Cone Type LSD
A
limited slip differential (
LSD)
is a modified or derived type of
differential gear arrangement that
allows for some difference in angular
velocity of the output shafts, but imposes a
mechanical bound on the disparity. In an
automobile, such limited slip differentials are
sometimes used in place of a standard differential, where they
convey certain dynamic advantages, at the expense of greater
complexity.
Early history
In 1932,
Ferdinand Porsche
designed a Grand Prix racing car for the
Auto
Union company. The high power of the design caused one of the
rear wheels to experience excessive wheel spin at any speed up to .
In 1935, Porsche commissioned the engineering firm
ZF to design a limited slip
differential that would perform better . The ZF "sliding pins and
cams" became available, and one example was the Type B-70 for early
VWs.
Benefits
The main advantage of a limited slip differential is shown by
considering the case of a standard (or "open") differential where
one wheel has no contact with the ground at all. In such a case,
the contacting wheel will remain stationary, and the non-contacting
wheel will rotate freely—the
torque
transmitted will be equal at both wheels, but will not exceed the
threshold of torque needed to move the vehicle, and thus the
vehicle will remain stationary. In everyday use on typical roads,
such a situation is very unlikely, and so a normal differential
suffices. For more demanding use, such as driving in mud,
off-road, or for
high performance vehicles, such a
state of affairs is undesirable, and the LSD can be employed to
deal with it. By limiting the angular velocity difference between a
pair of driven wheels, useful torque can be transmitted as long as
there is some traction available on at least one of the
wheels.
Types
Two main types of LSD are commonly used on passenger cars; torque
sensitive (geared or clutch-based or cone-based as shown in figure
at top of page) and speed sensitive (viscous/pump and clutch pack).
The latter is gaining popularity especially in modern all-wheel
drive vehicles, and generally requires less maintenance than the
mechanical type.
Torque-sensitive
The use of the word mechanical implies that the limited slip
differential is engaged by interaction between two (or more)
mechanical parts. This category includes clutch and cone and
helical gear limited slip differentials. For road racing, many
prefer a limited slip differential, because it does not lock the
two output shafts to spin at the same rate, but rather biases
torque to the wheel with more grip by up to 80%.
Characteristics
Geared LSDs respond to driveshaft torque, so that the more
driveshaft input torque present, the harder the clutches or cones
or gears are pressed together, and thus the more closely the drive
wheels are coupled to each other. Some include spring-loading to
provide some small torque so that with no / little input torque
(trailing throttle/gearbox in neutral/main clutch depressed) the
drive wheels are minimally coupled. The amount of preload (hence
static coupling) on the clutches or cones are affected by the
general condition (wear) and by how tightly they are loaded.
Broadly speaking, there are three input torque states: load, no
load, and over run. During load conditions, as previously stated,
the coupling is proportional to the input torque. With no load, the
coupling is reduced to the static coupling. The behaviour on over
run (particularly sudden throttle release) determines whether the
LSD is 1 way, 1.5 way, or 2 way.
If there is no additional coupling on over run, the LSD is 1 way.
This is a safer LSD, as soon as the driver lifts the throttle, the
LSD unlocks and behaves somewhat like a conventional open
differential. This is also the best for FWD cars, as it allows the
car to turn in on throttle release, instead of ploughing
forward.
If the LSD increases coupling in the same way regardless of whether
the input torque is forwards or reverse, it is a 2 way
differential. Some drifters prefer this type as the LSD behaves the
same regardless of their erratic throttle input, and lets them keep
the wheels spinning all the way through a corner. An inexperienced
driver can easily spin the car when using a 2 way LSD if they lift
the throttle suddenly, expecting the car to settle like a
conventional open differential.
If the LSD behaves somewhere in between these two extremes, it is a
1.5 way differential, which is a compromise between sportiness and
safety.
Clutch Type LSD
The clutch type has a stack of thin clutch discs, half of which are
coupled to one of the drive shafts, the other half of which are
coupled to the spider gear carrier. The clutch stacks may be
present on both drive shafts, or on only one. If on only one, the
remaining drive shaft is linked to the clutched drive shaft through
the spider gears. If the clutched drive shaft cannot move relative
to the spider carrier, then the other drive shaft also cannot move,
thus they are locked.
