Differential centre

BMW Differentials Issues in General

In our 30+ years of experience with BMWs we’ve seen certain differentials that are highly problematic, and then others that we don’t get call on (because they’re reliable.) But with the exception of a few, almost all BMW differentials will eventually fail with stress and poor maintenance.

The main killer of differentials are leaking seals and low oil. However, there are other things that can lead to differential wear such as:

• Unevenly worn tyres (particularly on xDrive cars.)
• Fitting larger wheels.
• Towing large loads with non-towing diff.
• Aggressive driving.
• Using non-genuine differential oil.

F25/F26, F15/F16 Front Diffs

The most frequent requests we get are for front differentials from X3/X4 and X5/X6. They are a common issue and can cost $4000-5000 new from BMW. We supply these diffs low-KM, second-hand at a more affordable price.

E90 320i 3.91 Ratio

The E90 320i 3.91 ratio differentials have been a big problem. They were terribly built and all end up making a grinding noise. We stock them when the noise isn’t too bad, but if you’re looking to completely fix the noise then we recommend getting your diff rebuilt or finding a new/aftermarket solution.

Overview

BMW uses multiple drivetrain systems in their vehicles, most of which utilise differentials. Broadly speaking, the role of differentials are to:

1. Distribute rotational power (coming from the engine) to individual wheels.
2. Enable different wheel speeds on each side, allowing the car to turn smoothly.
3. Control the rotational ratio between the input shaft and the output shaft.

RWD System

The RWD System (rear-wheel-drive) features a longitudinal engine orientation and uses one differential (differential centre) in the rear axle, which sits between the tail shaft and the output shafts.

RWD-Based xDrive System

The RWD-Based xDrive System (originating from the RWD platform) also features a longitudinal engine orientation, but uses two differentials: front and rear. In this system the transmission feeds into a separate transfer box, which then distributes power to the rear differential through a tail shaft, and the front differential through a shorter prop shaft. The front differential, which is typically bolted to the engine’s sump, is smaller in comparison to the rear diff as the system has a rear-wheel bias.

The FWD-Based xDrive System

The FWD-Based xDrive System features a transverse (East-West) engine orientation, that uses only one stand-alone rear differential—with the front differential internally integrated into the transmission. In this system the engine, transmission and transfer case are together under the hood. The transmission provides power directly to the LHF wheel’s output shaft—and indirectly to the RHF wheel’s output shaft, via an intermediate shaft. The transfer case, which is attached to engine (on the opposite side to the transmission) sits around the intermediate shaft and hooks into it using notches, and uses it’s gears to transfer rotational force from the front axis to the rear differential.

Understanding Ratios

The differential ratio is defined by the number of times the driveshaft (or input shaft) rotates for each rotation of the wheels (or output shafts). This ratio is a comparison between the teeth count on the ring gear and the pinion gear within the differential. This ratio plays a crucial role in defining the performance, fuel economy, and driving characteristics. Smaller ratios are typically more aggressive in character, but less fuel efficient. Whereas higher ratios can sacrifice performance for fuel efficiency.

Within each model generation there can be many variations of each differential. These variances can include different ratios, but there can also be non-exchangeable variants sharing the same ratio. The variance can be to account for different engine sizes, wheel sizes and towing capacity.

In addition to that, some models also offer LSD and QMV (electronically controlled) options.

The best way to determine fitment compatibility of a differential is to match the BMW part numbers from the catalogue. This can be done by decoding your vehicle’s VIN in a BMW EPC (electronic part catalogue.)

If you don’t have access to an EPC, provide our team with your vehicle’s 17-digit VIN—and we’ll check our stock for a compatible match.

There is of course high-interchangeability of differentials between vehicles sharing a common drivetrain system. This is particularly true between 1-4 Series F-Generation vehicles, and of course between X3-X4 and X5-X6. There is also some interchangeability between E and F-Generation vehicles as they share almost identical drivetrains components in many cases.