Gear ratio advice

[Trailstomper]

Hey everyone.
I'm planning a fat tire scratch build. I'll be using either a 8hp tecumseh or a Kohler 7hp power plant, 22.5x 10 inch, three wheeler tires and rims, and a 30 series torque converter. I am not too concerned about speed, 20mph or so top speed would be fine, but I want it to be able to climb hills and carry a bit of a load. What gear ratio should I shoot for? I don't mind using a jack shaft to achieve an acceptable level of torque.
Any and all thoughts are welcome.

Thanks,
Trailstomper

 

[panchothedog]

Regular gearing is 10 tooth on the shaft and 60 on the axel. If you changed
to a 9 or 8 tooth on the shaft you will be lowering the gear ratio by either
10 or 20 percent. Took our off road karts to Pismo Beach last summer and before leaving changed from 10 to 9 and you could notice the difference.
More power with a drop off of top end speed. We had 20" tires on one of the karts and 19" on the other .

 

[SAS289]

Regular gearing is 10 tooth on the shaft and 60 on the axel. If you changed
to a 9 or 8 tooth on the shaft you will be lowering the gear ratio by either
10 or 20 percent. Took our off road karts to Pismo Beach last summer and before leaving changed from 10 to 9 and you could notice the difference.
More power with a drop off of top end speed. We had 20" tires on one of the karts and 19" on the other .

Agree. Wouldn't it be even better if there was clearance for an even larger rear sprocket with a 22.5" tire?

I run 9/60 gearing on my mini bike with a 19" tire. Works great. And there's no need for a big power high RPM engine. With the stock 30 series driver all off road RPM is under 4700.

 

[Trailstomper]

So no need to go with a jack shaft for lower gearing with the big tires then? Maybe I should plan enough space in the frame to add one in case I decide it's needed later......

Thanks for the responses. When I start cutting and welding I'll start a build thread.

Any other wisdom is welcome.

Trailstomper

 

[panchothedog]

I'm sure you know if you buy a whole 30 series torque converter with mounting plate it has a built in jackshaft. With 22 1/2" you might be able to
run something bigger than a 60, as suggested by SAS289 maybe a 66 or 72.
With sprocket availability I can't see the need for a second jackshaft. A lot of work and added complexity.

 

[Trailstomper]

Yes, I am aware the TC has a built in jackshaft. I appreciate the advice. I think I will just try with the torque converter, but make my frame long enough to add a jackshaft for additional gear reduction if it seems necessary later.

Thanks,
Trailstomper