BMW E90 Rear Knuckle and Hub Differences: What You Need to Know

In BMW E90 models, rear hubs and knuckles are matched sets. The difference between 28-spline and 30-spline hubs is more than just the spline count—each has a specific knuckle to match its unique wheel bearing size.

28-Spline Hubs: Found in 325i and 328i models, 28-spline hubs use a knuckle designed for smaller wheel bearings. These knuckles are incompatible with 30-spline hubs due to the different bearing sizes.
30-Spline Hubs: Found in 335i, wagon, and xDrive models, 30-spline hubs pair with a knuckle that accommodates larger wheel bearings, supporting higher power levels and torque.

Because each spline setup has a unique wheel bearing diameter, upgrading from a 28-spline to a 30-spline hub requires replacing the entire knuckle assembly.

Rear Knuckle and Hub Spline Counts

325i and 328i (25/28i): These models come with 28-spline rear hubs and knuckles.
335i, Wagon, and xDrive Models: These models feature 30-spline rear hubs and knuckles, supporting higher torque and commonly used for LS swaps due to compatibility with upgraded axles and differential setups.

Wheel Bearing Specifications

30-Spline Hubs: Both the outer and inner diameters of the wheel bearing are larger on 30-spline hubs. The knuckle is designed to fit these larger bearings and accommodates the larger outer diameter of the parking brake.

Parking Brake and Knuckle Differences

Brake Sizes:

Rotors:
- 325i/328i: Uses a 300mm outer diameter (OD) brake rotor.
- 335i: Uses a 336mm OD brake rotor for increased braking power.
Parking Brakes:
- 325i/328i: Equipped with a 160mm OD parking brake shoe.
- 335i, Wagon, and xDrive Models: Use a 185mm OD parking brake shoe.

Knuckle Variations: The knuckle design varies based on the application:

The brake caliper mounting location differs to account for variations in hub spacing.
There are three versions of hub spacing, with each spacing difference compensated for by the knuckle design itself, allowing calipers and rotors to remain consistent.

Hub Width Differences in BMW E9x/e8x

The BMW E90 platform includes variations in hub width and track width across different models and production periods, including changes in the LCI (Life Cycle Impulse) update and differences in the 1 Series platform.

E90 vs. LCI E90 Track Width:

The E90 (pre-LCI) models have a slightly narrower track width compared to the LCI (post-2009 facelift) E90 models. The LCI update introduced wider hubs, increasing the rear track width to improve stability and handling. This change in hub width affects compatibility when swapping components between pre-LCI and LCI models, as the knuckle and hub assembly on LCI models is slightly wider.
The track width adjustment affects alignment and compatibility, especially in custom setups like LS swaps, where precise fitment and suspension geometry are crucial.

BMW 1 Series Hub and Track Width Changes:

The BMW 1 Series (E82/E88) actually has a 10mm wider track width per side than the pre-LCI E90, making it slightly wider overall. This additional width provides a broader stance, which may improve stability but requires careful consideration if swapping between 1 Series and E90 components.

Axle and Differential Compatibility Between BMW 328i and 335i Models

The BMW 328i and 335i rear axles are specific to their respective hubs, but the differentials can be interchanged between these models with some adjustments. Here’s a detailed look at axle specifications, spline count, and differential swap considerations.

Inner Cup Bolt Pattern: Both the 328i and 335i axles share the same inner cup bolt pattern for connection to the differential. While the bolt pattern remains the same, the spline count on the differential cups differs between models to match the spline count of each axle for proper engagement.

Spline Count and Hub Compatibility:

328i Axles: Use a 28-spline configuration that matches the 28-spline hub in the 328i. The spline count is specific to the hub rather than the differential itself.
335i Axles: Use a 30-spline configuration that pairs with the 30-spline hub in the 335i.

Differential Compatibility and Swapping:

Swapping Differentials: A 335i differential can be swapped into a 328i, or vice versa, as long as the differential and differential cups are swapped together. This ensures that the differential setup matches the spline requirements of the axles.
Axles and Hubs: The axles must stay with the chassis, as they are matched to the hub's spline configuration. This means that even when swapping differentials, the axles do not need to be changed, as they will continue to align with the hubs specific to each chassis.

Axle Lengths:

The axle lengths for both driver and passenger sides are the same across the 328i and 335i models. Although spline counts and differentials may vary, axle lengths remain consistent.

8.8 Rear End and 15-Inch Wheel Compatibility

8.8 Rear End: The 30-spline knuckle and hub are required for compatibility with most aftermarket rear axles, such as those offered by G-Force and DSS.
15-Inch Wheels with 30-Spline Hubs:
- To run 15-inch wheels with 30-spline hubs, use 25/28i brake calipers and rotors that fit 15-inch wheels but still provide the larger inner diameter parking brake required for the 30-spline hub.
- BMW X1 rotors are commonly used in this setup, featuring a 185mm parking brake with a 300mm OD that works with 25/28i calipers, enabling compatibility with 15-inch wheels.