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Old 08-14-2009, 09:47 PM   #1
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trying to figure out the meaning of why you should loosen or tighten the diffs front or rear
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Old 08-14-2009, 10:12 PM   #2
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Originally Posted by drive2survive View Post
trying to figure out the meaning of why you should loosen or tighten the diffs front or rear
The outer tire for a car that is cornering has to travel farther, so it needs to turn faster. A diff with 'perfect' action is one that allows the two tires to turn at different speeds, but they put exactly the same amount of torque on the pulley.

As a diff is less and less perfect, it will put small torque forces between the two wheels that act to equalize their speed. So, in a corner, the outer wheel will be slowed down by the inner wheel, and the inner sped up by the outer. The path of the car and traction demand that the tires turn at different speeds, but to resist the cross-axle torque generated by the imperfect diff, both tires must use a portion of their traction. That leaves less traction available for cornering.

In the rear you want the loosest diff setting that doesn't slip, because this will minimize lost traction.

The front is different, though. Front wheels are pointed in the direction you are trying to get the car to go, so applying throttle can make the car turn harder. The problem with a really loose diff here (and with rubber tires, any diff action at all) is that a cornering car has nearly all its weight on the outside tires, so the front inner tire has almost no traction available. When you apply the throttle, that front wheel sometimes can't support the motor's attempt to speed up the wheel. So, it breaks traction, and now the diff's ugly side rears its head. Since the outer wheel can only transfer as much torque to the ground as the inner wheel, the front wheels end up transferring very little longitudinal force to the ground. Now the front end isn't pulling the car around the corner much at all.

A tighter diff, one that transfers more torque across the axle as the wheels turn at different speeds, will have more pull around a corner on-throttle (corner exit). A solid front axle (spool) will generate maximum front end pull out of a corner. The sacrifice is that, the more like a spool the front diff is, the less lateral front tire traction is available off-throttle (corner entry and mid-corner).
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