LSD: the basic

[vince3004 0]

Voici un article intéressant que j'avais trouvé y'a 1 ou 2 ans de ça:

In a standard differential, if one wheel loses traction, it will get all
the power and will spin, while the wheel with traction gets nothing. The
idea of a limited-slip differential is to prevent all power from being
applied to only one driving wheel when traction is lost. There are numerous
types of limited-slip, positraction, locker,
etc. units.

The percentage number denotes the percentage of torque applied to the
slower turning wheel from
the faster turning wheel. In a straight line, both drive wheels turn at the
same speed, so no limited slip action is occurring. In a turn, or when one
tire is spinning more than the other (such as on snow or ice), with a
limited slip differential, 25, 40, or 75 percent of the torque applied to
the faster wheel is applied to the slower wheel,
effectively 'limiting slip'. A higher lockup percentage will cause
increased rear tire wear on the inside tire during cornering -- the tire
itself will have to slip slightly to counteract the limited slip's desire
to have both tires turning at the same speed. It will also increase
oversteer in wet or slippery conditions, but it will also increase
understeer in tight corners under dry conditions. This is simply due to the
fact that with a limited slip, the drive wheels tend to want to turn at the
same speed, making the car tend to want to go in a straight line. When it
is slippery, however, both drive tires will tend to lose traction at the
same time, increasing oversteer. The
advantages are less inside wheelspin when accelerating out of a tight
corner. This also translates into more horsepower to the pavement and
faster autocross times -- provided that the suspension is tuned for the
limited slip. The ability to accelerate out of corners without excess wheel
spin can be a great advantage.

Om a more technical note:

The limited slip percentage (S) is also called the locking factor. It
describes the maximum applied torque
difference between rear wheels compared with total applied torque.
Passenger car LSDs are usually in the
25-40% locking factor range.

Limited Slip Locking Factor or Percentage S:
(note: drive torque is torque applied to road surface)

S = ( Drive Torque Difference Between Rear Wheels / Total Drive Torque of Both Rear Wheels ) x 100%

Think of a situation where the two rear wheels are on different surfaces
with different coefficients of friction:

H = Higher traction, more torque can be applied to road surface
L = Lower traction, less torque can be applied to road surface

S = (H - L / H + L) x 100%

By rearranging the equation a little, you see that for a 25% LSD, the High
torque side can be as much as
62.5% of the total while the Low torque side can be as little as 37.5% of
the total.

25% LSD Example:


H = (( S+1 ) / 2 ) = (( 0.25 + 1 ) / 2 ) = 0.625

L = (( -S + 1 ) / 2 ) = (( -0.25 + 1 ) / 2 ) = 0.375

The H/L ratio, called the bias ratio, is easier for me to think about
because it quickly shows how much more
torque can be sent to the high side. With a 25% limited slip, it is
possible to have 1.67 times as much torque
applied to the high side. A 40% LSD works out to a 2.33 bias ratio.

25% LSD Example:

H / L = (( S + 1 ) / ( -S + 1 )) = (( 0.25 + 1 ) / ( -0.25 + 1 )) = 1.67 (Bias Ratio)

A locked differential has a 100% locking factor (infinite bias ratio)
because all torque can be applied to one
wheel (e.g. one wheel on ice or in the air). For a limited slip, the
initial preload, or break-away torque, allows
power application when one drive wheel is on ice or in the air. Open
differentials are another story (see
snow/ice write-up below).

In theory, an open differential has 0% locking factor (1.00 bias ratio)
because the torque to each wheel is
balanced (H = L). In actual practice, there is some bias because the
differential is not friction free.

Differentials reduce tire wear and help a car turn more easily by allowing
the rear wheels to travel at different
speeds while turning corners. The inside wheel must slow down (smaller
radius turn) while the outside wheel
speeds up an equal amount (larger radius turn). To balance the drive torque
at each wheel, more torque is
applied to the outside wheel, speeding it up, while less torque is applied
to the inside wheel, allowing it to
slow down.

Open differentials always work well turning. They also apply power very
evenly when both rear wheels have
adequate traction. However, the big downside, is their torque balancing
action when one wheel has much
less traction, such as in ice and snow.

The torque applied to the wheel with the most traction can only equal the
lesser traction wheel. Total applied
torque for both wheels is only twice the traction of the worst wheel.

Increasing Locking Precentage

How do they increase the locking percentage on a diff? Do they just pack
more shims in there to make the clutch plates tighter? Or do they re
engineer the ramp angles? I have also read that Metric Mechanic adds clutch
plates. In this case is the carrier machined to accept more plates?

The shim alters pre-load, and too much pre-load provokes understeer. It'd
be better to get the locking to
almost go away under braking & corner entry, and then come back for
power-on exits. The pre-load should be just enough for a smooth transition
between the action of the coast & power ramps, and to keep things
together when unloaded. But the shim is what people can do. The shim in
question is not the 'thrust washer'
that the shop manual refers to, but the 'spacer ring' which is shaped like
a Belleville spring washer as big as
the discs themselves. These are made of stiff spring steel around 2 mm
thick. These, as well as the 'dog- eared
plates' are selected to shim up the static locking.

