If you want to see a graphic demonstration of wheel slip, park a two-wheel drive car with one drive wheel on dry pavement, and the other on wet grass. If you try to take off quickly, the wheel on the grass will spin merrily while the wheel that actually has some traction will just sit there doing nothing. This is a by-product of the standard differential design that has been used by automakers for the last 100 years, but you can fix that with a limited slip differential.
The reasons behind a differential are simple: when a car is cornering, the wheels on the outside of the turn have to roll farther than the wheels on the inside of the turn. If you force both drive wheels to turn at the same speed, you’ll be dragging one tire or the other across the pavement every time you turn the steering wheel. That scrubs tread off your tires and slows you down. When you are at speed you don’t notice this so much, but at low speeds such as in a parking lot, your car can become impossible to turn.
A differential is made by creating a set of gears that allow one drive wheel to spin at a different speed from the other. When traction is more or less equal for both drive wheels, such as on a paved road, power is distributed equally to both wheels even while they turn at different speeds. The downside is that if one wheel loses traction significantly enough to cause the wheel to slip on the road surface, all the torque is then transmitted to that wheel until even grip is restored. Getting off the gas, which reduces power to the drive wheel, is the quickest way to stop the wheelspin, but it doesn’t do much for getting the vehicle going again.
Enter The Limited Slip Differential
What you really want is the best of both worlds – you want to make sure that both drive wheels get a share of torque, but without making your car hard to drive at low speeds. The solution is a limited slip differential that allows enough wheel slip to corner easily, but will also direct torque to both drive wheels to maximize traction when necessary.
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The primary designs used to create limited slip differentials are clutch-based, locking, torque sensing, viscous coupling, electronic, and spool.
Clutch-Based Limited Slip Devices
The difference between the various designs is that a clutch-based differential sends power to the slipping wheel based on input torque, which is power coming to the axle from the engine. The clutch may be a disc or cone design, but they engage when one wheel has more traction than the other and the incoming torque applies pressure to the clutch friction surfaces.
When high torque is applied to the differential and transferred to the axle, the spring-loaded clutch pack clamps the differential gears to the differential cage, limiting their relative motion. The clamping action is proportional to the torque delivered, which means that the higher the torque the greater the clamping load. By adjusting the tension on the clutch surfaces you can make a clutch-based limited slip differential tighter or looser, but full lockup is difficult to achieve, especially with higher torque engines.
Locking And Torque-Sensing Devices
In contrast, locking and torque-sensing rear differentials such as Torsen, Eaton Truetrac, Detroit Locker, or Quaife equalize the gripping and slipping wheels based on the difference in wheel speeds, regardless of engine power input. This is accomplished through gears that engage as wheel speeds change relative to each other. The gears allow for a certain amount of motion, but then bind up to force equal motion.
Even though several designs use binding gears to actuate the limited slip effect, there are differences between the various brands. For example, when a true locking differential such as a Detroit Locker engages, it will not unlock until pressure is released. With a locking differential you can break an axle and the car will still drive, whereas a torque-sensing differential will not.
Viscous Coupling And Electronic
It’s worth mentioning that you can find viscous coupling differentials in many production cars. This design simply uses thick fluids to encourage both wheels to turn at the same speed regardless of traction conditions. Manufacturers like viscous coupling because it’s inexpensive to produce, allows them to claim limited slip, and does not materially affect drivability. The limited slip differential in the early Mazda Miata was a viscous design, but Mazda shifted to a torque-sensing differential in 1994. Many Subaru Impreza models also used viscous coupling differentials in both the rear and center differential locations.
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There are also modern electronic differentials that use electronic inputs from the wheel speed sensors to actuate clutches while the traction control system applies the brake on a spinning wheel to equalize torque distribution. These work well, but often at the expense of torque transfer to the ground since they are designed for stability control, not performance.
Lastly we should mention the spool, or “welded” differential. Old school racers used to take their differentials out and weld up the internal gears to create a solid drive axle. For all intents and purposes, spool axles are locked together at all times. These units are mainly preferred for straight-line maximum traction in drag racing. Spool axles create a tremendous amount of drag when cornering, as one tire must be dragged around at the wrong speed.
On the left is the full spool and on the right is the mini or “poor man’s” spool.
The Special Case of All Wheel Drive
Cars with all-wheel drive are a special case when it comes to differentials because there are typically three of them. One for the front wheels, which behaves like any front-wheel-drive differential, one for the rear wheels that behaves like a rear-wheel drive differential, and a center differential to direct torque to the front and rear axles. Depending on year, make, and model, you may have a driver-controllable center differential and open or various limited slip designs in any of these positions. As long as you keep the same final drive ratios on both ends of your car, it’s quite possible to swap in limited slip devices in any of the three locations.
Choosing Your Limited Slip Differential
Selecting the best limited slip differential for your car is a matter of availability and personal preference. Obviously, a car used exclusively for drag racing has different requirements than a car used for track days and around town. In general, any limited slip device is better than an open differential, and you will see a noticeable improvement in corner traction and the ability to lay down power coming out of a corner. Check around and find out what others are using in similar cars to yours — chances are there is a clear favorite for your make and model.
TIP:While you’re playing with your differential, consider changing your final drive ratio. Production cars generally come with tall final drive gears. If you want better acceleration, a shorter final drive is a great idea, but don’t go too short or first gear will become annoying.