The spider gears mount on the pinion cross shaft which rests in
angled cutouts forming cammed ramps. The cammed ramps are not
necessarily symmetrical. If the ramps are symmetrical, the LSD is 2
way. If they are saw toothed (i.e. one side of the ramp is
vertical), the LSD is 1 way. If both sides are sloped, but are
asymmetric, the LSD is 1.5 way.
As the input torque of the driveshaft tries to turn the
differential center, internal pressure rings (adjoining the clutch
stack) are forced sideways by the pinion cross shaft trying to
climb the ramp, which compresses the clutch stack. The more the
clutch stack is compressed, the more coupled the wheels are. The
mating of the vertical ramp (80°-85° in practice to avoid chipping)
surfaces in a 1 way LSD on over run produces no cam effect and no
corresponding clutch stack compression.
The break-in period of clutch LSDs can be very specific.
Manufacturers give detailed instructions on how to break the
differential in. If these are not followed, the LSD may be
permanently harmed, in that it may engage and disengage erratically
due to irregularities on and damage to the clutch surfaces.
Essentially, the LSD must be worked hard to remove manufacturing
imperfections, then drained of the metal-laden oil.
Servicing consists of changing the oil after hard sessions to
remove metal particles, and eventually replacement of the clutches
or the centre. In any case, the oil should be changed on a regular
basis depending upon use. The fluid should be changed after about
seventy thousand miles or one hundred thousand kilometers (as
opposed to the open differential, where the oil could be left
unchanged for several hundred thousand kilometres).
Geared LSD
Geared,
torque-sensitive mechanical limited slip
differentials utilize
helical gears or
worm gears to "sense" torque on one shaft.
The most famous versions are:
Geared LSDs wear the gears and their supports rather than the
clutches of the clutch type and the cones of the cone type, but
both output shafts have to be loaded to keep the proper torque
distribution characteristics. Once an output shaft becomes free
(e.g., one driven wheel lifts off the ground; or a summer
tire comes over ice while another is on dry
tarmac when the car goes uphill), no torque is
transmitted to the second shaft and the torque-sensitive
differential behaves like an open differential.
Geared LSDs are dependent on the torque and not on the speed
difference between the output shafts. Such differentials may not be
adequate on extremely slippery surfaces such as ice (or thin air,
when a drive wheel loses ground contact altogether).
Geared LSDs may be used:
Speed-sensitive
Viscous
The
viscous type is generally simpler because it relies on
hydrodynamic friction from fluids with high
viscosity.
Silicone-based
oils are often used. Here, a cylindrical chamber of fluid filled
with a stack of perforated discs rotates with the normal motion of
the output shafts. The inside surface of the chamber is coupled to
one of the driveshafts, and the outside coupled to the differential
carrier. Half of the discs are connected to the inner, the other
half to the outer, alternating inner/outer in the stack.
Differential motion forces the interleaved discs to move through
the fluid against each other. In some viscous couplings when speed
is maintained the fluid will accumulate heat due to friction. This
heat will cause the fluid to expand, and expand the coupler causing
the discs to be pulled together resulting in a non-viscous plate to
plate friction and a dramatic drop in speed difference. This is
known as the hump phenomenon and it allows the side of the coupler
to gently lock. In contrast to the mechanical type, the limiting
action is much softer and more proportional to the slip, and so is
easier to cope with for the average driver.
New Process Gear used a viscous coupling of the
Ferguson style in several of
their
transfer cases including those
used in the
AMC Eagle.
Viscous LSDs are less efficient than mechanical types, that is,
they "lose" some power. They do not stand up well to abuse. In
particular, any sustained load which overheats the silicone results
in sudden permanent loss of the differential effect. They do have
the virtue of failing gracefully, reverting to semi-open
differential behaviour. Typically a visco-differential that has
covered or more will be functioning largely as an open
differential; this is a known weakness of the original
Mazda MX-5 (a.k.a. Miata) sports car. The
silicone oil is factory sealed in a separate chamber from the gear
oil surrounding the rest of the differential. This is not
serviceable and when the differential's behaviour deteriorates, the
VLSD centre is replaced.
Gerotor pump
This works by hydraulically compressing a clutch pack. The gerotor
pump uses the housing to drive the outer side of the pump and one
axle shaft to drive the other. When there is differential wheel
rotation, the pump pressurizes its working fluid into the clutch
pack area. This provides a clamp load for frictional resistance to
transfer torque to the higher traction wheel. The pump based
systems have a lower and upper limits on applied pressure, and
internal damping to avoid
hysteresis.The
newest gerotor pump based system has computer regulated output for
more versatility and no oscillation.
