need gear ratio help

1moretoy

I just bought another cart that has been heavily modded and has a clone swap already in it. I have been searching trying to find out how to change the ratio to get more low end torque. Here is the current set-up:

engine turns at 8.5:1
rear axle turns at 12.44:1

engine clutch sprocket is a 12 tooth
jackshaft input sprocket is a 15 tooth
jackshaft output sprocket is a 15 tooth
rear axle sprocket is a 15

with that set up, I am getting a final ratio of 1.25

the problem is, it has 26 inch tires. If I swap out the jackshaft input sprocket to a 48 tooth, my ratio changes to 4.00

My question is: will I have enough low end torque to pull the beast and will that be too much of a change. I do not plan on running at high speeds but do want it to climb and pull good.

BGW

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fstg21

The engine has a 8.5:1 compression ratio, not gear ratio.

The way its is currently setup the engine will turn 1.25 times for each turn of the input shaft of the transaxle, effectively making gear ratio 20% lower. Assuming the CVT setup goes to a 1:1 ratio as well at top speed. Lower ratio means numerically higher.

This means if the engine is running at 4000rpm only 3200rpm is getting to the axle. Which still has the 12.44:1 ratio which is fine for the large tires you have.

The setup should work well for offroad, just don't expect alot of top speed.

Changing the gears to give you a 1:1 jackshaft ratio or even 0.8:1 would pickup some top speed at the expense of torque.

As an example at 3200 RPM your theoretical top speed is 19.9mph, at 4000rpm it is 24.9.
0.8:1 would give you 5000rpm at the input shaft equating to 31.1mph, with a minor loss in torque.

A 4:1 jackshaft ratio gives you a 1000rpm shaft input speed and a top speed of 6.2mph

If you really want more torque, Ditch the jackshaft and run the cvt direct to the rear end, the way it is supposed to be.

What is the current clutch setup?

1moretoy

Thanks fstg21...I finally found a gear ratio calculator on line that works with the jackshaft set-up. (the reason for the jackshaft is: engine to axle mis-alignment) The 48 tooth sprocket would have been way to low creating a good puller but very slow. I chose to order two sprockets. I can swap them out to get what I want depending on the situation. One is 35 tooth and the other is 30 tooth. the interesting thing is you have to switch out the input shaft sprocket (not the output shaft sprocket or the axle sprocket) to get the ratio I am looking for. Right now it is just set up with a HD centrifugal clutch. I will probably put a torque converter on later once I get the ratio finalized.