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Old 09-03-05, 15:18   #1
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Configuring Gear Ratios

Hey guys,

This thread wont be useful to anybody who is not considering custom cut gears, this is only a guide to assist in a good gearbox combo setup. Most of the time people buy gearsets and the ratio is good but not fine tuned for your own application. Here is a list of variables which we will be using:

a = previous gear
b = next gear
c = shift revs
d = rev decrease (the revs you want the car to be at after you shift)

Just say for example, I want to make a gearset and my first gear is 2.70:1, my optimum shifting revs are at 6,500rpm, and I would like the engine to drop to around 5,500rpm. I could find the next optimum gear by throwing it into an equation. The equation looks something like this:

a:c = b:d

What we can do here is put all of the information we have into the equation.

ie: 2.70:6500 = b:5500

To find b, we use this simple rule:

(a * d) / c

When we put our info in we get something like:

ie: (2.70 * 5500) / 6500 = 2.28

So we have just found out our Second Gear (2.28:1). If We continue doing this we can finish the gearset with your own ratios for your own application. I'll finish the rest of this gearset for you to understand how it works and the results.

2.28:6500 = b:5500 .. (2.28 * 5500) / 6500 = 1.93
1.93:6500 = b:5500 .. (1.93 * 5500) / 6500 = 1.63
1.63:6500 = b:5000 .. (1.63 * 5000) / 6500 = 1.25

There you are, you have just finished your first gearset.

1st 2.70:1
2nd 2.28:1
3rd 1.93:1
4th 1.63:1
5th 1.25:1

This gearset is only an example but you may like to work with something like that, considering the rev drops are small, the car wouldnt be using all of its go, but many race cars use close ratios in the optimum range within the bandwidth itself, where most torque and power is made. In some cases you may calculate a custom gearset and the final number will be 0.xx, dont be alarmed, this only means that the gear is an overdrive gear. But, if you didnt plan on an overdrive gear then I would recalculate the gearset.

Anyway I hope you enjoyed it.. I'm rather board at the moment and I thought "Well I might as well post some rubbish on CT because I have nothing better to do"

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Old 09-04-09, 00:00   #2
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Very usefull thread... Well done mate
Common sense is not so common...
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Old 09-04-09, 08:00   #3
Aaaaaaand it's gone.
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Excellent thread, well done Sean.

Have you ever had a set of custom cut gears made before? I imagine it wouldn't be cheap.
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