Differential Upgrade

(archive link: http://www.bimmer.org/5series/messages/archive/msgsy2000w46/98037.html)

Posted by JIMMY on November 17, 2000 at 15:37:23:

Limited Slip Differential (LSD) Lock %

From: Jim El Nabli <mailo:nabli@ibm.net>
To: E36M3 Mailing List
Reply-To: nabli@ibm.net

Based on a little research here's your answer:

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. Most BMW LSDs are 25%.

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

               Drive Torque Difference Between Rear Wheels
         S =    ------------------------------------------- x 100%
                  Total Drive Torque of Both Rear Wheels

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

                H - L
         S =   ------- x 100 %
                H + L

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:

                       S + 1      0.25 + 1
             H =    -------  =  --------  = 0.625
                           2                2

                   -S + 1       -0.25 + 1
             L =    -------  =  --------  = 0.375
                          2              2

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

 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. I have not measured any, but it looks as though BMW has the same
angle for both coast & power
ramps. BMW did use shallower ramp angles on euro high % LS. (Anyone want to
sell an original 75% factory

Due to the BMW's design, the ramp angles can't be modified. The ramps are
located on the 'pressure covers'
that are also used to retain the spider gears. The back side of the gears
fits into the rounded interior section
of the left & right pressure covers. This may be so the gears assist in
their spreading action.

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!

> Gruppe;
> I have been curious about this but have not seen information regarding
> this.
> Korman and others advertise differentials for our cars that have lockup
> of 40% instead of the usual 25%, which, I believe is what is stock on
> the 1995 and newer M3s.
> What does it mean to have a 40% or for that matter a 25% lockup diff?
> What are the advantages or disadvantages one would see on track,
> autocross course, or street of one versus the other?
> Though this question doesn't affect my car, does this change the way or
> the effectiveness of the ABS systems or the traction control systems at
> all?
> Thanks for any explanations,
> Marc
> 1995 M3 with stock LSD diff and 25% lockup.