Limited Slip or Posi-Trac or Trac-Lok or Detroit Locker or...

[blykins]

This is a subject that most people don't understand. Your old car would leave a 2-mile stripe because it had an open rearend. Open means that there isn't anything valuable except for spider gears in the differential case. When one starts spinning, the spider gears turn (in the same way as they would if you were making a hard left or right-hand turn), and you transfer all the power to one tire. Very bad...especially on slick surfaces such as rain or snow...or when you're racing.

There are many rearend names out there. Limited slip and positraction rearends are essentially the same in their function. Limited slip rearends usually are viscous coupled...i.e. they act like a torque converter. If one tire spins, it creates a viscous situation which in turn applies a moment force to the other side...causing it to turn also. As you would think, the amount of traction to both tires is directly related to ring gear or axle RPM.

Positraction units usually have mechanical locking units...Most of them use clutch packs. In the same way, they lock up due to mechanical friction instead of fluid friction. The more one side starts to spin, the more friction it creates between the clutches, and it all hooks up, causing both tires to spin at the same rate. You can vary this clutch friction...causing the clutches to hookup immediately, or have some slip to them. You can set them up so tight that you would have trouble turning.

Locking rearends actually have some sort of locking teeth that pop in and out of each other. When turning, they disengage each other allowing the tires to turn differentially. When going in a straight line, they will lockup, transferring 100% power to both tires. They are very noisy however. They pop and crack and shutter when they disengage/engage. They are also bad for most types of road racing, because you would be in an open-rearend situation (spinning one tire) at the apex of a curve...then it would immediately lockup without warning, and you could get some power-on oversteer.

Detroit Lockers are of this type...and there's also a locker that you can engage/disengage with an air pump controlled in the cockpit.

Most other diffs (Auburn, Trak-Lok, etc.) are positraction units and have clutch packs in them.

There are also "spools" available, which pretty much connect each axle to each other...allowing NO slip at all between them. Good: you get power to both tires 100% of the time. Bad: It's very hard to turn.

Hope this clears some fog. It is a pretty messy topic.

Brent