View Full Version : Limited Slp Diffs - What and How

06-09-2011, 06:12 PM
This from an old University buddy, who is now a Chief Engineer at Aston Martin Lagonda no less:cool::cool:

Lets start with a normal "open" diff. I'm assuming you know that the basic function is to allow torque to be transmitted to the wheels whilst allowing them to rotate at different speeds. It's a bit difficult to explain how it does this without pictures (particularly as I is an engineer) but I'll give it a quick go! The torque comes into the diff on a pinion gear that engages onto a ring gear that is connected to a cage that contains a cluster of four bevel gears.......OK I give up have a look on Youtube!

The problem with this diff is that if one wheel has less grip that the other this wheel will spin and no torque will go to the wheel with grip. A LSD uses either clutches or a viscous liquid (you know that stuff you dig out of the ground) to limit how much one wheel spins with respect to the other by transmitting torque to the other wheel. The benefits depend on the type of car. In a 4x4 they make sure that if one wheel loses traction then torque goes to the other wheels and you stand a chance of getting out of the mud / sand / snow. In a performance rear wheel drive car the main advantage is giving traction during cornering. When you go round a corner the weight goes onto the outside wheels, if you then try to accelerate out of the corner the inside wheel is prone to spin. The LSD limits this spinning and pushes torque to the outside wheel that has more grip. The down side is that if you put too much torque through and the outside wheel spins as well and unless you are quick you disappear into the undergrowth!