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
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.
