Question: I am currently running 48 pitch gears and I know the tooth sizes that perform well in 48 pitch, but I want to change to 64 pitch. How do I convert the the number of teeth on gears from 48 pitch to 64 pitch?

Answer: Good question. The short answer is: take the number of teeth on the 48 pitch gear and divide that number by 1.5 and then muliply that answer by 2. Here's and example:
(81 tooth 48 pitch) 81 ÷ 1.5 = 54 x 2 = 108.

The answer of 108 will be the tooth size to use in 64 pitch. In that example we get a whole number after converting, 108. Not all of the numbers will work out this cleanly, lets take an 86 tooth gear for example:
86 ÷ 1.5 = 57.333 x 2 = 114.666.

We need to round this number off to the nearest whole number, 115 (a common 64 pitch spur gear size).
Just reverse the equation to convert 64 pitch to 48 pitch. 108 ÷ 2 x 1.5 = 81.

Question: Will converting the number of teeth from 48 pitch to 64 pitch ( or vice a versa ) change my gear ratio?

Answer: Hmmm...another good question. The answer is no. Lets take a look with an example using a 21T pinion and an 81T spur. The gear ratio is found by dividing the spur by the pinion,
81 ÷ 21 = 3.857

a gear ratio of 3.857 to 1. ( for every 3.857 revolutions of the pinion the spur gear makes 1 revolution ). After converting the 21T and 81T 48 pitch gears to 64 pitch as shown above we come up with 28T pinion and 108T spur 64 pitch. So when we divide the spur by the pinion, 108 ÷ 28 we get 3.857 again. The same gear ratio.

Question: What is the advantage of 64 pitch over 48 pitch?

Answer: Finer gear ratio changes. Here are some examples with 64, 48 and 32 pitch:

64 pitch

28T by 108T = 3.857
27T by 108T = 4.000

a difference of .143

48 pitch

21T by 81T = 3.857
20T by 81T = 4.050

a difference of .193

32 pitch

14T by 54T = 3.857
13T by 54T = 4.153

a difference of .296

It's useful to note that in the three above sets of examples, that you could take the gears of the first example in each set, set the gear mesh, and then interchange them with the other two examples without readjusting the gear mesh.
Question: How big of a clutchbell can I run on my RC10GT without modifying anything (like filling or grinding)?

Answer: A 20 tooth clutchbell should fit with a 66 tooth spur. The easiest way to figure this out is by adding the teeth on the clucthbell and the spur together to see where it falls within a range of 80 total teeth to 86 total teeth.

Example: 20 tooth clutchbell + 66 tooth spur = 86 total teeth (right at the top limmit of the range)

If the total is between 80 and 86 the combination of clutchbell to spur will not matter, just don't go over 24T on the bell and 67T on the spur.
The stock size is 15T + 66T = 81.

Question: How do I figure out my final drive ratio (FDR)?

Answer: If you have an on road car that only has 2 gears, divide the smaller gear (pinion) into the mating gear (spur).

Example: 108(spur) ÷ 24(pinion) = 4.5:1 (FDR)

If your car has a gearbox you need to know the gearbox reduction ratio. (There are some gearbox ratios listed below for some popular cars). We'll use the Associated B2 ratio of 2.400:1 for an example. Divide the pinion into the spur then multiply by the gearbox ratio.

Example: 85T (spur) ÷ 16T (pinion) = 5.3125 x 2.4 (gearbox) = 12.75 or 12.75:1(FDR)

What this means is that the motor makes 12.75 revolutions to every one revolution of the tire.

Asso. B2 (gearbox ratio) = 2.400
Asso. T2 (gearbox ratio) = 2.600
Asso. B3 (gearbox ratio) = 2.400
Asso. T3 (gearbox ratio) = 2.400
Asso. GT(gearbox ratio) = 2.600
HPI RS4 belt (gearbox ratio) = 2.133
HPI RS4 NITRO (gerabox ratio) = 2.166

If you do not know the ratio of the gearbox on your car call the manufacturer and they can tell you the ratio for the make and model of car you have. Write it down and store it on your r/c toolbox for future reference.

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