Electronic
Electronic limited slip differential systems use speed sensors,
anti-lock brakes, and
microcomputers to electronically monitor slipping. If any of the
wheels on an axle is rotating unusually faster than the others, the
computer will assume that it is slipping and will briefly apply
brakes to it, slowing the spinning wheel down and causing the
opposite wheels to gain speed and keep traction.
One advantage of this system over mechanical is that the vehicle
steering and control is less affected. It also generates less
stress on the drive train compared to a mechanical locking device,
making it particularly suitable for the vehicles with
independent suspension. It can also
be tuned for specific applications on and off road and at different
speeds.
A disadvantage is that it is less predictable when going over an
obstacle, as the system needs time to react. Also, the wheel with
traction will only have half of the available torque applied to
it.
The Mitsubishi
Active Yaw Control
(AYC) electronically controlled rear differential uses a
conventional open differential with an added planetary gear set to
rotate two hollow shafts around the left hand drive shaft, one
running at +15% speed, one at -15%. These can be progressively
locked up to the left hand drive shaft via a hydraulic clutch pack
under CPU control, increasing or decreasing the torque on that
wheel in relation to the other. This allows a certain amount of
rear wheel "steering" to provide stability control and performs the
function of an LSD.
Other related final drives
Spool
A spool limits differential rotation to exactly zero. A spool
consists of a pinion & ring gear only, the center is solid, the
axle is one piece. A mini-spool is similar, replacing the usual
differential center with a solid piece, retaining the factory
axles. A spool is not a differential at all, but is used to achieve
a similar effect to an LSD on some street and race cars. This
arrangement is popular in the motorsport of
drifting where copious wheelspin is
advantageous. Those that use the car for "drifting purposes" often
generate their own "custom differential" by welding-solid a
standard open differential to produce a home made spool.
It is also preferred by drag racing enthusiasts for 2 main reasons.
First, you will always put the power to each wheel equally to
facilitate a straight launch which is much safer. High horsepower
vehicles with a spool generally are still aiming straight even with
wheel spin. However, turning is more difficult because of the lack
of the required speed differential (outside tire has further to go
and thusly has to spin faster to compensate). The second reason,
they are strong. Because of the solid center design and lack of
side gears, cross pin and axle gears, a spool can take much more
abuse. This is especially important for drag racers that are
shocking their drive trains with hard launches and sticky tires.
Lastly, the third member can be more reliable and lighter as a
whole than a comparable limited-slip drag racing third
member.
Locking differential (Detroit Locker/Lokka)
A
locker locks both wheels
under normal conditions. If a wheel is
externally
forced to rotate faster than the differential centre (i.e., the
outer wheel in a corner) the mechanism unlocks that wheel and
allows it to turn freely (but only so long as it rotates faster
than the centre). Thus in contrast to other LSDs,the locker has the
unusual characteristic of
only applying drive torque
through the inner wheel in corners when decelerating or under
neutral throttle.Drive shaft input torque causes the pinion cross
shaft to lock the centre more firmly, resisting the unlocking
action. As the two actions of the mechanism are contradictory, the
car will unpredictably alternate between one-wheel and two-wheel
drive under power in corners, giving difficult to control handling.
As the vehicle goes down a straight line it is locked positive, and
as the vehicle enters a curve it is still locked positive. As the
vehicle goes farther into the curve, the gear unlocks causing it to
jerk. It can be very noisy and is often used in off-road
4WD applications. The traditional American racing
differential is a Detroit Locker.
Selectable locker
Air Locker
Normally functioning as open differential, a selectable locker can
be locked by the driver. Compressed air, mechanical cable, electric
actuator or hydraulic fluid activates the locking mechanism.
Generally used by street cars that also drag race, the car drives
to the event open, and locks the differential on the strip.
Selectable locking differential is often used together with
electronic systems for off-road driving.
Factory names
In the 1950s and 1960s many manufacturers began to apply brand
names to their LSD units. While Packard pioneered the LSD under the
brand name "Twin Traction" in 1956, the most famous of these was
Chevrolet's "Positraction". Since then,
Positraction (often shortened to "positrac" or merely "posi") has
become a
genericized trademark
for LSDs.
Other factory names for LSDs include:
References
- The Motor Vehicle K.Newton W.Steeds T.K.Garrett Ninth Edition
pp549-550
- KAAZ Technical Help - retrieved 2009-02-09
External links