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.
