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The Lowdown On Limited-Slip Differentials

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Get the popcorn; it’s time for a movie. The matinee: “My Cousin Vinny”! If you’ve seen it, you know why it’s the matinee. If not, I won’t ruin it. It’s an enjoyable 1992 comedy that, believe it or not, is related to the information in this article. It’s worth a watch.

Who’s heard of one-wheel peel? That’s what it’s called when you decide to “burn rubber” and only one wheel spins, leaving rubber particles from that tire onto the road. It’s not an official term, rather one of joking intention among the performance crowd. However, it doesn’t mean anything is wrong. It’s actually perfectly normal operation for an “open” differential, so for now just remember that term; it’ll all come together down the page.

To understand limited-slip, you first must understand normal differential operation. Every vehicle has a differential. The differential is the component in the drivetrain that transfers the power output from the transmission to the axles that drive the wheels. It’s necessary because wheels travel at different speeds when turning a corner. The inside wheel turns slower because it’s essentially traveling in a smaller circle, whereas the outer wheel is traveling in a larger circle so it must turn faster to cover the increased distance.

There was a time when virtually every automotive platform was a front-engine, rear-axle design – so I’ll call this “traditional,” for sake of reference. In a traditional drivetrain, the power output from the transmission travels via the driveshaft to the rear-axle assembly. The rear-axle assembly mounts to the rear suspension and consists of the housing itself, with the differential assembly and both rear axles inside. Most full-size trucks and vans, as well as front-engine rear-wheel-drive cars, still use this same design (see Figure 1).

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Figure 1

The rear-axle assembly often is referred to as just the “rear axle,” “rear end” or “differential,” but most of the time someone is just looking for some of the parts. As technicians, we call it that too. It’s easier and a generally understood reference, but it’s nothing a few key questions can’t handle.

The power inputs the rear axle via the differential pinion gear. The pinion gear drives the ring gear, which is bolted to the differential carrier. The carrier houses the spider and side gears. There are two side gears, which are splined to the axles but rotate freely against the carrier (see Figure 2).

The spider gears are supported by a center pin that rotates with the carrier, but they also have the ability to spin around that same pin, so the spider gears can move in two different ways. Picture a vehicle traveling in a straight line at a steady speed. Under these conditions, the spider gears do not spin; they remain stationary on the pin, but they rotate with the differential carrier and, in turn, cause the side gears to rotate. In other words, in this example, the ring gear, carrier, spider gears and side gears are all rotating at the same speed and the axles are being driven, both at the same speed.

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Figure 2

When the vehicle goes around a corner and a different axle (wheel) speed is required from side to side, the spider gears will spin on their mounting pins in order to allow the differential action to occur. The spider gears are still rotating with the ring gear and carrier, transferring the direction of motion through the side gears to the axles, but their ability to spin on their mounts allows them to “walk” around the side gears at the same time, allowing one axle to rotate at a different speed. A differential requires only one spider gear for operation, but for added strength most have two, and some heavy-duty vehicles can have even more.

Open Differentials

In independent-suspension automotive platforms that include front-wheel drive and mid- or rear-engine rear-wheel drive, no separate rear-axle assembly is needed, and the differential is an internal part of the transmission. Combining the two is what gave us the term “transaxle.” In place of the axles, these vehicles utilize CV (constant-velocity) shafts or half-shafts (very short driveshafts).

Regardless of traditional or transaxle, the differential inside operates the same. A standard differential is considered an “open” design. While this allows the wheels to rotate at different speeds when needed, an operating characteristic of an open differential is that it only transfers power to the wheel that spins the easiest.

If one wheel is on ice and one is on dry pavement, for example, the wheel on ice will spin, and no power will be transferred to the wheel on dry pavement. The same affect is what causes a car under heavy acceleration to “burn rubber” with only one wheel. On a completely dry road surface and during normal acceleration, an open differential is fine, but add these other factors and traction becomes a problem.

