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Old 11-22-2010, 11:53 PM
robbie robbie is offline
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Default Speed Formulas - Gear Ratio, Tires, RPM

The parent site of this forum has a very handy speed calculator, which I've used many times. The only problem is that the calculator doesn't solve the equation for any other variable besides vehicle speed. I've rearranged the equation in four different ways so you can solve for any variable, including tire diameter, engine RPM, gear ratio, or vehicle speed. So if you know any three numbers and want to find the fourth one, use one of these equations.

In the equations, the gear ratio is stated as a single number GR, where an actual ratio is written as GR:1. Use a single number in the equation. Vehicle speed (S) is in miles per hour. Tire diameter (D) is in inches. Engine speed is RPM. 337 is a constant that needs to be there to convert between minutes and hours, and inches to miles. Just factor it in and don't worry about it.


Remember that the number to the right of the = sign is the one that you are looking for, and the ones to the left are the ones you already know, so fill them in with real numbers.


If you want to find vehicle speed:

D x RPM
________ = S
337 x GR



To find the gear ratio:

D x RPM
_________ = GR
337 x S



To find tire diameter:

337 x GR x S
__________ = D
RPM



To find engine RPM:

337 x GR x S
____________ = RPM
D


To calculate a gear ratio from a pair of sprockets, take the rear sprocket tooth count and divide by the engine sprocket tooth count. If you have a jack shaft, start with the wheel sprocket, divide by the sprocket that drives it, then multiply by the sprocket driven by the engine, and finally divide by the engine sprocket. This will give you a number which you substitute for GR in these equations.

For example, 11 teeth on the engine and 60 on the wheel will give you a ratio of 5.45 : 1, so you would use the number 5.45 to replace GR in the equations.

10 teeth on the engine, 17 on the jack shaft input sprocket, 9 for the jack shaft output, and 54 on the wheel would give you the equation GR = 54 / 9 x 17 / 10. This comes out to a 10.2 : 1 ratio, so GR = 10.2

Here's the same thing formed into a handy equation to find what sprockets to use on your jack shaft. E is the engine sprocket, W is the wheel sprocket, JI is the jack shaft input sprocket, and JO is jack shaft output.


W x JI
________ = GR
E x JO
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Old 11-23-2010, 04:16 PM
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This is sticky and lock material IMO. Don't let it get spammed into oblivion and onto 20 pages of posts.
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Old 11-24-2010, 10:01 AM
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these would prob be handy for people, calculates stuff like torque, pitch dia, chain tension, length to reach top speed, if your axle will snap, ect
http://gokartguru.com/engineering_tool_kit.php
and this is good for helping people with js ratios, and it has a few little things like estimated chain length.
http://www.compgoparts.com/Technical...Calculator.asp
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Old 11-24-2010, 07:32 PM
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Handy- Thanks!
Sticky? Please?
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Old 02-28-2011, 03:13 AM
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good to know
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Old 02-19-2013, 02:34 PM
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I made an (imho) nifty Excel spreadsheet with the above equations, how would I upload it here for others to use?
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Old 02-19-2013, 05:33 PM
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PM sent. And thanks!
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Old 04-25-2013, 12:07 AM
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Quote:
Originally Posted by robbie View Post

10 teeth on the engine, 17 on the jack shaft input sprocket, 9 for the jack shaft output, and 54 on the wheel would give you the equation GR = 54 / 9 x 17 / 10. This comes out to a 10.2 : 1 ratio, so GR = 10.2
What are the Input and output sprockets on the jackshaft for?
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Old 04-25-2013, 01:16 AM
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Quote:
Originally Posted by dreazoian View Post
What are the Input and output sprockets on the jackshaft for?
They are used for compound gear reduction. If you did not have them you would need a huge axle sprocket to achieve the same ratio as what the smaller jackshaft sprockets can give you.
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Old 05-28-2013, 04:34 PM
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This is the formula to find the max speed, you can play around with gear ratio to find what speed best suits you

I'm doing the formula for a torque converter (for clutch replace total ratio with just the gear ratio)

Driven sprocket / drive sprocket = sprocket ratio
Sprocket ratio / torque converter ratio (high or low) = total ratio
P.S. use high for max speed
tire diameter x pi (3.14) = tire circumference
Tire circumference / 12 = tire circumference in feet
RPM / total ratio = axle RPM
Axle RPM x tire circumference in feet = feet per minute
Feet per minute x 60 = feet per hour
Feet per hour / 5280 = miles per hour

I hope this helps!
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Old 05-28-2013, 08:15 PM
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Generally speaking, one would not bother calculating for a CVT. One, the engagement ratio is of little consequence, when the high ratio is calculated and implemented properly. Two, some CVT's top out at 1:1, making the CVT entirely invisible in the equation. Three, the most common CVT's that DON'T macx out at 1:1, feature a 10% overdrive. An added bonus for sure, but can be easily factored after the fact.

But thank you for posting!
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Put down the wrench, and come out with your hands up!- Me!
Wrench, Wheel, Wreck, Repeat...
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Old 05-29-2013, 01:43 AM
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Quote:
Originally Posted by turkeyfellow91 View Post
This is the formula to find the max speed, you can play around with gear ratio to find what speed best suits you

I'm doing the formula for a torque converter (for clutch replace total ratio with just the gear ratio)

Driven sprocket / drive sprocket = sprocket ratio
Sprocket ratio / torque converter ratio (high or low) = total ratio
P.S. use high for max speed
tire diameter x pi (3.14) = tire circumference
Tire circumference / 12 = tire circumference in feet
RPM / total ratio = axle RPM
Axle RPM x tire circumference in feet = feet per minute
Feet per minute x 60 = feet per hour
Feet per hour / 5280 = miles per hour

I hope this helps!
Good to know. Thanks
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