, 2 keys and collar"

I keep losing the concept after 2 sprockets. So please typ slow,

I was trying to avoid a gear box, but this ain't lookin good.
Lets take our target to be 18.75:1 total reduction. I think you can get a clutch for your honda GXH50 with a 12t output so you input 12 into the first field of the calculator. then you want to get at least 3:1 to your jackshaft maybe even 4:1 so for 3:1 you would input 36 and for 4:1 input 48 into the second field. then you would have another 12t driver on your jackshaft so input 12 into the third field then your final driven sprocket at the wheel lets say a 56t so input 56 into the forth field.
so when you input the 36 for 3:1 to the jackshaft you end up with a total reduction of 14:1 (not enough) but this set up would give you a max speed of 39.8mph @ 7200rpm but you would burn up the clutch in very short order on the starts. With the 48t 4:1 to the jackshaft you end up with a total reduction of 18.67:1 (target 18.75:1) max speed 29.8mph @ 7200rpm.
To get to the target total reduction with the 36t 3:1 to the jackshaft you would need a 75t final driven sprocket at the rear wheel.

So with the calculator you can play with the various sprocket combinations and see your speeds at any given rpm. Buy inputing your idle rpm plus approx 300-500 rpm you can see how fast you need to get the bike going to save the wear and tear on your clutch.

ocscully
 
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