Most 4x4 owners will tell you that you need a locked diff for proper off-roading. Is that true, though? We’re going to explain the differences between a locking differential, a limited slip differential, a spool, and the variants in between all of them. First, we must start with the basics.

While it’s true that you want to be able to put as much power down as your tires can handle, that’s not always true all the time. There are times that your wheels must turn at different rates and its usually while cornering. So, how can we achieve both? Full torque to the wheels while in a straight line but some difference while turning? We do that with the differential.

What is a Differential?

While you’ll see an entire axle or the full ring-and-pinon package referred to by most laymen as a “differential,” what’s really doing the action is what’s attached to the ring gear. That’s the differential. Its job is to take the torque that’s being applied by the ring gear from the pinion gear and apply it to both tires.

The standard differential for most daily drivers off the showroom floor is the “open” differential. It’s not open like as if nothing is there. Rather, it’s described that way because there isn’t always a locking device to apply torque to both wheels. Instead, a set of gears attaches to the carrier – known as a pinion gears – and the axles – known as side gears.

When traction is equal between the two tires, the entire carrier rotates and turns the tires. The gears are not locked together but since there is no speed or traction difference between the two axles, the pinion gears force the side gears to rotate with the carrier.

When you go through a turn, the pinion gears can rotate in opposite directions. This allows the carrier to apply torque to the axle that is moving faster while rotating around the side gear that is slower. It’s also this action that also allows it to spin the tire that has the least traction and rotate one wheel that is in the air while the other is on the ground.

This video made in the 1930s is a great way to explain it visually.

What is a Limited Slip Differential?

For trucks, SUVs, and Jeeps, the next step up is the limited slip differential – also known as the LSD. The first ever LSD is credited to ZF when they designed a solution for Ferdinand Porsche when he was working with Auto Union’s Grand Prix team in 1935.

Their “sliding pins and cams” system was created to fix this problem and was even used in the Type B-70, the VW Kubelwagen and Schwimmwagen. It didn’t really limit slip except when two wheels were moving in the same direction, but while cornering it would only send full power to the slower-turning of wheel of the axle. Looking at it, it mentally rings of images of the “automatic locker” – which we’ll describe in the next article – in how it initially operates with the disadvantages of the limited slip that we’ll describe in this article.

Limiting Slip In Different Ways

There are multiple types of LSDs: clutch, Torsen, electronic, and electronic-brake. You’ll hear them all called by different names from different manufacturers, but nearly all LSDs are the same within their design types. For example, a Ford Traction-Lok, a Chevrolet Positraction, and a Dodge Sure Grip are all clutch-type LSDs. Different names, same thing.

The purpose of the limited slip is to allow the wheels to turn at different speeds, just as you would with an open differential. However, when the vehicle begins to accelerate it works to make the wheels turn at the same speed. This allows you to use the vehicle’s power to accelerate out of a corner or even work to keep power to the axle with the best traction. How each LSD does this is different.

Clutch-Type LSD

The way most clutch-types work is entirely mechanical but also like how a multi-plate clutch works in a high-performance car. The carrier is split into two pieces rather than the single piece you see with an open differential. Inside, you’ll see a set of clutches and gears. The gears look like the ones in the open differential but with two extra pinion gears and they work in the exact same way. Well, up to a point.

To each side of the side gears are plates, frictions, and pressure rings. The pinions extend out and contact the pressure rings. The openings of the pressure rings that the pinions pass through are shaped to move the pressure rings against the clutches and “lock up” the axle.

The shape determines if it’s a one-way (where the action only happens in one direction), 1.5-way (where the action happens mostly in a single direction but can also half-lock in the opposite direction), and two-way (where the action happens in both directions). The “direction” is usually acceleration, but it can be determined by who builds the LSD and for what reason. Though, we have never seen a one-way LSD that only worked on deceleration only.

The “lock up” happens as the clutches and plates squeeze against each other, which each clutch and plate set pressed against the other increases the amount of torque it takes to cause them to turn. So, much like a multi-plate clutch on a high-performance car, you increase the clamping force as you add more clutches and plates to the mix. The plates are also driven by the carriers, which is why you see teeth on them.

