TORQUE BIASING DIFFS
These are the least harsh of all LSDs because they never ‘lock’ both wheels together. A complex arrangement of the sun and planet gears means that as soon as one wheel starts to slip the torque is biased away from that wheel.
The most famous of this type of diff is Quaife’s ATB (Automatic Torque Biasing) unit, although manufacturers like Wavetrac and Blackline have similar products too. An ATB diff may look complicated, but the principal behind how it works is simple. Rather than relying on plates to operate, an ATB unit uses gears to bias the torque away from the spinning wheel. ATBs differ from conventional diffs because they use a series of helical pinion gears which mesh with each other as well as the sun gears.
Usually a diff will have a sun gear which meshes with a planet gear, which in turn meshes with the other sun gear. This is why with a conventional type, open diff when you spin one wheel, the other wheel spins in the opposite direction. 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 which means that both wheels turn in the same direction.
Under normal conditions the pinion gears mesh and both wheels are driven at the same speed. However, when one wheel starts to slip the friction from 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 will work in all cars, they’re favoured by FWD owners due to the controllable power delivery and reduction in steering wheel snatch and torque steer. They also work incredibly well on cars like the Mk3 Focus RS, which behaves like a FWD until the ECU sends power to the rear wheels when required.
ATBs behave in a safer and more predictable way than others making them very 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, again making them an ideal fit-and-forget solution for fast road cars.
One downside, however, is that ATBs do require both wheels to be in contact with the ground to work effectively. This means they are perhaps better suited to road use than certain types of motorsport where there is a chance of one wheel loosing contact with the ground (rallying, or bouncing off kerbs, for example).