In any form of auto racing that involves turning corners, the quickest car will always be the one that can lay down more power sooner and stay on the throttle longer than the rest of the field. For the autocross/track day/road racing crowd, chassis science plays a huge part in getting traction into and out-of the corner, but so does that lowly device called the differential. In fact, it has a dramatic affect on all aspects of the art and physics of high-speed cornering.
It may seem a bit counter-intuitive that a device buried deep in the bowels of the rear-axle assembly could (or would) affect corner entry, but the truth is – it does. So it’s worthwhile to take a closer look at the myriad of differential choices that are offered to the aspiring track hero.
Road racing and autocross are all about applying power as soon as possible upon corner exit. A limited-slip differential is generally ideal for this, although lockers have also been used successfully in certain applications, or where the rules require them. For the purposes of this story, we’ll limit the discussion to front-engine, rear-wheel drive machines and keep the story focused on how the differential reacts in these vehicles. There are actually four main types of differentials (one really isn’t a true differential) and we’ll address all of them. In no particular order, the different types are: open differential, spool, locker, and a major category of limited-slip differentials with several design variations.
Before we jump into design differences, let’s first describe why a differential is required for most cars. When a vehicle enters a corner, the inside rear tire will transcribe an arc that is a shorter distance than the outside tire. If a solid connection is made between the two rear tires (like a spool), the inside tire must slip or spin in order to accommodate the greater distance the outside tire must travel. If you’ve ever had to push a race car equipped with a spool through a tight turn, you know how difficult that is. That resistance is the tire not slipping and the entire driveline binding because there is no slippage.
It might be worthwhile to mention here that all forms of differentials designed to improve traction are really band-aids of sorts. The real issue is that under acceleration, there are multiple forces at work that affect traction — even in a straight line. As torque is applied to the rear axle, the pinion attempts to climb the ring gear, which imparts a counter-clockwise motion to the rear axle housing (as viewed from the rear). This lifts the right rear (passenger side) tire while planting the left rear.
This may seem counterintuitive since the body motion on most vehicles is to lift the left front and plant (or squat) the right rear. While this is true, the axle housing is operating in the exact opposite motion. This means that a limited slip device is transferring some portion of the torque away from the right rear and applying it to the left rear – assuming the car is accelerating in a straight line.
With an open differential, the side gears allow the two tires to turn independently. This open design consists of a ring-gear carrier with two main side gears splined to the axle shafts connected with two smaller spider gears. This system allows the two axles to spin at different speeds. This is the simplest form of differential and while it does its job very well, it also incorporates an inherent flaw.
In a cornering situation, especially in autocross where the turns offer a very tight radius, applying power to an open differential instantly spins the tire with the least amount of traction. The open differential simply directs power toward the path of least resistance. This is almost always the inside tire, because body roll tends to unload that tire. If you’ve ever driven a car with an open differential on an autocross or road course, you know how frustrating it is to not be able to apply power in an aggressive fashion on corner exit. The unloaded tire just spins. For early road racers, because this inside tire lift was difficult to counteract, they solved that dilemma by simply employing a spool.
The spool is the simplest traction device possible — it is a solid connection between both rear axles, eliminating the differential completely. Its simplicity is especially helpful for drag racers since they don’t care about corners. But for those of us enamored with apex aiming, the spool presents a few unfortunate negatives. Many circle track cars use a spool, but also employ stagger through the use of a larger outside tire which compensates for the corner radius, so both tires travel the same distance — in the corner.
This obviously doesn’t work as well in a road race or autocross situation, since the car must deal with both left and right turns. This really makes the spool a non-player for turning in both directions, including the street. Enterprising car builders quickly created the next variation.
Before we get into lockers, it’s also important to mention that spools tend to dramatically affect vehicle handling. Because both rear tires are splined together, a spool-equipped car (with equal height rear tires) will induce dramatic understeer on corner entry. The front tires will turn into a corner, but the rear tires will keep driving in an outward vector. Conversely, on corner exit, a spool will tend to make the car oversteer quite heavily. The chassis can be tuned to compensate, but without radical changes, these are still band-aids to cover up a more serious issue.
The locker differential was designed to mitigate some of these issues. The locker differential can be viewed as a spool that can be “unlocked” in the corners, allowing the tires to turn at different speeds to accommodate the radius of the corner. The locker differential remains unlocked as the car transitions through the apex and then at some point locks up again as the car finishes the turn and power is applied.
