New LSD

I sent a spare Alfa rear axle up to Charlie Skinner at Cloverleaf Transmissions in Cumbria today to have it fitted with a “Gripper” LSD (Limited Slip Differential).

A differential allows driven wheels on the same axle to rotate at different speeds – this is essential for proper handling going round corners – the inside wheel always travels less distance than the outside one. The characteristics of a normal diff detnote that power will be transmitted to the wheel with the least mechanical resistance to motion. This means under hard cornering, it is possible to “spin up” the inside wheel (which is subject to less weight transfer) than the outside one, meaning it is more difficult to get the power down out of tight corners, as it is wasted driving (and thus spinning) the inside wheel which has the least grip.

Conversely, a limited slip differential can sense the wheel with the most mechanical resistance (and hence grip) and transfer power to this wheel instead. Indeed, it was possible to specify an Alfa GT with a LSD from the factory way back in the early 70’s – and I’ve had one fitted to my GT Junior for a couple of years now. Seeing as it’s a rear wheel drive car, fitting an LSD means it’s possible to powerslide the car out of corners (see photo taken on the way home from Le Mans 2005

The original Alfa diff created its locking effect using clutch plates – these eventually wear out, and the diff needs rebuilding – and this happens even more quickly when you increase the engine power. With the track use my GTJ gets, the diff is now pretty much toast and has lost its locking effect – out of Goddards at Donnington I was spinning up the inside wheel again.

The gripper diff I’ve bought doesn’t rely on friction plates for its locking effect – it’s a geared mechanism that requires no maintenance. The other advantage is that it has two different “ramps” depending if you’re on the power (up to 70% lockup) or on the over-run (up to 40%). The standard Alfa unit was just 25% from the factory regardless. This allows more precise positioning of the car on the approach to corners, reducing the tenancy of LSD diffs which can feel like they are pushing the car straight on into corners on initial turn in.

I’m also taking the oppertunity of reducing the final drive whilst having my diff rebuilt from the long 4.1:1 of my current setup (great for motorways, but barely get into 4th gear on the track) to 4.55:1 (max top speed of about 120, but much better acceleration). This should work much better on the track – I’ll let you know how I get on.