The torque path has the ramps driving the differential pinion's shaft, and
the ramp angle leverages the clamping force applied. Less angle on
the ramp offers a mechanical advantage for applying the clutches =
more lock.

Adding clutches increases surface area for clamping = more lock available.
You would have to machine the
housing to fit them, so it could weaken it.

Hope this is enough!

[vince3004 0]

Du Club4ag.com:


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 cars that maintain higher cornering speeds.
On every car a basic differential is fitted to to distribute the engine's power to the driving wheels. The basic differential gear (d1 below) supplies the power to wheels which is/are loaded with least resistance. This allows the cars wheels to run at different speeds in a turn with least resistance.
However, when a car is cornering close to it's limit, the car will exhibit roll, a leaning to one side, causing the inside tires 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 tires regain traction and start to translate the power to the ground.
An LSD is used to alleviate this wheelspin.


Basic Differential Gear system (d1)


The Limited Slip Differential Gear
An LSD is constructed similarly to the normal differential. (d2 below)


(A limited slip differential gear figure d2)

As you can see, the pressure plate rings have the side gear, pinion, and the pinion gear locked inside, and behind each pressure rings are a number of clutch plates. When torque is applied to the differential, the differential case will spin 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 to the wheels. On acceleration and deceleration, it provides even grip and on neutral power, it frees up for less drag and easier turning.


1-Way, 2-Way, 1.5-Way.......... What Does It Mean?
Some manufacturers make LSD's in different configurations and are commonly classified as 1 way, 2way, and 1.5 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 either acceleration or deceleration. The 1.5 way is a new term used to describe the 2 way cam which enables different lock up rates during the two directional forces. The 1.5 distribute positive lock stronger under acceleration than 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. It is also effective for front drive cars which need extra stability during braking.


A 2- way limited slip differential - d3


A Comparison of LSD
There are many brand and grades of LSD but here's how you rate them.
The OEM supplied LSD, standard and optional on many cars today are 2 pinions. This design has very low positive lock and is designed to provide some sporty feel to 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. Barely does doughnuts...!
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 is determined by a number of components. The cam profile, clutch plate quantity and size, initial torque of the pre-load spring, as well as the lubricants.
Contrary to what manufactures want you to believe, TRD-Toyota Technocraft, Mugen-Honda Racing, RalliArt-Mitsubishi Rally Operations, MazdaSpeed-Mazda, STI-Subaru Technica Int., NISMO- Nissan Motorsports, all have their LSD's made by the same supplier, a Hitachi subsidiary. This is because the majority of the normal differential gears are supplied by them. They use the cheap factory casing and re-work the inside. This limits the size and number of the gears, and clutches thereby sacrificing true potential. They claim 4 pinions but have only 3 or 4 small clutches on one side. They use very high cam profile to put high loads on the clutches so that it can achieve high lock up. The problem with this approach is that the pre-load on the clutches are higher and tires have constant drag. Also, since the clutches are small and few, the load is larger per given surface area, translating into higher operating temperatures and fast wear. Another major disadvantage to this type of LSD has to be the abrupt lock-up, making on-off conditions less predictable and controllable.



The true high-quality Limited Slip are supplied by Kaaz and Cusco, who supply majority of racing teams throughout Japan. The Cusco and Kaaz differentials have on the average 10mm oversize clutch diameter and computer stamped oil channels which hold lubricants during operation. The plates number from 6-8 and have very wide selection of pre-load springs to choose from. Since the clutch has to press together less to achieve very high lock-up, the LSD generates less heat and wears much longer. All components are computer ground and shell casing is not stamped as in the OEM and Branded Hitachi units. Over the long run, these units will cost less to operate due to the lower maintenance required in terms of rebuilding and replacement of clutches. Most of all, these LSDs are much more controllable, having a wider, more progressive lock-up and better positive lock up at the limit. They are more streetable because it won't suddenly spin you around in low traction conditions such as rain and snow. You can, but you can also be in control...

To add to this, Kaaz differentials have higher quality tolerance levels, represented by computer balanced shell casing and gears, and initial pre-load springs are matched to vehicle applications. The reason for small application list is that they go through 3 years of N1 and Gr. A racing to design the differential design and specifications. I prefer the KAAZ because of cheaper entry price tag too, made even better by introductory pricing in the US markets. Consider it the ultimate in LSD.

[MathXRS 0]

t' est vraiment en feu a soir pour nous donné de l info ! jvais finir bilingue

[vince3004 0]

Muhahaha , j'ai décider de mettre plein d'info pour rendre le forum plus instructif

[Dinolobe 0]

voici un vidéo qui explique très bien comment fonctionne un diférentiel de base. très imagé pour les personnes visuelles

[peugot406 1]

c kand meme bien le video ! i va rester le LSD ! XD