The Basics of Limited-Slip Differential Performance

Click Here to Begin slideshow

We’ve made a great deal of progress with differentials since the 1970s, 1980s, and even the 1990s. Days of running a spool - also known as a locked differential - to manage big power are well behind us, as are the times when drivers could use their aggressive throttle technique to dramatically turn a car and hold on by the scruff of its neck. Differentials have various uses, but in this instance, we’ll talk about how they are used to make a car corner quickly and safely.

Cars nowadays are outfitted with differentials that are specialized, sensitive, and adjustable. They can be tuned to get a car to find that perfect medium between stability and incisiveness. In other words, they help a car move through a corner without too much oversteer or understeer, and they should be viewed through the variables of acceleration lockup, deceleration lockup, and preload.

What lockup basically refers to is how closely two driven wheels on a shared axle will spin to one another’s speed. Since the inside and outside wheels travel over different lines and are sometimes subjected to different cornering forces, they will naturally rotate at different speeds. The limited-slip differential’s job is to limit how much these two vary in their rotational speeds. Basically, when two wheels spin more-or-less at the same speed, they can provide certain handling traits, like understeer or oversteer, depending on which axle they are attached to, and sometimes having them spin at slightly different speeds is preferable.

Finding a strong sense of drive off the corner is the objective here. However, it has to be balanced with enough lockup to allow for the car to follow the intended line. In this case, once the car has passed the apex and the throttle is depressed, response at this point is the focus.

In the case of a rear-wheel drive car, if the car begins to push or understeer out of the corner, it’s not locking enough. If, however, it’s oversteering wildly - fun though it may be - it’s locking too much. Ideally, the car will lockup just enough to turn and accelerate without becoming wildly loose or unstable. This characteristic is most useful on tight tracks or street circuits with plenty of sharp corners that need a bit of rotation throughout.

Now, the opposite is true for front-wheel drive machines. Finding too much push or understeer at the corner exit when the throttle is depressed means the differential is locking too much, and the inside tire isn’t allowed to spin a little more freely, which will help with rotation. This is because the front-wheel drive car must use the driven wheels to turn and accelerate simultaneously. Since the engine is sitting on top of the driven wheels, and front-wheel drive racers tend to have less power, there’s less threat of wheelspin, but it’s completely possible to fry the inside wheel at lower levels of lock.

This is where it gets a little trickier. As acceleration lockup accounts for turning and acceleration at the exit of a corner, deceleration lockup deals with braking and cornering. The objective again is the right balance of precision and stability: not too much understeer and not too much oversteer, respectively. Increasing the amount of locking will help minimize oversteer and keep the car predictable and stable while slowing and turning. The faster the corner, the more valuable this differential setting becomes, and it’s a necessity if a series of fast laps are in order. Predictably then, too much locking means the car continues to go straight under braking, since the inside wheel will not “unlock” and spin at its own speed, which would allow the car the turn.

There are more variables to consider, like weight distribution and amount of grip available, which will determine just how the differential should be adjusted, but these are the basic rules. Finding the sweet spot is always a challenge and very hard to do. However, being competitive requires a bit of fine-tuning the differentials so that they work for the majority of the course, and prevent an accident. As always, driving ability is needed to do the final tuning if the car isn’t quite right, but this setup exercise, as with any, will yield massive rewards if done well.