This design has certain disadvantages, some of which can be compensated by driver technique. Adding as much power as the tires can hold on corner exit may not be possible since the differential at the early stages of corner exit is still unlocked – especially on tight radius corners in an autocross situation. Of course, once the differential is again locked, you have both tires offering near-equal ability to accept power to accelerate the car off the corner.
Locker differentials do have advantages, including their simplicity which also means they are less expensive and can last longer than limited-slip differentials. However, for a street car that is aimed at autocross with occasional forays on track day, there are better options.
Moving to limited slip differentials (LSD – the non-hallucinogenic kind), there is a whole story worth of choices. Overall, there are three main types of LSDs: a clutch plate version, a cone style, and the gear style (often referred to under various brand names such as Torsen-Gleason, Gold or Platinum Track, Eaton Truetrac, and others).
LSDs offer multiple advantages for performance street applications as well as autocross and track days. The limited slip is a precise name for what these units achieve, as they attempt to minimize tire spin on the unloaded tire and add that torque to the more heavily loaded tire. This is even beneficial for non-performance situations such as wet and snow-packed roads.
Let’s start with the cone-style limited slip. This device uses a pair of male cones that use differential speed to apply a load to the axle with the greater traction. The device is fairly simple and performs well. This style of LSD was OE for example on third-generation Camaros and Firebirds. Auburn is probably the best known brand name associated with the cone style LSD and offers units for most of the popular axle systems.
Auburn also offers an Electronically Controlled Traction Enhancing Differential (ECTED). This retains the Auburn cone-style device, but applies the torque via electronic control using a simple switch controlled by the driver. There’s also an ECTED Max that go directly to a locker style as well. The standard Auburn units can also be tuned with shims to be more aggressive.
The clutch-style limited slip is by far the most prolific and popular version, with decades of use across many different manufacturers. The clutch-style LSD uses a series of clutch discs that are splined to each of the two large side gears in the differential. Large steel plates in between these clutch discs are fixed to the differential case. In most cases, either a series of four small springs, or sometimes one large wave-type spring, is used to apply tension to the side gears to help load the clutches.
When traction changes from on one side, this spins the differential spider and side gears, which pushes outward on the side gears, loading the clutches even further. This style of limited slip transfers the torque between tires very smoothly, often with a minimal impact to the suspension under hard cornering.
The clutch-style limited slips are perhaps best known under the Eaton brand. There are several other companies offering similar designs. Much like the Auburn, this style can also be tuned or adjusted to increase or decrease the load on the clutches to suit both traction and driving styles.
It’s also possible to increase the load on the clutches to the extent that the car can develop a push or understeer condition. A too-tight setup will also tend to make the car oversteer on corner exit, much like a spool. However, torque bias can be altered with a tighter setup, which can potentially be a tuning advantage.
Clutch-style LSDs also require a special gear lube with a friction-modifier additive that prevents the clutches from chattering, which can be annoying in slow speed corners. The clutch discs are also subject to wear. Rebuild kits are available and are easy to service, but this does require removing the unit from the rearend housing, which is not generally a DIY driveway service procedure.
The third type of LSD is the gear style that substitutes lateral gears for the clutches. Quaif calls its version automatic torque biasing (ATB). The gear-style LSD is well represented by several different companies including the Eaton Truetrac, Torsen, Quaif, and the Platinum Track unit sold by Larson Racing Products. This style of limited slip has become extremely popular in the last decade for multiple reasons.
The main advantage of the gear-style LSD is that it requires no service, no special gear lube, and it generally offers a much longer useful life compared to the clutch or cone-style versions. Torque bias is established from the factory and cannot be changed unless returned to the manufacturer. Other advantages include a very smooth and stable application of torque movement to the tire with the most traction. This is virtually seamless to the driver, yet offers traction advantages that are especially helpful on corner exit if the chassis is properly adjusted.
If you’ve made it this far into this dissection of the various performance differentials, you can see there are plenty of options. It may be noticeable that this discussion has made it appear that the gear-style differential has superior characteristics, but this is hardly the last word in balancing traction coming off a corner. It would be helpful to get the opinion of racers with similar vehicles and setups as to which differential they choose to run. This will likely bring up topics we have not had the opportunity to include in this story.
If there is one aspect that should be apparent from this discussion, it’s that the choice (or even modification) of a rear differential in terms of torque bias will have an immediate effect on the way the car handles, both in corner entry and corner exit. It’s best to remember this during the selection process. Whichever system you choose, the suspension will likely need to be fine-tuned to take maximum advantage of its design. The driver will be the ultimate arbiter in that tuning discussion with how the differential affects both corner entry and corner exit.