Diff Ratio Good Link

[RT104GT]

This is a link to a large forum about BMW final drive ratios and the rationale which should resolve many of the issues rasied about what diff to use in what four cylinder and six, as BMW have a huge fleet of different cars.
And different engines.


http://member.rivernet.com.au/btaylo...ferential.html

SOme interesting discussions such as this reprint about how to test your ratio in the car easily..............................


Number of Teeth on Ring Gear
Rear End Ratio = -------------------------------
Number of Teeth on Pinion Gear

It is usually inconvenient to count the teeth. Luckily rotating
the wheel and counting driveshaft revolutions works just a well.

The ring gear is bolted to the differential housing. The rotation
of the differential housing is directly related to the rotation of
the rear wheels.

If the rear wheels are both rotating at the same rate, they match
the differential housing rotational speed. If one wheel is held
stationary, the other wheel rotates at twice the differential housing
speed. The moving wheel rotates at double speed because of its
own motion plus the extra forward motion from the differential
spider gear rotating along the stationary gear of the fixed wheel.


Left wheel + Right wheel Rotations
Diff Housing Rotations = ----------------------------------
2

So if one wheel is held still, it takes two rotations of the other
wheel to equal one rotation of the diff housing (and ring
gear).

Drive Shaft Rotations
Open Diff Rear End Ratio = 2 x ----------------------
Rear Wheel Rotations

From the example given (one wheel held still):
48 driveshaft turns for 25 wheel turns

48
Open Diff Rear End Ratio = 2 x ------- = 3.84
25

For limited slip differentials, both wheels move together when
they are off the ground because of the preload on the clutch
pack. This preload break-away torque should be about 45 ft-lbs.
(Of course, if you hold one wheel, the limited slip acts just like the open differential
- -- it's just much harder to turn the free wheel).