**Pulleys for Jackshaft>??

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skiz

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I put a 5hp briggs in a 250 rm dirt bike frame. I have a hilliard 3" (v-belt) centrifugal clutch on the engine shaft, and attached a
9" v-belt pulley on the rear wheel. Bike won't even move....gear ratio is way off. Guess I have to make a jackshaft.
What size v-belt pulleys would I put on the jackshaft to get the gear ratio about 6-1 ? No chain sprockets....only v belts.
I'm lost with this.
Help!
Thanks.
 
Poboy kartman

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V-belt pulleys are as easy to figure as tooth sprocket. 9"÷ 3"= 3:1!!!!

Your looking for a 6:1 ratio.

Welcome to the forum...

Please fill out a profile location so we can better help you.
 
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Hello and welcome.
A quick calculation... You'll need a 6"js in, 3" js out. So that makes 6:1.. (3-6-3-9)
This is also based on if you have a tire less than 12"od.

Also do you have space for these pullies and can you find belts this short?
 
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mckutzy

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Ahh yes I missed that part.
On the other hand, I have a 7:1 ratio on my bike with 24" of rear tire. However, it is a chained driveline.
 
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skiz

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The rear tire is an 18".
I found a place that has very short belts.
Just talked to my friend and he said go from 3" engine clutch pulley to a 9" pulley on jackshaft, then from there go from a 3" pulley to the 9" pulley on rear wheel. I don't know how to figure out what the final gear ratio would be on that. All the online calculators deal with sprocket teeth and chains.
Thanks!
 
Poboy kartman

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skiz said:
The rear tire is an 18".
I found a place that has very short belts.
Just talked to my friend and he said go from 3" engine clutch pulley to a 9" pulley on jackshaft, then from there go from a 3" pulley to the 9" pulley on rear wheel. I don't know how to figure out what the final gear ratio would be on that. All the online calculators deal with sprocket teeth and chains.
Thanks!
Click to expand...

That's 6:1.... but you may want to go higher if you can...the smaller the first pulley, or the larger the second, the higher the ratio...

Reducing the drive pulleys is probably easiest...but that shortens your belts...while enlarging your driven pulleys lengthens it.....but you have to have room fot the larger pulleys....
 
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skiz

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The website for the calculations only works with sprockets with teeth....not belt pulleys.
After measuring...I can only add pulleys in this order.....3-6-4-9 inches. I have no idea how to figure out this gear ratio with 4 numbers. Two numbers I can do. And then if I still need more.....I will add a 12" v-pulley on the rear wheel. I guess.
Thank you!
 
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skiz

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I'll change that 4" pulley to a 3"....and change the rear wheel pulley to a 12", and looks like it might get the ratio up to 8-1. My property has some hills so I'll need that higher ratio. Now I have to construct a mount for the jackshaft.
Thanks guys....I appreciate your input!
 
Poboy kartman

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skiz said:
I'll change that 4" pulley to a 3"....and change the rear wheel pulley to a 12", and looks like it might get the ratio up to 8-1. My property has some hills so I'll need that higher ratio. Now I have to construct a mount for the jackshaft.
Thanks guys....I appreciate your input!
Click to expand...

Yes...I don't know where my brain was yesterday. That would put you at 8:1.
 
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mckutzy

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Howabout some pics of the bike..
I'm have ing a hard time seeing how your going to mount this pully to the rear wheel, as it is a "dead" axle rear end. Pullies like these are meant to mount to a shaft or axle and spin with that.
 
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skiz

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What I do is drill holes in the pulley where the original sprocket was bolted, then put original bolts back in and bolt it to the wheel hub.
ok...tomorrow I will take a few pics.
 
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