How Limited Slip Diffs Make You Faster
Over the years, I've found that limited slip diffs (LSD's) are some of the least appreciated performance parts you could get (or upgrade) for a car. LSD's make a big difference, though, because they can vastly improve how early you can get back on the power and, therefore, how good your corner exit is.
That last bit is very important if you're driving on track and want to get a good lap. You don't have to just take my word for it, though. F1 royalty Sir Jackie Stewart puts an emphasis on the importance of corner exit. When Captain Slow was sent to him to cut 20 seconds off his lap time (Top Gear Season 8 - Episode 5), Sir Jackie told him: "the exit of the corner is FAR more important than the entry of the corner, with regards to smoothness."
You really need to nail the exit. And to get a better appreciation for and understanding of LSD's, you first need to know how open diffs work and where they fall short. If you're not sure, brush up on Differentials 101 in my recent post about Why An Open Diff Doesn't Work On Track. I won't get too much into the working mechanicals of a typical LSD, just the principles and how it affects the car.
How does it work?
The most common LSDs have traditionally been mechanical (i.e. no electronic wizardry) with a limited slip mechanism to resist one slipping wheel. That is slowly starting to change now, but these ones are still very common. They are typically open differentials at heart with modifications or additions (although some are more complex than that). Those modifications are designed to resist a speed variance across the differential. The result is a limit to how much faster one wheel can spin relative to the other, overcoming the limitations I mentioned for an open diff.
An example would be a set of clutch packs that progressively engage when there is a speed variance across an axle (i.e. one wheel is spinning faster than the other). The clutch packs progressively engage to transfer more torque to the wheel with more traction, thereby limiting slip. Another example is using a viscous fluid that effectively gets "thicker" as one wheel excessively spins, virtually locking half-shafts to the diff case.
There are other types too but, in essence, the differential is locking the two half-shafts/wheels together to an extent. That extent depends on the design and specification of the differential - typically referred to as a percentage (%) number and occasionally as a Torque Bias Ratio (TBR). Remember in the post about open diffs when I said an open diff provides virtually equal torque to both wheels? The % number and bias ratios mean LSD's can distribute torque unevenly side-to-side, which is what you want.
The Power Struggle
The % number is the difference in torque (in % of total) the diff can provide between the two axles. A 25% LSD, for instance, can provide the side with more traction 25% of the total torque and the rest is split equally. That means one wheel can get up to 62.5% of total torque if it has more traction, instead of always getting 50% with an open diff. Equally as important, the low traction wheel only gets 37.5% of total power instead of 50%, so you don't overpower it as easily.
Spec E30 BMW 325i clearly demonstrating weight transfer mid corner with the inside front wheel lifted clean off, the reason you want to take advantage of more grip at the outside wheels/tires - Graham MacNeil ©
TBR is similar. It's the ratio between the torque sent to the outside wheel relative to the inside wheel. A 2:1 TBR, for instance, provides two times as much torque to the outside wheel, or 67% of total and the inside wheel gets 33%. That means you can send more torque to the outside wheel to take advantage of more grip while simultaneously reducing torque at the inside wheel with less grip to avoid slipping.
LSD's Can Get Confused
One tricky disadvantage to the traditional LSD, though, is losing grip on snow, ice, or heavy rain. With a limited slip diff, you are more likely to get moving because you can better utilize available grip at the driven wheels. But once you are moving, the diff could get confused by road conditions.
Because the diff is "dumb" and just sends power away from a slipping wheel, if you are driving and one wheel begins to slip due to poor road conditions (i.e. hydroplanes, hits a patch of ice, etc.), the other wheel gets more power. That extra power could cause the OTHER wheel to slip and the first wheel that was previously slipping and then got limited gets more power. This results in back and forth shifting of power that could cause the car to "fish tail" and make it more difficult to catch the back end if it starts to go.
Another way LSD's can get confused is slow-pace driving out of a corner. They can't tell the difference between turning and a slipping wheel. If you're going around a turn, the outside wheel is spinning faster than the inside wheel because it has to "travel further" as a result of taking the longer way around the turn (explained in more detail in the last post linked above). But from the diff's perspective, one wheel is faster (outside) and one wheel is more difficult to spin (inside). The diff will start to lock up in response, thereby transferring torque to the inside wheel.
