Changing Ratios

[jumpjg]

Although I've only ridden my Goat a few times, it's obvious that the overall drive ratio needs some modification. It has a (presumably) 7 hp Briggs, a (presumably) series 40 Comet Torque Converter, and two jack shafts back to the rear wheel.

The Comet site says that a series 40 has a range from ~ 2.33-1 to 1-1. The reduction between the 1st & 2nd jack shafts is 15-35 (2.33-1), and the reduction between the 2nd jack shaft to the rear wheel is 11-40 (3.64-1). I wrapped a tape measure around the rear tire & measured a little over 51" - I'll round this down to 50 to make calculations easier. It was interesting to note that the original ratios incorporated the "hunting tooth" principal, whereas the tooth counts do not divide evenly. This design concept ensures that the sprocket teeth do not mate up with the same place on the chain on a regular basis, thereby spreading the wear over the whole system.

Using my calculator, that's 8.48-1 from the T/C driven pulley to the rear wheel. Coupling that with the T/C's 2.33 reduction yields 19.77-1...yea, it will pull stumps! Using an engine governed speed of 3600 rpm, the low ratio (high numerically) yields 8.8 MPH, and the high ratio a whopping 20.5 MPH. Given that I'm only planning on riding this mini off road I don't want to gear it for Bonneville, but certainly I can step it up some. It's a big & heavy machine with no suspension, but it's a 7 hp or thereabouts for goodness sake!

The Goat's sprockets are not keyed onto the shafts; rather, they are welded on. It does appear that the driven pulley is not welded onto the first shaft though, so it may be easier to just replace the shaft with a keyed shaft vice grinding off the old sprocket and welding on a new. And that way I can easily return it to the original configuration.

I was thinking of changing the 1st to 2nd jack shaft reduction to something like 24-35. Plugging those numbers into the equation yields 13.8 MPH for low & 32.1 for high. Or perhaps a bit taller; a 26-35 ratio yields 14.9/34.8 MPH and a 28-35 yields 16.1/37.5 MPH. Now I'm starting to think about a hi/low range gearbox!

All insights and opinions appreciated - thanks!

Joe in St Louis

 

[minidragbike]

All the rotating mass you have your motor spinning is causing you to loose power/speed/RPM/torque/etc. Rotating mass is the main reason for moving slower then someone else with less mass.

A better way to get the speed you want is to modify your motor. Simply turning 4,000 RPM instead of 3,600 might satisfy your need for speed. The problem with gear changing is that there is no guarantee that you will pick up more speed with higher (numerically lower) gearing. Changing the gearing for more speed also put more load on your motor. Basically saying that if your turning 3,600 RPM now you might only turn 3,300 with the gear change. The motor responds like you just added a heavier flywheel.

4,000 RPM won't hurt you, and your motor will love you for it.

 

[jumpjg]

Yes, when you add up the flywheel, crankshaft, and the Comet series 40 clutch, there is quite a bit of rotating mass. I can see where the mass thing is important for drag racing/acceleration, but it shouldn't have any effect on torque. Take a look at tractor design - huge rotating flywheels to maintain momentum. This is more in line with what the Goat is designed for, and what I have planned for it's use - off road trail riding. I want to have the flexibility of low speed grunt for hill climbs, but also be able to go at a reasonable speed on the fire roads without taching it out. On the other hand, I agree with you on the Rpm point - a few more revs may well fill in the gap. I was just using 3600 RPM as a data point for my calculations. I'd be interested in a few mods, intake, carb, exhaust, thinner head gasket, milling the head, etc, but I need to get it going first.

Appreciate your insight!

Joe in St Louis

All the rotating mass you have your motor spinning is causing you to loose power/speed/RPM/torque/etc. Rotating mass is the main reason for moving slower then someone else with less mass.

 

[Yellowhand]

We had a Tote Gote back in the 60's...3hp model...and it worked great for it's intended purpose- hauling an adult rider and some cargo through very rugged terrain at extremely slow speed.

