WHAT IS AN ATB?
There are many different types of limitedslip differential available, and all try to achieve the same goal – to maximise the available amount of traction at each driven wheel.
ATBs (Automatic Torque Biasing) units are the least harsh of all LSD types, because they never lock. A complex arrangement of the sun and planet gears means that when one wheel starts to slip, the torque is biased away from that wheel.
The most famous ATB diff is that made by UK engineering company, Quaife, although some OE manufacturers and other transmission specialists offer similar products.
An ATB diff may look complicated, but the principal behind how it works is actually quite simple. Rather than relying on friction plates to operate, an ATB unit uses gears to bias the torque away from the spinning wheel. To do this, an ATB uses a series of helical pinion gears, which mesh with each other as well as the differential’s sun gears. Typically, an open diff will have a sun gear which meshes with a planet gear, which in turn meshes with the other sun gear. This is why when you spin one wheel, with a conventional open diff the other wheel spins in the opposite direction.
However, with an ATB diff the sun gear meshes with a series of helical pinion gears, which mesh with another set of helical pinion gears, which then mesh with the other sun gear. This effectively adds an ‘idler’ gear, meaning both wheels turn in the same direction.
Under normal driving conditions the pinion gears mesh and both wheels are driven at the same speed. However, when one wheel starts to slip the friction caused by the drag of the pinion gears in their pockets enables the ATB to transfer more torque to the other wheel with traction.
ATBs are one of the most user-friendly LSDs because the amount of torque transmitted to each wheel constantly varies. While ATBs work in all cars, they’re favoured by FWD and 4x4 road car owners due to the controllable power delivery and reduction in steering wheel snatch and torque steer.
ATBs behave in a safer and more predictable way than other diff types, making them well suited for road use.
Also, because of the amount of surface area in contact with each other, ATBs are among the strongest LSDs available, making them an ideal fit-and-forget solution for many fast road cars.