The Quaife ATB (Automatic Torque Biasing) differential is a unique type of limited-slip diff that substantially improves traction for powerful vehicles using only gear mechanisms. This exceptionally clever design revolutionised the BMW LSD market - offering a superior level of performance and warranties compared to its competitors.
Quaife is now the world leader in differential technology. Successfully used in all forms of motorsport including F1, rallying, circuit racing, rallycross and drifting, they are also ideally suited to high-performance road cars. Quaife even supply FORD as original equipment for their Focus RS range!
All standard BMW and Mercedes cars are fitted with the basic "open differential" - a cheap and robust device that allows the driving wheels to turn at different speeds (enabling the car to navigate around corners). The problem with the open diff is that it can only ever apply the same amount of torque to both driving wheels, so when a wheel slips, it will only put through the amount of torque it requires to power that spinning wheel (very little!). The car spins away power in these instances.
Additionally, BMW and Mercedes use various electronic traction systems that "curb" power to the drive wheels upon traction loss (indicated by the flashing traction light on your dashboard). The Dynamic Stability Control system (DSC) is designed to "curb the engine output and stop slip on the wheels", a feature that is certainly considered a frustration by most. These stabilisation measures are good features for day-to-day drivers, especially in slippery conditions. However for those of us who like to drive 'enthusiastically' there are clever differentials that will transfer torque to the wheel with more traction rather than take it away... As demonstrated in the above video, a limited-slip differential enables the transfer of power across the drive axle, allowing power to the wheel with more grip. What it doesn't show you is the unique way in which the Quaife ATB limited slip differential does this...
How does a Quaife ATB differential improve performance?
The Quaife unit uses sets of floating helical cut gear pinions that run in pockets and mesh during normal driving, acting as an open diff. Should one of the driving wheels start to spin however, the helical gears start to generate a torque bias thanks to the axial and radial thrust of the helical gear pinions in their pockets. The result is an instantaneous and progressive transfer of torque away from the spinning wheel to the one with more traction, always varying accordingly and automatically.
With the DSC and traction systems switched off during traction loss, you will feel the Quaife transferring the power to the wheel with more grip in a smooth and predictable manner. Any movement at the rear end is controlled and intuitive for the driver. With the traction systems switched on, you will rarely ever see the light flicker on your dashboard as the Quaife allows you to power through any loss of traction, working in perfect harmony with all of the BMW's standard equipment.
The car ultimately becomes easier to control and is able to apply its power to the tarmac with maximum effect. Acceleration, cornering speeds, handling, steering, and overall dynamics of the car are dramatically improved! The Quaife is capable of varying the torque split from 0-80%, and does this quietly and smoothly in operation. It does not affect any of the traction systems on the car, and does not affect the manufacturer's warranty. All backed by its own worldwide lifetime guarantee.
Chris Harris of AUTOCAR Magazine demonstrates the Quaife's abilities in our 335i demo: