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Old 06-11-2013, 07:57 AM   #1
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Default What is Overdrive???

I heard some 1/8 scale onroad racers talking about overdrive and how it had a positive effect on the handling of their 1/8 scale 4wd onroad cars. Can anyone tell what overdrive is and what you have to do to set it up on your car?? And can you set up a offroad car so it has overdrive ??
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Old 06-11-2013, 08:41 AM   #2
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Overdrive : front tires are rotating at faster rate per revolution than rear tires. Front tires are actually pulling the car.

Underdrive : front tires are rotating at slower rate per revolution than rear tires. Rear tires are actually pushing the car.

1:1 : front tires and rear tires are rotating at the same rate per revolution.

Depends on car setup, most people are using 1:1 and underdrive. Underdrive cars are easier to turn @ tight corners and sweepers.

I'm not sure with overdrive or underdrive applications in off road buggies. You could simulate underdrive setting with different gear ratios at the front gear box. However, I'm not sure if that will work effectively because off road buggy cars have center differential. Maybe you will be better off with torsen differentials to limit the diff slip.
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Old 06-11-2013, 11:32 AM   #3
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Quote:
Originally Posted by asw7576 View Post

Depends on car setup, most people are using 1:1 and underdrive. Underdrive cars are easier to turn @ tight corners and sweepers.
may I know you are mention in 1/10 or 1/8?
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Old 06-11-2013, 11:47 AM   #4
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For 1/8th onroad, we usually use a little overdrive. Anywhere from 1.01 to 1, to 1.05 to 1. Sometimes on high bite tracks, you might go 1 to 1 or even underdrive just a tick. It is mainly set by tire size though many manufacturers offer an optional pulley to make a large change. With most cars, somewhere around an 8mm split (front tires 8mm smaller in diameter than the rears) will give 1 to 1. 6 or 7mm splits are common, I like overdrive so I often run 4 or 5 to get a 1.03 to 1.04 overdrive.

To understand what it does, you need to understand a tire's traction circle. A tire has a certain amount of traction available. This traction can be used for forward acceleration or lateral acceleration (cornering) or some fraction of each. The more of one you demand from the tire, the less is available of the other. So if you increase overdrive, you are using more of the front tire's traction for forward acceleration and therefore it has less available for cornering. So more overdrive will provide less steering under hard acceleration (usually corner exit) while less overdrive or even underdrive will provide more steering on exit.

In a nutshell, I find the effect to be similar to rear toe-in except that overdrive effects entry less.

Offroad cars use rubber tires of a fixed diameter so fine adjustments in overdrive are going to be hard to do. The only way would be to change the gearing in the front and/or rear diff. I'm not sure how this would work with a center diff. I do know that some electric 4wd offroad cars have had gearing options for overdrive.
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Last edited by wingracer; 06-11-2013 at 02:40 PM.
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Old 06-11-2013, 02:29 PM   #5
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Thanks , wingracer and ASW.
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