Introduction to LSD
The Normal Differential
As we well know, a race car driver wins races from
shaving lap times by cornering faster. Because of this
many teams and constructors try to make their cars that
can maintain higher cornering speeds.
On every car a basic differential is fitted to distribute
the engine's power to the driving wheels. The basic
differential gear (Figure 1) supplies the power to the
wheels which is/are loaded with least resistance. This
allows the car's wheels to run at different speeds in
a turn with least resistance.
Figure 1.
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However, when a car is cornering close to it's limit,
the car will exhibit roll, a leaning to one side, causing
the inside tire(s) to lose forward traction and lateral
grip. The wheels lift and cause excessive spin because
of lack of downward force or weight distribution. This
wheel spin is useless for acceleration until the tire(s)
regain traction and start to translate the power to
the ground. Limited Slip Differential (LSD) is needed to alleviate this wheelspin.
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Limited Slip Differential
An LSD is constructed similarly to the normal differential
(Figure 2).
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As you can see, the pressure plate rings have the side
gear, the pinion and the pinion gears locked inside
and behind each pressure rings are a number of clutch
plates. When torque is applied to the differential,
the differential case will rotate and throw the pinion
into the pressure ring cam. The pressure ring is then
pushed out against the clutch plates thereby squeezing
them together. This in turn causes the wheels to gradually
lock together, depending on the power applied. This
effect limits the wheel spin during hard cornering and
applies power to the wheels evenly when more power is
applied. On acceleration and deceleration, it provides
even grip and on neutral power, it frees up for less
drag and easier turning.
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Figure 2.
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Different Type of LSD Configurations (1 Way, 1.5 Way & 2 Way)
Some manufacturers make LSD's in different configurations
and are commonly classified as 1 way, 1.5 way and 2
way. This designation reflects the design of the cam
groove which enables the LSD to function differently
under different loads. A 1 way differential means that
the cam is shaped in such way as to have positive lock
only when accelerating. The 2 way is constructed in
a way to have positive lock motion in both acceleration
and deceleration mode (Figure 3). The 1.5 way functions
almost same as a 2 way but provides less lock when decelerating.
The 1.5 way can provide more forgiving balance when
braking than a full 2 way setup, although it is less
effective for true racing applications, it provides
easier operation for beginners in throttle off conditions.
1.5 way is also very effective for front wheel drive
cars which need extra stability during braking.
Figure 3.
A Comparison of LSD
There are many brands and grades of LSD in today's
market.
The OEM supplied LSD, standard and optional on many
cars today are 2 pinions. This design has very low positive
lock and it is designed to provide some sporty feel
to the showroom cars. Because of the nature of this
low lock design, it is just slightly more effective
than not having one at all. Not a choice for true performance
drivers.
A performance LSD should have at least 4 pinions. This
is the design used for racing and rallies around the
world. The positive lock ratio and linear lock characteristics
are determined by a number of components. The cam profile,
clutch plate quantity and size, initial torque of the
preload springs and the lubricants.
A viscous type, torsen type and helical type are some
of other designs used as OEM equipment or optional LSD
unit. These designs are commonly used since they are
less aggressive and easier to maintain than the clutch
type racing LSD unit. In order to achieve the maximum
traction control, most OEM performance divisions and
aftermarket manufactures use the clutch type design
for their LSD unit.
