Limited Slip Differentials
What does a Limited Slip Differential do?
An LSD is installed to maximise wheel traction. These benefits are seen most readily in competition, particularly where the car is attempting to accelerate on the way out of a bend or corner, or in an autotest or motorkhana where an LSD gives an ability to steer the rear end of the car accurately under wheelspin conditions.
Under normal driving conditions, a standard diff tries to equalise the driving torque between the two driving wheels. This works well on an even and regular surface.
However, when the road surface under the two wheels is significantly different or the weight loadings differ (such as when cornering), one wheel has a reduced ability to transmit the torque to the road surface and traction is reduced or completely lost by that wheel.
If traction is lost by one wheel the differential will turn that wheel at a greater speed and no driving torque is applied to the other wheel. This creates a ” no drive” condition.
A LSD reduces this lack of drive condition. LSD’s work by clutching the differential sidegears to the differential cage. The clutch unit will have a relatively small amount of grip when no power is being transmitted, allowing the gears to move within the cage and so the whole unit operates more-or less as a standard diff. When full power is applied, these clutches become heavily loaded and the amount of clutch grip increases substantially, effectively locking up the whole internal diff cage assembly and making the car behave as though it has a solid axle from wheel to wheel. At part throttle openings, the effect will be somewhere between these two extremes. If you think about all this, you will see that the term “limited slip” is an accurate description of its operation.
Under normal road conditions there will be no change in driving characteristics.
In summary, you can think of it like this: A standard diff feeds the power to the wheel with least traction, whereas a limited slip diff feeds it to the wheel with most traction.
How can I tell if I need an LSD?
If you are competing and your car is not fitted with an LSD, you will notice a tendency for the engine to rev faster in the corners than on the straight for a given speed, and to leave only one wheel mark off the starting line. This indicates that one of your wheels is spinning and that your available traction has been greatly reduced.
If your vehicle seems to “go nowhere” under power or people comment on smoke from your inside rear tyre, there is no doubt that your traction, can be improved by fitting an LSD. This translates directly to more BHP on the road! In that sense it provides excellent value for money in the extra BHP stakes – on point in adding more power if you aren’t getting it on the road!
In general, the LSD provides improved traction in all conditions, and makes for a lot more fun under slippery conditions. If you are likely to use your vehicle to its limits you will get good value from an LSD.
An important additional advantage of an LSD is that it will save you money in the long term if you get frequent wheelspin. Under these conditions, the lubrication in a standard diff is not up to the job, with the result that the differential gears and thrusts wear rapidly. Even worse, a spider pinion can seize to its shaft causing something to break. This can be the pinion itself, or the pin securing the shaft.
If the pinion shatters, chips of it inevitably wedge themselves between the crown wheel and pinion, cracking a tooth or two. If the securing pin breaks, sometime later it will work loose, then gradually work its way out until CLUNK it jams on the housing or main pinion with catastrophic results. B series tube axles are very prone to this event, while A series usually shatter the spider pinion.
Either way, prolonged wheel spin can be very expensive as well as slower!
Will an LSD affect handling?
Yes, for the better. Handling is enhanced with an LSD as the effect when the rear wheels lose traction is much more even and controllable. Up to that point, the LSD will accentuate a slight understeer under power, oversteer on overrun. This provides a the opportunity to “fine-tune” your line through a corner with the use of the accelerator.
How do I tell if I already have an LSD?
Raise one of the drive wheels off the ground with the hand brake off and the car out of gear. Try to rotate one wheel in the forward direction. If it rotates freely then it has a standard differential. If it doesn’t move, or moves with difficulty then the diff is a limited slip type.
Are there any downsides to an LSD?
LSD’s usually result in increased wear of the rear tyres. This arises when cornering under power. When cornering, the outside wheel has further to travel than the inside wheel so needs to turn faster – this is the whole reason for a differnatial of course. If the diff partially or fully locks up some degree of slippage has to occur instead, loading and wearing the tyres more.
But if you ask me, it’s worth it, LSD’s are really worthwhile when competing and make for a lot more fun in autotests or motorkhanas. For that reason it’s the first thing I do to every car I buy!