Torsen and Gear-type LSDs

Torsen is the name of a limited slip system designed by Vernon Gleasman and manufactured by the Gleason Corporation – now JTEKT Torsen North America, Inc. It’s an acronym of TORque-SENsing. If you look at a Torsen LSD, it looks like a voodoo operation of gears. It’s quite a bit simpler than it looks and is based on the principal of worm gears. Most importantly, how a worm gear can’t drive the worm wheel in large reduction ratios.

The way it locks up is when there is an imbalance of torque between the axles. The amount of torque difference it takes to lock up is determined by the Torque Bias Ratio (TBR) and different Torsens can have different ratios. For example, a 3:1 TBR Torsen can transmit torque of up to 75-percent on one side and 25-percent on the other side of the axle.

Once that axle with the higher torque load begins to slip, it will try to speed up. When it does this, the tractive torque of the slipping wheel is transmitted to the other wheel that isn’t slipping. That tractive torque is multiplied by the TBR and begins to try and speed the slower wheel up.

Since a worm gear can’t drive a worm wheel, the remaining torque from the slower wheel contributes to the angular acceleration of the faster wheel but without any added torque, thus never really speeding it up until it can manage more torque. This means that the slower moving wheel will always receive the most torque from the ring and pinion. This also means you can accelerate out with a greater amount of power out of a corner versus a clutch LSD.

Problem is, once a wheel is in the air, the Torsen acts like an open differential since no torque can be transmitted back to the wheel on the ground. There are two ways around this, though. First is to use a parking brake to add drag into the axles. This “tricks” the Torsen into multiplying the drag torque into its TBR. Once this happens, the grounded wheel will take this torque multiplication (subtracted by drag torque) and allow the grounded wheel to move the vehicle.

The HMMWV/Hummer – which had a Torsen diff in the front and rear axles – used a similar principal but used the main brakes automatically when it knew it had a wheel in the air.

The other way is used in the T-2R RaceMaster Torsen and by using a preloaded clutch built inside it. This did the exact same thing as the brake torque multiplier, but didn’t require you to use the main or parking brakes to make it work.

The other issue is that a Torsen is direction specific and must be built for the rotation it’s intended for. A two-way clutch-type LSD could be used in either a front or rear differential, but a Torsen must be made for a front or rear differential.

Detroit/Eaton Truetrac

If you’re thinking this sounds exactly like the Detroit or Eaton Truetrac, you’re not wrong. They are both gear-type LSDs and work under the same principles. It also means it has the same weaknesses and work arounds. The difference comes down to how they are designed and how the worm gears are turned for differential operation.

Electronic LSD

This is exactly like the clutch-type LSD, however, instead of using the pinion shafts and load rings to create a clamping force, an electric device such as a stator or a fluid pump forces the frictions and plates together.

The more advanced version of this is the Torque-Vectoring Differential. The differential is a conventional open unit with two side gears and two pinion gears. Outside of the housing sits an electric motor that actuates the clutch packs and a planetary gearset. The gear set multiplies the torque sent to the wheel the clutch pack is activated on. This is effective enough that the tires can act like they are being turned in opposite directions and why it takes the name “Torque-Vectoring.”

Electronic-Brake LSD

There are many who would accuse this system of not being a “true LSD.” That’s because it doesn’t use any of the methods mentioned above but uses something that already comes on modern vehicles: the brakes and the ABS. No gears, no clutches, no other electronics and it’s a standard, open differential with only four gears.

This is an inexpensive way for OEMs to apply a limited slip system on their trucks and SUVs and is based on the traction control systems they already tie into with the ABS. However, instead of trying to keep a vehicle going in the direction the driver intended, a Brake LSD is used only to limit slippage of the driven wheels. When you think about it, it’s no different in operation. It slows down the wheel that’s moving too fast because it’s slipping to transfer torque to the other wheel that isn’t. It just uses the brakes instead.

The problem is that the vehicle’s ECU must detect that slippage and that means it’s not going to act as fast as anything that uses a clutch pack or Torsen TBR. It’s also limited to the brakes holding torque, which can change as the pads and rotors heat and cool. In the rear, that won’t be as drastic of a change. The front, however, can be as most of your brake power goes to the front wheels and they are used the most.

Next Article

With those advantages and disadvantages in mind for all those limited slip options and how that open differential works, you’re probably already looking for a locker differential. Well, not so fast as there are many, many options for lockers and even a few for spools. So, in the next article, we’ll discuss all those as well as looking into the “Lincoln Locker.” Those who know, already know what we’ll be referencing there.