# SprocketsFiguring out Gear Ratio?

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#### RocketPenguin

##### New Member
All I have been able to find is how to calculate the ratio of two gears, but near to nothing on more. I saw one thread where a guy kinda explained how it's done, involving multiplying one number by another, but i found that very confusing.
Kept looking for a little bit, and either found nothing, calculators that were way too complicated for me, or very vague explanations on how its done.

Could someone care to elaborate/point me in the right direction?
That, and i need to find a variable for such a situation:
I have 6 teeth on my engine sprocket, which goes to a jack shaft that has X amount of teeth. From the jack shaft, i have a sprocket with 13 teeth, which leads to a tire of 24" that has a sprocket with 47 teeth. How many teeth do i need for sprocket X to achieve a ratio of 1:13~1:16 (Or whatever is recommended for a chainsaw bicycle. I heard some say 1:14-1:15, others as low as 1:18, and others as high as 1:10)
Thanks!

#### Slogger

##### Member
With 6/24, to 13/47 on a 24" = 14.46/1 drive ratio. X= 24 tooth
With X=22, you'd have a 13.26/1 ratio.
Make X=26, you get 15.67/1.
This is according to the downloadable and cool "ratio calculator" program somewhere on the forum. I kept it.
If your engine could pull 6000 RPM at these ratios, your top speed would be either 27.3, 29.6 or 32.3.
~Regards

#### Sidewinder Jerry

##### Well-Known Member
Here a few factors need to be known. Engine size and power rating, your weight and primary riding terrain. The larger you are and the steeper the hills and the smaller the engine the greater the reduction will need to be. I'm 250 lbs; there's lots of 20%+ grade hills where I live. The engine is a 33 cc. I use a shift kit. The reduction range is 49.21-18.82:1. Without the lowest reduction there's no way I'd be able to get up the hills here.

To get the ratios you want sprocket x=22-27.

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#### RocketPenguin

##### New Member
With 6/24, to 13/47 on a 24" = 14.46/1 drive ratio. X= 24 tooth
With X=22, you'd have a 13.26/1 ratio.
Make X=26, you get 15.67/1.
This is according to the downloadable and cool "ratio calculator" program somewhere on the forum. I kept it.
If your engine could pull 6000 RPM at these ratios, your top speed would be either 27.3, 29.6 or 32.3.
~Regards

Thanks! So there is a calculator somewhere on this forum for it? I'll look around for that. How do you calculate this stuff on paper, in case I change sprockets and need to calculate again without a calculator? The engine I have can pull around 10k to 13k at full throttle.
EDIT: Found the calculator. Thanks!

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#### RocketPenguin

##### New Member
Here a few factors need to be known. Engine size and power rating, your weight and primary riding terrain. The larger you are and the steeper the hills and the smaller the engine the greater the reduction will need to be. I'm 250 lbs; there's lots of 20%+ grade hills where I live. The engine is a 33 cc. I use a shift kit. The reduction range is 49.21-18.82:1. Without the lowest reduction there's no way I'd be able to get up the hills here.

To get the ratios you want sprocket x=22-27.

I weigh 170lbs, the bike and me in total would be around 200lbs. Depending on where I drive it, I will anything from minor hills all the way to 40° inclines. Probably won't be riding it on those steep hills though. Engine I am using is 42cc, has between 2-4 HP.

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#### The_Aleman

##### Well-Known Member
I have a Huasheng 49CC with a 5:1 4G, SBP shift kit, and a 5-speed Sturmey-Archer (X-RD5)

I've drawn up a couple pics to demonstrate how to use Jim Sitton's ratiocalc program with jackshaft and gears (in this case, an internally geared hub or IGH).

Pic 1 is the basic figuring of overall reduction with the jackshaft.

Pic 2 is the speed with each gear at my Huasheng's HP peak. Since the 5-speed is an IGH, one must use the percentage values of reduction.
In my case, Sturmey-Archer claims my X-RD5 has percentages of 66/75/100/133/160%.

Whether you have an IGH or standard gears with a derailleur, it is not difficult to figure what your ratios are using this program.
IGH reductions are posted by the manufacturer and gear clusters can be manually counted or looked up online.

Hope this helps!

#### Sidewinder Jerry

##### Well-Known Member
I weigh 170lbs, the bike and me in total would be around 200lbs. Depending on where I drive it, I will anything from minor hills all the way to 40° inclines. Probably won't be riding it on those steep hills though. Engine I am using is 42cc, has between 2-4 HP.

You must be planning to do some extreme off roading going up a 40° incline. That's an 89% grade. The two steepest streets in the world are a 37% grade. Which is a 16.65° incline. Your weight and the size of engine will probably need at least an 18:1 reduction. Which means sprocket X will need to be a 30t.

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#### RocketPenguin

##### New Member
You must be planning to do some extreme off roading going up a 40° incline. That's an 89% grade. The two steepest streets in the world are a 37% grade. Which is a 16.65° incline. Your weight and the size of engine will probably need at least an 18:1 reduction. Which means sprocket X will need to be a 30t.

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

Last edited by a moderator:

#### RocketPenguin

##### New Member
I have a Huasheng 49CC with a 5:1 4G, SBP shift kit, and a 5-speed Sturmey-Archer (X-RD5)

I've drawn up a couple pics to demonstrate how to use Jim Sitton's ratiocalc program with jackshaft and gears (in this case, an internally geared hub or IGH).

Pic 1 is the basic figuring of overall reduction with the jackshaft.

View attachment 57537

Pic 2 is the speed with each gear at my Huasheng's HP peak. Since the 5-speed is an IGH, one must use the percentage values of reduction.
In my case, Sturmey-Archer claims my X-RD5 has percentages of 66/75/100/133/160%.

View attachment 57538

Whether you have an IGH or standard gears with a derailleur, it is not difficult to figure what your ratios are using this program.
IGH reductions are posted by the manufacturer and gear clusters can be manually counted or looked up online.

Hope this helps!

Kinda confused about your pictures... Mainly 1st step and 2nd step. With a jackshaft, wouldn't I just calculate as the first Drive Gear/Sprocket Teeth being the amount of teeth on the clutch (6), the Driven Gear/Sprocket Teeth being the amount of teeth it leads to on the jackshaft, and the second set of Drive/Driven Gear/Sprocket Teeth being from the jackshaft to the tire?

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