12/1/2004
By: Roger Hewson
| A lot of ground has been covered in the first two installments of You Need To Hook It Up. Part One covered how important chassis dynamics and weight distribution are and after Part Two everyone should have a better understanding of how important tires are to traction. The next move is to go about making the tires’ job of putting power to the ground as easy as possible for the amount of power you have. |
| Peg legs are for pirates In Part Three we’re going to tackle how the choice of a limited slip differential and different types of limited slip differentials will affect how efficiently the tires put the power to the ground. Remember that the more efficiently you put power to the ground, the more bang for your buck you get out of the horsepower at the flywheel. |
| Next to tires, a limited slip differential or LSD is going to do more for hooking your car up than anything else. It lets you divide the power between two tires instead of trying to get it all down into one- that also means double the tire size and contact patch. For example, instead of trying to run 300 horsepower through one tire you’re sending 150 horsepower through two in an ideal world. That does a lot in the game of trying to minimize wheel spin. |
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A good LSD is not going to give you twice the traction in reality, but there will be a huge difference because if set up right. A big advantage of having an LSD is consistency which makes it ideal for bracket racers. Because you are able to put the power down more effectively, launching the car becomes easier and more repeatable. It is also a must for high-horsepower applications since a pair of 10-inch wide slicks does a lot more good when both are seeing power. |
| How does an LSD perform such magical acts? First it helps to understand how a standard non-limited slip (or open) differential operates. One of the jobs of the differential is to take the energy coming from the transmission and direct it toward the tires. With an open differential, the power is channeled through either the right or left tire. When that one tire loses traction, the other side does nothing to compensate for the spin. Open differentials are more cost effective for the OEMs so they come standard in most economy cars. That’s fine for the average drive to the grocery store, but it’s bad news in any kind of race or serious driving. |
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The way the energy travels is also known as following the path of least resistance and it is always the way power goes in an open differential. It is the reason why a vehicle with an open differential will spin its tire on ice, mud, or water when the other is on dry pavement. It’s also why the redneck in the pickup only spins one tire when he tries to do a burnout in the K-Mart parking lot. |
| A limited slip differential helps direct the power back to the other side where it can be turned into forward motion and not wasted in wheel spin. Now that we have a little understanding on how open differentials work and why they are bad we can discuss the basics of limited slip differentials and why they are better. |
The limited slip differential does exactly that, it limits wheel slip by directing the energy (which would have been wasted in wheel spin) and dividing it up between the tires to provide more traction through a series of clutches, gears or fluid couplings . Clutch type units are identical in design to an open differential except they control the flow of power with a set of high-friction, multi-disc clutch packs for the left and right axles and a spring set between them to provide extra ‘lock’ by putting the clutch packs under tension. The tighter or looser the clutch pack, the harder or easier it is for the wheels to turn different speeds. The same goes for the amount of friction in the clutch plates, how many plates, tension in the springs, etc. Once again a basic explanation, but the theory is the same for any clutch type LSD. One of the biggest advantages of the clutch type LSD lies in the flexibility of its setup- if you want less lock up run a smaller number of friction plates in the clutch packs. If you have a problem with spinning only the left or right tire run the clutch packs more aggressively with more clutch discs to produce more friction if you can. |
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The down sides to a clutch-type diff are that it does require periodic rebuilds once the clutch discs wear out. The more aggressively the clutch packs are set up, the quicker they are going to wear. They can be noisy too. The more lock you have dialed into a clutch type LSD, the more noise they make while turning. They tend to emit a low pitched groaning noise whenever the wheels are turned at low speeds. On a front wheel drive car a clutch type LSD tends to add steering effort at low speeds with a really aggressive, race type clutch setup. |
| Ray Nakadate of KAAZ differentials says the proper setup for a clutch-type diff depends on how much time you spend at the track. “For a street driven car, I would set up the diff at 60 percent lock so it would be better suited for street and track use,” says Nakadate. “This way the car performs well at the drag strip and is quiet on the street. If the driver is more serious about turning good times at the track 100 percent lock is the only way to go.” |
| Gear driven limited slip differentials work in a completely different manner. They rely on gear sets which are able to distribute the power between the tires as needed. If the left tire has significantly more traction than the right the differential will transfer a percentage of power to the left tire. There are left and right angled gear sets which overlap with each other slightly inside the differential to provide the power transfer. The angle (also called the helix angle) cut into the gears determines how quickly and how much power gets transferred between the left and right tires. Gear driven limited slip differentials are constantly shifting power around to achieve the best traction possible. |
| A gear driven LSD is quiet, smooth, and quick reacting to most situations requiring torque transfer between the left and right tires and are constantly searching for the best traction possible. Another plus to many of the gear driven differentials like the Quaife unit for example is they are significantly stronger than the open and clutch style differentials and they do not suffer from any significant wear even under severe conditions. Quaife America is so sure their LSD will not fail they offer a limited lifetime warranty on the unit even if it is used in competition. |
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The gear driven units do have a down side though. Units like the Quaife require some kind of preload on both tires at all times. What this means if you lift one tire in the air, or have a tire hydroplane in the burnout box water, the diff will go open and spin one tire because there is not enough preload on the gears for the differential to operate. Gear driven differentials can also torque-steer without notice and with a lot of force if they suddenly find grip at one wheel on a low traction surface. What happens is the diff sends the majority of power to one tire because it finds grip there and the car violently pulls to that side. |
| Figuring out which differential suits you best is up to you. Each differential has its up and downs and depending on your particular situation and habits, one will suit you better. If we use our friend with the Civic from Part Two as an example, he would be a prime candidate for the Quaife or the KAAZ set at 60 percent lockup because the car is mainly street driven. He or she would not want to put up with the increased steering effort and the noise of the KAAZ or other clutch type differentials at 100 percent lock on a daily basis. The Quaife may still be the friendlier of the two on the street for a medium horsepower front-wheel-drive car. |
| Regardless of which LSD you pick there is no denying the immediate benefit in traction. Limited slip differentials are a must in any performance car especially if you are going to take it to the track. An open differential will be a thorn in your side for as long as you have it. You will need to run bigger tires to compensate for the lack of traction, launch less aggressively, shift easier or deal with a number of things which will slow you down. It’s no fun doing the one wheel peel, open differentials aren’t called ‘peg legs’ for nothing. |