How Differentials Work

Torque Biasing Differentials

Limited slips are reactive. They limit over-speeding of the inner wheel by coupling it to the outer, more heavily loaded wheel. In contrast, "torque biasing" differentials are proactive. They send increased torque to the tire that is more heavily loaded prior to wheel spin. Two makes are in use in race cars, the Gleason-Torsen and the Quaife. Gleason made a few diffs for race cars in the '80s but are no longer producing them except for some Japanese street car manufacturers. The Quaife is virtually identical in operation and is still being produced.

Torque biasing operation is based on the fact that worm gears transmit torque efficiently only in one direction. In practice, side gears are coupled to each axle and mate with several pair of overlapping planetary worm gears. These worm gears float in the housing and when power is applied, the gears are pushed outward, causing friction between their tips and the recesses inside the case, thus resisting rotation. This allows more torque to be biased toward the wheel with more traction.

We have two means of adjusting torque biasing differentials. The first is by the helix angle and pressure angle of the gears; the second is by varying the number of beveled washers exerting outward pressure between the end gears. However, most racers who use them have no extra gears, so the only practical adjustment is the preload or the end gears.

While they have the advantage of being proactive. torque biasing differentials are not without faults. First, they are quite heavy and have a lot of mass concentrated just inside the case, giving them quite a bit of rotational inertia. Second, a fair amount of power is turned into treat due to the friction of all the gears inside the case, and they never fully lock. Finally, they send more torque to the more heavily laden outside wheel which, opposite to the effect of the Detroit Locker, gives the car a power-on over-steering tendency.

Here's a break-down view of a Quaife.

Fully Locked Differentials

A differential which is fully locked cannot really be properly called a "differential," but it occupies the same physical space in the axle, so we will consider it to be a fourth type. The simplest fully locked unit is merely a steel machined part connecting each axle to the other and to the ring gear. This part is called a “spool." Spools are light, have little polar moment of inertia, no frictional losses and prevent inner rear wheel spin - so why are they not more popular? The answer lies in the fact that, while the torque biasing diffs and the limited slips help to reduce wheel spin out of the corners to varying degrees, we do still need some provision to allow wheel speed differentiation under braking (especially trail braking) entering a turn. A spool cannot do this. The limitations of a fully locked differential don’t bother many oval track racers, who overcome it with stagger, i.e., a difference in tire size between the inside and outside rears. They rely on this stagger to reduce or negate the wheel speed differential normally required. Thus, they are quite happy with Spools. However, as a note of caution to those who only occasionally, run ovals, the stagger must be perfect to do this, and even then, a great deal of tire drag invariably occurs down the straights.

When they cannot obtain another type or find that it is out of reach financially, many road racers use a slightly different form of a spool: the welded open diff. To do this, an open differential has the spider gears welded together to prevent them from turning. This effectively locks the unit up completely, although it is heavier than a true spool.

Exotic Differential Types

Many other types of differentials have hit the market in the last few years, and some are quite ingenious. One is a Salisbury type with an integral oil pump that pressurizes the clutch packs once a wheel has begun to over-speed. Another uses a computer-controlled DC motor to apply load to the clutches. Still another uses eccentrics instead of gears. To completely replace traditional differentials there are now some fluid couplings around as well. Time will tell if any of these shake out as an evolution in differential design, or are merely good ideas that did not work. For now, none of these exotic differentials are in widespread use, and few of us have much likelihood of getting to play with them anytime soon. So the question becomes, of those differentials we can utilize, which are best and how should we use them?

How the Differential Affects Handling

Open differentials are required in FVee, FFord, FF2000 and S2000. To reduce wheel spin, some competitors have tried shimming the side gears to within an-inch of their lives to require more breakaway torque to initiate spinning. Tech inspectors take a dim view of this, however, and one competitor lost a Runoffs, win a few years ago in a Fford by doing this. With an open differential, roll stiffness must be biased towards the front. It is not uncommon in these classes to find 80 percent of the roll stiffness at the front. This allows most of the lateral weight transfer to take place at the front and keeps the inside rear more firmly planted, reducing the possibility of wheel spin. This is also the reason that many competitors in these classes find the front tires go off before the rear. Droop limiters at the front also help to reduce inner rear wheel spin in open-diff cars. They are usually adjusted to allow only a fraction of an inch of droop travel before the inside front tire is lifted off the ground. When this occurs, the unsprung weight of that corner becomes sprung weight and being cantilevered so far out to one side, some of this weight is then carried by the inside rear tire, preventing the onslaught of wheel spin. Getting corner weights perfect is probably more important on open diff cars for a similar reason. If the one rear tire carries 30 pounds less weight than the outer, it will be much more likely to spin when it is on the inside.

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