Racing tractor jackshaft setup

This is the jackshaft on my homemade tractor.

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To decide what ratio is ideal for your application you first must understand its a torque vs. top end decision. The top speed of something like a go cart or my racing tractor is affected by REAR TIRE SIZE, ENGINE RPM, ENGINE HP, and WEIGHT. My tractor is a good example of small changes having profound effects. When the ratio was 16:1 it was too fast and the little 3.5 hp couldn't turn the tall back tires, and push the 300 pounds my tractor and i weighed. With a 25:1 ratio, it has enough ass to take off and will go maybe 10-15 mph with the motor wide open(no governor). By doing a quick google search you should be able to find a forum talking about something similar to what your working on. The heavier it is and the taller the back tires are, the higher the ratio needs to be to have enough low end power, unless of course you substitute the ratio for a more powerful engine.

If you have a motor with a governor remember, the rpm-range of your motor is going to be smaller. A briggs 5hp revs to something like 3600 governed and 5000 not-governed. (I've never blown one up. I have a 5hp gocart i've had for 10 years, a 7hp racing tractor, and my newest 3.5 hp racing tractor all not-governed flatheads. Flathead briggs motors are super hard to blow. They just hit a spot where they can't rev any more and I hold them there for 5-10 seconds, and since the valves are along side the piston instead of over top of it, there is no valve-to-piston clearance issue. OHV motors I wouldn't suggest spinning as high.)

Now, onto a simple way to figure out your ratio:

Using the tractor again, I started with a 4" at the motor to a 4" at the jackshaft. That gave me a 1:1 ratio because the jackshaft turned 1 revolution every time the engine turned 1 revolution. From there, the jackshaft had a 5" that went to a 3.5" on my rear-end giving me a 0.7:1 ratio (simply divide the driven pulley by the driving pulley to find ratio. in this case 3.5/5). Multiplying the two ratios together gives you the final ratio. So for the tractor it was 1:1 x 0.7:1 which equals a 0.7:1 final drive. Since I knew the rear-end of the tractor was 23:1 I multiplied that number in. So, 0.7:1 (which is my input into the rear-end) multiplied by the 23:1 axle speed is 16.1:1.

In the current setup, I start with a 4" at the motor, to a 5" at the jackshaft. A 1.25:1 ratio. Then, a 4" pulley to a 3.5" pulley at the rear-end. giving me a 0.875:1 ratio. Altogether a 1.09375:1 ratio spinning the rear-end. So factoring the rear-end's 23:1 in it has a final drive ratio of 25.15625:1.

Or if you want to change tire size:

Smaller tires to larger tires = higher top speed but may need more gear to spin them.

Larger tires to smaller tires = more bottom end and acceleration but will top out quicker.

A side note:

Larger motors, like 12-20 hp briggs I'd stay away from too much overreving because you are dealing with alot more reciprocating weight. Small weight changes mean alot when they are spinning fast. Also, like I said, OHV motors I'm afraid of overreving them too much because of the clearance. My neighbor has my identical 7hp on a tractor with a gas pedal. It's rigged to the throttle blade and we have both driven them the hell out of them. My tecumseh OHV gocarts still have their governors.

It's not a bad idea to run some slick-50 in the oil for an hour, drain it, and replace it with straight oil. They don't have bearings and oil pressure like a car engine so the teflon probably is a good idea. My one 5hp briggs has had it every 15 hours for the past 10+ years and still goes wide open.

Maybe you'll blow your motor. It will probably be cool, I don't know, I have yet to see it.

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