They designed the Goat for extreme off-road use...even off-trail use...hauling a hunter and his gear into the mountains far from any roads, and the stock gearing and tire choice really reflects that intent.

The Tote Gote and the Cushman Trailster don't have a rear seat, even though there was room for one because that's where you were supposed to put the deer after you had field-dressed the carcass, and I spent several hunting seasons in the Pine Ridge area of Nebraska, riding through the hills while sitting on a dead deer and trying not to get poked by an antler.

OK, enough reminiscing...I love the Goat and the Trailster...lots of fond memories...

You can get roughly twice the top speed from a bike using a variable-ratio torque converter versus a centrifugal clutch using the same size sprockets, and your idea to alter the sprocket sizes to make the ratio a little more fireroad-friendly and less of a mountain climber sounds like an excellent choice if you're not intending to take any deer for a ride with you.

I'd also like to offer up an idea, but I'm not suggesting you do it, although I think you probably could if you wanted to...anyone who is aware of the 'hunting tooth' principle knows quite a bit about drivetrains...

We experimented with close ratio transmissions mounted in minibikes during the 70's...there were a number of rider mower manufacturers making these, and many had simple four-bolt mounting patterns and chain drive input and output sprockets.

I mentioned in a previous post that we had built one that was made to be shifted during operation, using a simple foot operated idler on the drive belt, but the primary use was to achieve a simple minibike drivetrain that had an extremely low stump-pulling ratio range that could be used off-trail, but when the rider reached a road, they could stop for a second and simply shift to a higher gear range, then head down the road faster.

Unfortunately, by the 70's the minibike industry was all but dead...Cushman had long since ceased production of scooters and was making the Trackster as the primary product for extreme off-road usage, and all the things we made ended up just being toys to play with on the weekends...but tons of fun, though...and extremely useful for really getting into the unbroken areas.

 

[minidragbike]

Yes, when you add up the flywheel, crankshaft, and the Comet series 40 clutch, there is quite a bit of rotating mass. I can see where the mass thing is important for drag racing/acceleration, but it shouldn't have any effect on torque. Take a look at tractor design - huge rotating flywheels to maintain momentum. This is more in line with what the Goat is designed for, and what I have planned for it's use - off road trail riding. I want to have the flexibility of low speed grunt for hill climbs, but also be able to go at a reasonable speed on the fire roads without taching it out. On the other hand, I agree with you on the Rpm point - a few more revs may well fill in the gap. I was just using 3600 RPM as a data point for my calculations. I'd be interested in a few mods, intake, carb, exhaust, thinner head gasket, milling the head, etc, but I need to get it going first.

Appreciate your insight!

Joe in St Louis




Rotating mass does effect torque a lot. Your talking about rotating inertia, keeping the speed up, slowing something down that's heavy, or momentum of a heavy object. All of these are the same, and they don't effect torque at all.

It takes torque to get all these parts moving, before inertia even comes into play. All ROTATING PARTS HAVE A EFFECT ON TORQUE Most guys say more power to the wheels, but torque comes on way before HP comes on. And hill climbing needs torque to get the load moving.

The rotating inertia works like this. If you had a 300 lb guy riding your minibike at 20mph it will take him longer to stop to a dead stand. Being that the extra weight is harder to slow down then if the rider was 150 lbs. The bad part about "Rotating" inertia is that it's rotating with the speed of the motor. This puts around triple the amount of regular load.

Nothing actually changes with the torque reading. Basically saying that you "wont" have now gain a extra lb of torque by adding a light weight aluminum flywheel. But the rider will feel as though the minibike is pulling them much easier then before.

I have been doing this for years. Light flywheels, small rims and tires, less chain, smaller gears. The motor will now run as though you have lost 30 lbs of body weight. It will now get you moving faster, and allow you to rev a little bit more RPM on top end.