What's the diff: Part 1

Image by Dave Green

What's the diff: Part 1

SCCA / SportsCar Magazine

What's the diff: Part 1


Before we dive into RallyCross tuning tips, you’ll first need an understanding of differentials – so here it is

With the wide variety of limited-slip differentials available for most popular RallyCross vehicles, selecting the diff that’s right for your application and driving style can be difficult. Luckily, we’re here to help – or at least clarify things. To that end, this month we’re going to cover the various types of differentials and how they operate, and next month we’ll be back to cover what vehicles come with factory LSDs and what type of LSD may be the best choice for your particular application.

A differential’s basic job is to allow one drive wheel to rotate faster or slower than the other while turning. When a vehicle turns, the outside wheel is making a larger arc than the other and thus traveling a longer distance, so it must rotate faster. This is what an “open” differential allows under all circumstances. This is great for initial turn-in, especially when no torque is being applied, but not so much when you’re applying torque to accelerate. In contrast, a locking or limited-slip differential limits the difference in speed between the output shafts (your axles), and by proxy, the differences in wheel speed.

There are three different types of LSDs in terms of operation. They include 1-way, 2-way, and 1.5-way units. A 2-way differential will limit wheel speed differences under acceleration and deceleration. A 1-way differential will only do so on acceleration or deceleration. A 1.5-way differential limits differences in wheel speed under both conditions like a 2-way, but at different rates.

In addition to the types of LSDs, there are numerous different styles of LSD. Some of the common styles include clutch or cone types, helical gear (like the Torsen), viscous, electronic, and fully locked or welded differentials.

A clutch or cone LSD uses a stack of clutch discs to limit wheel slip. The more the clutch stack is compressed, the more “locked” the wheels become.

By contrast, gear-driven or mechanical LSDs utilize worm or spur gears. As torque is applied to the gears, they are pushed against the walls of the differential housing, creating friction, which in turn limits differences in wheel speeds. A viscous LSD operates using friction generated by discs suspended in a fluid. As the wheels change speeds in relation to one another, the discs will be pressed together with the fluid between them. The friction generated by this is what provides the locking action.

An electronic LSD, or “E-Locker,” typically utilizes an internal construction similar to an open differential combined with a clutch pack. The clamping force of the clutch is then electronically engaged or disengaged based on surface conditions and/or a preset value. Some systems allow the user to control and adjust the differential’s limiting torque, like in Subaru’s DCCD system.

Finally, a fully locked or “welded” differential allows no difference in wheel speed between the output shafts. If you ever hear or read a reference to a “welded differential,” this means the spider gears inside of the differential have been welded together so they can no longer spin, thereby locking them in place, which forces both wheels to spin at the same speed.

And, with that basic grasp of the topic, next time we’ll help you make the right selection for your car and driving style because, well, differentials aren’t cheap, so you’ll want to do it right the first time.

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