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Old 01-31-2003, 09:53 PM   #1
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How to calculate gear ratio for 1/12???

What do u do to claculate the gear ratio for a 1/12 pan car??? ........

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Old 01-31-2003, 10:36 PM   #2
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This is easy man

divide the spur gear teeth but the pinion and thats it since they are direct drive !
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Old 02-01-2003, 04:50 AM   #3
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No that is wrong. For 1/12 scale you want to figure out what is called "rollout". You need to measure the dia. of your rear tires and plug it into a formula. I have it written down but I am at work right now and do not have access to it. I will post the equasion I use it works very well.........
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Old 02-01-2003, 06:25 AM   #4
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This page has a great rollout chart program. Just put in your dimensions and then print the page!

Rollout chart
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Old 02-01-2003, 10:01 AM   #5
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Tire diameter X 3.1416
----------------------------- = rollout
FDR (spur/pinion)

So if the tire diameter is 60MM


You use a 100 tooth spur and a 25 tooth pinion


188.47/4.0= 47.1mm rollout

This is a hypothetical equasion and you will very rarely ever use a 47.1mm rollout on a 1/12th car......

To figure out a pinion for your given rollout you want to.........

your tires have worn down to 55mm

you still use the 100 tooth spur.......

tire diameter x 3.1416
---------------------------- = FDR
desired rollout of 47.1

55x3.1416= 172.788


you need a final drive ratio of 3.67

to get the pinion size for a final drive ratio of 3.67 with a 100 tooth spur you need to

----------- = Pinion

100/3.67= 27.2

so you will need a 27 tooth pinion to get close to a 47.1mm rollout

simple huh????

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Old 05-02-2005, 04:21 AM   #6
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This is all handy stuff to know but I think what he may have been asking was something I was hoping to find out also: where do you start in terms of calculating what gear ratio to aim for and is it dependent upon your motor?

I have an Associated RC12G with a 9 turn motor and no gears whatsoever (ebay purchase) and so far nobody can tell me much about where to start. I can now determine rollout based on my tyre diameter but what size spur do I buy? What size pinion? Isn't a smaller pinion better for low-end torque and a larger best for higher top speed? What's the happy medium, or at least a starting point from which to begin testing based on your local track(s)? Does the number of turns of the motor have a bearing on the gear setup?

Hope anyone can help with this.


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Old 05-02-2005, 10:09 AM   #7
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Final ratio and rollout are two seperate things.
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