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Home » Drivetrain » Limited Slip Differentials
Limited slip differentials help the output shafts to rotate at different speeds while regulating the maximum difference between them. The systems are used instead of the regular differentials due to the dynamic advantages (improved handling and performance) that they offer. This development can be traced back to the 1930s when Ferdinand Porsche commissioned ZF, a German engineering firm to develop a differential that could minimise wheelspin on the Auto Union’s racing car, Grand Prix. It was after Ferdinand noted the excessive wheelspin on one of the vehicle’s rear wheels at speeds as high as 160 km/h. Subsequent use of this differential was witnessed during the Second World War in the military Volkswagen like the Schwimmwagen and the Kubelwagen. Today, the benefits of this differential continue to be exploited as models like Chrysler, Ford, Chevrolet and others incorporate it into their vehicles. Limited slip diff also acts as a safety feature by offering more control over the vehicle’s power delivery system. It explains why limited-spin diff is fitted on performance vehicles like sports cars, which generate a lot of power. Other uses of the system include adding traction to 4×4 cars driven rough terrains and reducing undesirable torque steer on front-wheel hatchbacks.
The type of limited slip diff fitted on a vehicle depends on the drive system used. For example, four-wheel cars and rear-wheel drives will use a two-way limited slip diff. Here’s an overview of the different types:
Basically, there are three kinds of input torque states- the over run, load and no load. Load condition means that the coupling is equal to the input torque while no load allows the coupling to be reduced to a static condition. As such, an unexpected throttle release determines if the limited slip diff is one-way, two-way or 1.5 way.
The one-way differential is ideal for front-wheel-drive vehicles as it has a limiting effect on one direction when accelerating. The rationale for using it on FWD is that the one-way differential unlocks as soon as the driver lifts the throttle to acts like the standard open differential.
As earlier mentioned, two-way differentials are suitable for rear-wheel drives and 4WDs. This differential has the same limiting torque in reverse and forward directions to provide a reasonable level of limiting during engine braking.
1.5-way differential creates a differential effect when accelerating and slowing down. However, for this type, the slip is not the same in both directions, making it more useful than one-way differentials and letting the vehicle to use the engine braking feature when slowing down.
This differential uses helical gear SD, a cone and a clutch. The input torque of the engine determines the engagement force of the clutch. As the engine applies more torque, the differential decreases and the clutch grips harder. Torque sensitive differentials are responsive to the torque produced by the driveshaft. Thus, the more driveshaft input torque produced, the harder the cones, clutches and gears are pressed together. Some differentials are fitted with spring loading to produce reasonable torque and reduce coupling on the drive wheels. The Quaife ATB Helical LSD Differential – 6 1/2″ Front Differential is a perfect example of torque-sensitive differentials. It features the latest CAD design and rids of the harsh torque steer characteristics created by other gear-operated differentials.
The clutch type has thin clutch discs where some are attached to the drive shafts and others to spider gear carriers. The cone type clutch features a pair of cones that are pressed together to obtain the same effect. Spider gears are mounted on the pinion shaft to form cammed ramps. If the ramps form a symmetrical shape, the differential becomes a two-way, and if it is saw-toothed, it is a one-way differential. The ramps may also adopt a sloped arrangement to create a 1.5-way differential.
This kind uses thick oil to produces the same effect as limited slip diff. However, viscous differentials wear out faster than mechanical ones. Additionally, the oil may become excessively hot and lose effectiveness.
The differentials provide more control over power delivery compared to conventional open differentials. When negotiating corners, open differentials use gears to manipulate the wheels to turn at various speeds. However, when the car delivers a lot of power, the open differential may be overcome and lose grip. The power generated will look for the path that has the least resistance; in this case, the tyre with the least grip. As such, if the driver is using a powerful car, it can cause the unloaded tyre to spin away while others continue to hold grip. The inclusion of an LSD system provides a mechanism in the form of cams and clutch assembly that counters this natural flow of power. It redistributes the torque to the wheel that has the most grip. As a result, the unloaded tyre does not spin away, and the car has a firmer grip on the road when cornering.