Helical LSDs

  The Quaife ATB Helical LSD differential is a "different beast" so to speak. It uses all gear design for ultimately smooth transfer of torque to the proper places. This is not to say it does not use friction, just no fixed torque clutch or brake plates.

The essence of this design is high angle helical gears. When both wheels have equal traction the differential works in normal fashion. This is a torque sensing differential rather than speed sensing. As such it does not try to make the inside wheel turn same speed as the outside wheel. As long as both wheels have traction it transmits equal torque to both wheels regardless of relative wheel speed, very similar to an open differential. One nice feature here is very little affect on understeer or over steer with throttle changes.

When one wheel starts to slip the helical gears slide endwise with a force proportional to the torque on the wheels (not fixed torque and not proportional to speed). This applies a load on a sidewall friction plate to generate resistance to bias the output torque accordingly. This is in effect applying a braking force to the wheel that has less traction, but the reaction force is transferred to drive the other side rather than energy being lost as friction heat. Changing angle of the helical teeth changes the limit of torque bias. These can be designed with torque bias of 3:1 or up to 5:1. So if one wheel begins to lose traction the other wheel can receive up to 5 times as much torque as the one that is slipping. It also has the advantage of being able to transmit nearly all of the propshaft input torque to the wheels, proportioning torque appropriately up to the point where both wheels might spin at the same time.

Unfortunately five times zero is still zero, so when one wheel hits a patch of wet ice it will still spin, and the other wheel will be limited to five times the torque as the one on ice. This works well as long as the low torque wheel still has some grip and some available torque, but if you lift a wheel in a fast turn it will immediately revert to zero torque all around like an open differential. As such, this works best with well tuned suspension that can keep the drive wheels planted on the pavement. If you happen to be aware of the total loss of traction when it happens, and you understand how these things work, you can step gently on the brakes to stop spinning of the unloaded wheel. This will then apply up to five times as much torque as the light braking effect to the wheel that is not spinning, which will in turn overcome the light braking force and push the vehicle. Competition drivers may soon develop the technique of "left foot braking" while they keep the right foot on the throttle. (Incidentally this torque sensing differential works quite well in conjunction with a modern antilock braking system).

Another significant advantage of the Quaife design is that the all gear unit will have very little wear, so it is maintenance free and comes with a lifetime warrantee. They are relatively expensive to purchase, but value may come with the results. You can buy these off the shelf for installation in your MGA or early MGB banjo style rear axle. Just specify the number of splines required in the sun gears to match the splines on your half shafts.
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