From a performance standpoint, if you lose traction, you lose acceleration. The traction of two tires is always better than one, so if your street machine exhibits one-wheel-peel, be prepared for the ribbing to begin. In the era of the “traditional” automotive platform and muscle cars, an open differential wasn’t adequate for the amount of power you had. A solution was needed to allow these cars to get traction from both rear tires, and the “limited-slip” differential (LSD) was the answer.

Limited-Slip Differentials

LSDs are also known by the familiar GM trademark “positraction,” but it’s the same thing. A limited-slip differential has clutch packs located between the side gears and the differential carrier. Remember, in an open differential, the side gears are splined to the axles, but they spin freely against the carrier. The power in an open differential always flows from the rotating carrier through the spider gears into the side gears. But, if you were to lock the side gears to the carrier, the power would then flow from the carrier directly to the side gears and the axles, eliminating the spider gears but losing differential action.

This is where the clutches come into play. The clutch packs consist of friction discs and steel plates. The discs are splined to the side gears and the plates have tabs that lock them onto the carrier.

There is a spring between the side gears that keeps a slight tension on the clutch packs, keeping them partially engaged at all times. However, when differential action is required going around a corner, the clutches are able to slip just enough to allow for the action to occur.

Here comes the tricky part. Look at the shape of the spider and side gears in Figure 2 on page 53. By nature, when force is applied to these types of gears, they attempt to force themselves apart during rotation. When the differential carrier begins to spin, the spider gears rotate as well, and the natural action of the gears forcing themselves apart presses the side gears toward the carrier, fully engaging the clutch packs. Now, with the side gears effectively locked to the carrier, power can be transferred directly to the axles and wheels.

The more traction a wheel has, the more force it will take to move it, and the more force will be pressed against the clutch packs. So, if one wheel begins to slip, a greater force will be applied to the clutch pack on the opposite wheel with better traction. The process goes back and forth instantaneously, allowing a wheel to slip for only a limited amount of time before power is transferred to the other. The result is traction from both wheels, and a burn-out that leaves two tire marks. Pretty cool, huh?

Due to the additional cost, the majority of cars and trucks come standard with open differentials. LSDs generally have always been an option. That’s been changing with the advancement of all-wheel-drive technology, and those vehicles will come standard with a limited-slip type of differential.

There are many different types and designs of differentials that exhibit the same operating characteristics, including those designed for all-wheel-drive vehicles, so don’t be surprised to see a lot of different stuff.

Selling Parts for LSDs

When selling parts for an LSD, the two biggest factors are identification and fluid. Even as easily as you can identify a vehicle through its VIN, there are still a number of them – especially trucks – that can have different rear-axle assemblies that are not positively identified by the VIN. It often takes considerable research to figure out what you have, and as technicians we always spend the time to figure it out. But, just as often, we rely on counter professionals who may have more information than us.

The biggest problem is that rear-axle identification is often done by a small metal tag or a stamping into the axle housing. These are commonly rusted away, and sometimes disassembly is required before you can make a definitive identification. Over the years, I can honestly say this is the one area of auto repair that has posed the biggest challenge with getting the correct components. There have been times I’ve been certain of what I had, but I was flat-out wrong!

The fluid you sell is critical, and you always should use exactly what the manufacturer recommends (again making identification important.) Not only does the viscosity differ between different units, but the additives also have a direct and immediate affect on clutch operation. The wrong fluid can cause wear to the clutches or cause a chattering when turning.

Seals are important during repairs, and generally there is only the pinion seal, axle seals and sometimes a rear housing cover gasket, although these have mostly been replaced by silicone-type sealants. The biggest detail with seals is to sell the best quality you can. A tip that’s good to share with your customer – especially if they’re replacing seals – is to make sure the axle vent is operational.

Axle vents commonly get plugged with dirt, especially on trucks. The axle assembly will generate heat during use, causing the air inside to expand. If it’s not able to vent, the pressure can overcome the seals and force fluid out. Don’t get caught in that trap. That should cover the basics of LSD operation. The rest I’ll leave to the movie.

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