That means the inside wheel, which is the unloaded low-grip wheel, gets more torque, the opposite of what you want. Worst still, it generates a steering moment in the opposite direction of the turn; think of all the manufacturer talk about torque vectoring sending more torque to the outside wheel to help you turn. You are getting the opposite here. It's a small amount and LSD's are designed to minimize lock up at small speed variances like that for this very reason, but it's still there.
Fortunately, they start to work properly on a track once you feed in more power. As you feed in more power, you start to overpower the inside wheel. As you overpower it, it begins to slip. Eventually, the inside wheel speed will exceed the outside. But since you have a limited slip diff, it is progressively locking to minimize that and, this time, it's transferring power to the outside wheel.
Good, so how does it help?
Imagine that you've just nailed the apex of the corner and you're starting to feed in the power as you unwind the steering wheel. Your outside wheel is loaded due to weight transfer and your inside is unloaded. What you want is give more power to the outside since it has more grip and less to the inside. An open diff can't do that but a limited slip diff can.
As you feed in the power and the inside wheel begins to slip, the diff progressively locks. This forces more power away from the inside wheel so it doesn't continue to slip and sends that power to the outside wheel where you can use it. You can go faster by using more power earlier in corner exit and, due to limiting inside wheel spin, you won't lose traction as easily which means you can better maintain your available grip. The higher the bias ratio mentioned earlier, the more the diff can lock. More lock means the diff can send more power away from the inside wheel and to the outside wheel.
So More Lock is Better?
You don't always want the highest number possible. A higher number does let you get back on the power earlier with more power sent to the outside wheel. And limiting slip of a low traction wheel is great, as it can be the difference between accelerating and backing off the power when exiting a corner on a track. The downside to more lock up is understeer on a RWD car, even when the diff is working as intended. This is a result of three factors:
1. Locking up to any degree provides less speed differentiation than no lock up at all, which we've established is required for the car to turn. If you've ever driven a 4x4 with diff locks on dry pavement, you'll know exactly what I mean (better not do that, though, because it puts a lot of stress on drivetrain components).
2. Putting more power down means more weight transfer to the rear end, which results in less grip at the front end. Less grip in the front is more understeer. If you've ever driven a 911, understeer due to rear weight transfer is very pronounced since most of the weight is at the back to start with.
3. You can maintain your grip for longer due to no inside wheel slip. If the rear axle can hold on for longer, you'll increase understeer.
With that said, a car without a limited slip diff will generally be slower than a car with one. This is because you can get back on the power much sooner with a diff like that and more aggressively, which will let you shave a lot of time. As a result, you'll find that most good RWD cars actually have LSD's, such as Corvettes, BMW's, Camaros, Mustangs, Subaru BRZ/Toyota 86, Miatas, etc.
The only exception would be a momentum RWD car that has more traction than power and, therefore, could not excessively spin its inside wheel if it wanted to. You make the most of it by maximizing corner speed everywhere.
Know Your Car and Your Track
And there are ways to get around the understeer. You can tune the suspension to reduce that understeer so you typically only notice the understeer on a car that had a limited slip diff added but is otherwise unchanged. As with all things in motorsports, you need to learn your car, learn your setup, and learn your track. A power track could allow you to better take advantage of high amounts of lockup. A track with a bunch of high speed sweepers might lend itself to maximizing corner speed and minimizing understeer, plus high speed corners bring downforce into the equation so you could increase your grip to make up for lower lockup and traction performance.
It gets even better on a FWD car, since you only have the first two factors (i.e. lock up and weight transfer) against you. The third is actually helping you. LSDs on the front let you maintain grip for longer on the front axle, which is less understeer. And even before the diff is working as intended, torque transferred to either front wheel generates a positive steering moment. That means a limited slip diff typically curbs understeer on a FWD car even if all else stays the same, especially if it's a high horsepower car. The one caveat is that the axle locking can make it difficult to steer, if aggressive.
There has to be a better way!
Of course, as with every technology, engineers are always working to improve it. Limited slip differentials are no exception and there are several other ways to limit slip and distribute torque that address the downsides of a traditional LSD. Stay tuned for the next post, where I will explain other LSD technologies including gear-type, electronic (like the BMW M2 pictured), brake-based, and torque vectoring!