Originally Posted by mtbprelude

I think you will find this helpful: (Originally posted by Jamie Sculerati elsewhere)

"The reason there's a differential in the first place is because the wheels travel through different size arcs when the car turns. Since they're both attached to the same car, that means they rotate at different speeds -- since the outside wheel travels further, it rotates faster than the inside. The differential makes that possible -- try to turn a car with a completely locked diff, and the outside wheel overdrives the inside, resulting in lots of tire scrubbing and hop.

There is a downside. Although we talk about torque and power from an engine, those are really measured against a resistance (normally, a water or mechanical brake, hence the term "brake horsepower") -- the reality is that we get a given crankshaft rotation speed for a given throttle input. The gearbox slows this down some, and eventually, it ends up turning the axle. Thus, for a given throttle position, you get a given wheel rotation rate. If there's resistance, the force of the rotation (torque) applied through the tire tries to overcome that until the wheels are rolling at the proper speed.

That means if one tire is on a slicker surface than another, or is unloaded and not generating as much grip, it'll reach the right rotation speed before the tire with grip -- and you go nowhere. A limited-slip differential seeks to limit the difference in the axle (and wheel) rotation rate, forcing the engine to apply torque through both tires. up to the bias set in the diff -- a 100% bias is a locked differential. Most street LSDs are set at 15-20%, which is enough to keep a lightly-loaded wheel from spinning in a turn, or to start moving when one tire has little traction. Race LSDs can be set higher -- and some do run with locked differentials.

Honda's Active Torque Transfer System (ATTS) is a slightly different animal. The ATTS takes a batch of inputs -- wheel rotation rates, steering angle, yaw rate -- and decides whether the car needs help to turn. If yes, the ATTS actually speeds up the outside wheel up to 15%, which pushes the nose around a little faster. This is exactly the opposite of the way an LSD works! The ATTS won't function as an LSD, either -- unload the inside wheel, or put one drive wheel on a slick surface, and ATTS won't do a thing for you."