How Differentials Work

(adapted from an article that I don't remember where I got it from!)


FINE PRINT: The information presented here is merely meant as an opinion. Anything that you might do, or any actions that might result from using this information is your own blooming fault. There is no way that this information is guaranteed to be 100% correct anyhow.




Nothing Is for Free

Automatic "differential speed regulation" between the axles comes at a price. A reduction of traction at one wheel will be felt by the open diff as a demand for increased speed at the wheel with the lesser traction, and reduced speed at the wheel with greater traction. A traction imbalance can be created by either a slippery patch of pavement, a lot of lateral load transfer, or the application of sufficient engine power to cause wheel spin - or any combination of the three. Regardless of the cause, when such an imbalance occurs, the differential applies all the torque fed through it to the spinning wheel - and little or no torque to the tire with better traction. This can cause a number of handling problems. 

Cars with open differentials tend to exhibit noticeable inner wheel spin at the exit of slow to medium-speed corners. Since all the engine's torque is being applied to the spinning wheel, the car is coasting out of the turn until sufficient load is returned on the inside tire to regain traction. This particular problem has caused an ingenious type of rear suspension on FVees to become prolific. Called the "zero roll" rear suspension, it does not mean that the rear of the car does not roll, but rather, that no roll stiffness is provided by the rear. All of the roll resistance is provided by the front suspension, which keeps the inner rear always loaded by at least the amount of unsprung weight at that corner. While this unsprung weight may only 50 pounds or so, that may be enough to prevent open diff wheel spin on cars with limited power.

In addition to this lack of acceleration out of a turn, the spinning inside rear keeps traction low long enough for the rear of the car to come out. Much like lifting in a turn, this promotes an over-steering condition. This condition can also result at the outside rear when a FFord, FF2000, Sports 2000 or other car using an open diff puts two wheels in the dirt at the exit of a turn. The reduction in traction at the outside rear creates over-steer when the driver is already out of road. Great sensitivity is required to ease a car back onto the track when this happens.

With these disadvantages, it might seem strange that some Indy car engineers use open diffs on long ovals. Larry Curry once remarked that an open diff "frees up the car" which in engineering terms probably means they can miss the stagger a bit and get away with it. (More on stagger later.) Open diffs are only used on the long ovals like Indianapolis and Michigan, where wheel spin is not likely to occur.

Limited Slip Differentials

In an effort to solve the problem of the open differential gear train designers have come up with a number of solutions over the decades. One of these uses several clutch plates coupling the axles to the bevel gears. It was made famous by General Motors during the muscle car era as the "Posi-Trac,' but the proper name for this type is the 'Salisbury Differential.' In the early Salisbury units like the Posi-Trac, the amount of wheel spin allowed is controlled by the preload on the clutches. The theory was great, but in practice the preload was difficult to get right and changed as the plates wore. Many refinements to the Salisbury differential have been added by other manufacturers over the years, including one by Xtrac, the Hewland Powerflow and a new diff called the Variloc introduced by Taylor Race Engineering. Although there are differences among them, all use ramps splined to the inside of the housing which load the clutch discs through the end gears when power is applied. The loading of the discs is therefore controlled by the ramp angle employed.

Since we want differential action entering a turn and need to reduce wheel spin when the throttle is applied, the ramp angles are different between the coast and power sides. The coast ramps are usually 80, 83 or 90 degrees to load the clutch plates little, if at all, allowing speed differentiation under braking. The drive ramps can be anywhere between 30 and 60 degrees depending on how much lock-up is needed out of the corners. Additional adjustments can be made by the type and stacking order of clutch plates used and by preload on the plates. These units can be set up anywhere between an open differential to something approaching fully locked, both entering and exiting turns. Although they are very effective when properly set up, some power is lost as heat when the plates are slipping.

In the '60s. Hewland began shipping many of their larger gearboxes with a differential they called the “Cam and Pawl" It uses inner and outer concentric fluted cam with pawls that wedge between them when power is applied. These fully lock the differential and they are non-adjustable. Since all of the parts that limit slip are made of hardened steel, the wear rate is relatively high and it takes constant maintenance to keep them working correctly. When slightly worn, they lock both wheels together, but then slip a tooth or two, causing a series of jerks. When severely worn, they act like open diffs. Many vintage cars that use Hewland gearboxes have extensively worn Cam and Pawl differentials. In other words, open diffs.

Another type of limited slip differential that Ford popularized is the "Detroit Locker." It uses a ratcheting mechanism that fully locks the differential when power is applied and unlocks it when the throttle is lifted. Although that action may be a bit abrupt, the real disadvantage appears when part throttle is used. In these situations, it cannot decide whether to lock or unlock and jerks until full power is applied. This can greatly affect traction in the mid-part of a turn. The Detroit Locker supplies torque to the wheel that is rotating the most slowly. In normal cornering, this is the inside wheel. Consequently, that inner tire drives the front of the car toward the outside of the turn, causing comer exit under-steer. Compared to other types of limited slips, the Detroit Locker has few recommendations except by a few diehard road racers who continue to use them- and by NASCAR.

Limited slips require a reference wheel speed. If the inner rear is lifted off the track due to rear droop limiters, shocks too short, etc., no speed limiting will occur until it touches down again with a chirp.

  Next Page