Sprockets Figuring out Gear Ratio?

[Sidewinder Jerry]

My bad, over estimation. 35° Is probably the steepest I have, everything else is under that.

You seem to be confusing degree with grade. A 35° hill would be close to a 78% grade. Still way steeper than any paved street in the world. If you meant 35% grade not likely either. San Fransisco steepest street is a little over a 31% grade. Here in the mountains we have some close to a 30% grade. What you've probably got at best is between a 15-25% grade.

Here is a good way to understand it, say from axle to axle is 45". If you lift the bottom of your front tire 15" off of the ground that's approximately how steep a 33% grade hill would be.

The 18:1 reduction is more of a level ground or no greater than 10% grade hills. If you do have steep hills between 15-25% grades, a 25:1 reduction may be more of what you need meaning a bigger gear on the rear wheel. Of course a shift kit could solve the issues all together.

For a 25:1 reduction you'd need 6t engine to jackshaft 30t; 9t jackshaft to rear wheel 45t.

 

[RocketPenguin]

You seem to be confusing degree with grade. A 35° hill would be close to a 78% grade. Still way steeper than any paved street in the world. If you meant 35% grade not likely either. San Fransisco steepest street is a little over a 31% grade. Here in the mountains we have some close to a 30% grade. What you've probably got at best is between a 15-25% grade.

Here is a good way to understand it, say from axle to axle is 45". If you lift the bottom of your front tire 15" off of the ground that's approximately how steep a 33% grade hill would be.

The 18:1 reduction is more of a level ground or no greater than 10% grade hills. If you do have steep hills between 15-25% grades, a 25:1 reduction may be more of what you need meaning a bigger gear on the rear wheel. Of course a shift kit could solve the issues all together.

For a 25:1 reduction you'd need 6t engine to jackshaft 30t; 9t jackshaft to rear wheel 45t.

Yea, I meant a 33% grade hill. They are pretty steep, most vehicles have issues getting over them.
Probably won't be riding on them if its that much of an issue.
Mk, I'll aim for something around there. Thanks!

 

[butre]

calculating compound gear reductions is elementary school level math
it's easiest if you can reduce the second number all the way down to 1 rather than reducing them both to whole numbers, ie. if you have a 44 tooth driven side and a 12 tooth drive side you would reduce it to 3.66:1 rather than 11:3. reducing the second number down to 1 means you don't have to do any math for the second number. it's as simple as putting 44/12 into your calculator and adding :1 to the end of your result.

lets say you've got 3 sets of gears, first is 30 tooth driven side and 10 tooth drive side, second is 40 tooth and 10 tooth, third is 50 tooth and 10 tooth.
that means you have a 3:1 reduction, followed by a 4:1 reduction, followed by a 5:1 reduction. From there it's basic third grade level math, the ratio is (3*4*5):1. so you multiply 3*4*5 and you get 60, meaning the overall reduction is 60:1

In the case of a china girl engine, you have an internal 4:1 reduction and an external 3.66:1 reduction if you use the sprocket that comes with the kits giving you an overall reduction of 14.6:1, which without compound reductions would require a sprocket as big as your wheel.

I like to use this website to calculate potential top speed, you can punch in the first number into the teeth on axle field and the second number into the teeth on clutch field. http://www.csgnetwork.com/minibikepotentialspeedcalc.html

using that you can find out that the absolute fastest an unmodified kit china girl bike can go is 34 miles per hour. puts a big hole in a lot of sellers claims

 

[Sidewinder Jerry]

If I'm understanding you correctly Rocket Penguin you already had the following gears a 6t for the engine, a 47t on the rear wheel and a 13t for the jack shaft you wanted to know what other gear to use on the jackshaft. For your weight, engine size and the steep hills where you live you'll need a 25:1 reduction for 24" wheels. Keep the 6t for the engine and the 47t for the rear wheel. Order a 9t and a 30t for the jackshaft. Not much use for the 13t keep it as a back up. Then 6t engine to 30t jackshaft. Followed by 9t jackshaft to 47t rear wheel. This will give you a little over a 26:1 reduction. You should be able to pull any of the hills with a little assistance at the most. Without a shift kit its a compromise in steep hill country.

 

[RocketPenguin]

A little bit has changed since the post, I currently have not purchased either of the sprockets that will be going on the jackshaft. I have also decided I am not modifying the rear hub, as I have more leeway because I am not purchasing two sprockets instead of one. The goal is to have the rear shifter also work after I am all done, to provide a method of shifting.
With that said, I have the 6t on the engine, nothing yet on the jackshaft for either direction, and the rear hub has 28t, 24t, 20t, 17t, and 14t.
Doing some calculations, if I have the engine sprocket (6t) go to a 54t sprocket on a jackshaft, leading to a 10t sprocket that goes to the 28t sprocket on the tire, I should be within 25:1.

 

[Stoneman]

Does anyone have a copy of this program?