BMW Differentials
Normally the cars are equipped with open differentials, just like cars with rear wheel drive, differential case (differential briefly) and final drive (gear pair) are placed in one housing.
1-pinion
2-crown
3-case
4-bolt
5-lock ring
6-stop disc
7-diaphragm spring
8-planetary gear
9-bevel gear
10-bevel mate shaft
11-lock ring
Front wheel drive cars have even 3 in 1. Differential, final drive and gearbox. Open differential is the cheapest and easiest to maintain, so it’s preferred for OEM. For simplicity, total housing differential with main transmission will call “differential” or we’ll be call it for short “Diff”. As we know at BMW rear wheels are driven. Here’s a typical BMW E36 Diff:
BMW e36 diff
BMW Limited slip Differentials
BMW chose to use LSD friction type(clutch type) produced by ZF. All BMW models until E36, E34, E32 could be produced with the option “sperrdifferential”. Over the years, they were installed and other types as Visco LSD, locking type (EH Differential), Torsen… but this was rare. Here I will explain what is the difference between open differential and LSD. Differential character is determined by planetary gear. This mechanism is designed to differential case. Here is an example of open case type and limited slip case.
open vs limited slip
The main problem of the open diff is the loss of traction. When one drive wheel is in the air or on ice or road surface with different grip, the car loses it mobility and power. Rotates only the wheel which has no traction. Americans call this effect (or defect) “one wheel peel”. Anyone who has driven a car has experienced this defect, even if he did not know that’s is happening. I personally find it very unpleasant. But I’m not impartial. Limited slip differential (LSD) can transmit a certain torque to the wheel that got traction. This can be done by using friction discs (#9 in the picture), installed in LSD case:
1-limited slip case
2-cover
3-cover bolt
4-stop disc
5-stop disc
6-little diaphragm spring
7-diaphragm spring
8-dog ear disc
9-friction disc
10-shells
11-planetary gear
12-bevel(spider) gear
13-Bevel mate shaft
The is a lot of practical advantages when we talk about LSD. At first, the limited slip differential is very useful in poor road conditions such as snow, mud… LSD makes possible different road performances, donuts and having a lot of fun . The fun is guaranteed if you decide to take a turn sideways. As a man who enjoys sports driving and track racing LSD is necessary, and when it comes to motor sports is MUCH needed. This applies not only to drift, but for all track and rally disciplines. In drag racing, for example, the presence of LSD makes the start much better. The things can be almost perfect…
It is possible to make LSD with an higher percentage lock and a different gear ratio, according to the field of application. The most amazing thing about LSD is the fact that it works completely automatically. Don’t need any human intervention, pressing buttons, etc. As someone says: self-blocking differential. However it requires a little more attention to service. If in the open diff not fundamentally change the oil (Long life), the limited slip differential requires a special oil, which must be changed frequently, depending on how you use the car and what is the main purpose.
There are three sizes BMW differentials, classified according to the external diameter of the crown, measured in millimeters:
typ168 (little pumpkin). Used for 4-cylinder engines for models E30 and E36 including 320i.
Typ188 (medium pumpkin or medium case). The most commonly used type in the six-cylinder engines, and small V8 (530i). As an exception E30 M3
Typ210 (very large body). M3, M5, M6, V8, V12. In general, all motors larger than 3500 cm3