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.
Sean. Certified speed crazed mowron.
Team Shepherd USA
Last edited by wingracer; 06-11-2013 at 01:40 PM.