From an engineer’s point of view, wheel bearing design and selection can be a balancing act between durability, cost and fuel economy. Large bearings may be good to compensate for holes and heavy loads. But, there’s only so much room in the hub and knuckle. In addition, the larger bearings probably increase the rolling resistance due to the larger sealing surface. On the other hand, if the bearing diameter is too small, it may not be able to withstand heavy loads, plus there may not be enough room for the axle. The two technologies are being used in new vehicles and aftermarket replacement hub units that allow engineers to fit better bearings in a smaller package.
Better design was needed to accommodate large diameter wheels, which applied heavier bending motion to the hub flange. In addition, many of today’s vehicles have twice as much power as the new models a decade ago. This extra force can put stress on the axle splines and stubs.
Axial (Face) Spline Hub.
Most drive wheel bearing hubs have a male ribbed shaft that slides through a matching female ribbed hole in the hub. Everything is held together with the axle nut. This design has been used for over 60 years.
Splines have limitations in terms of hub size, amount of power, and load that can be transferred to the wheels. Increasing the number of splines and the diameter of the stub shaft can make the hub unit too large for the knuckle and increase rolling resistance.
Recent hub designs drive the splines from inside the bearing to the rear of the hub. The axial or face-spline hub has a gear-like appearance on the rear of the hub assembly unit and can be found on the BMW X5 and X3 SUVs.
Matching axial gear teeth are formed at the ends of the CV joints, and the whole assembly is held in place by long bolts.
The axial spline arrangement allows for higher torque transmission than the traditional CV spline method, mainly because the gear teeth operate over a much larger pitch circle. This design allows for a smaller diameter hole in the center which can increase bearing rigidity and durability. It can also reduce brake pedal pulsation and travel.
These units are pre-assembled and pre-loaded from the manufacturer. Preload is regulated by rolling the inner races with tons of pressure.
Asymmetric/Hybrid Bearing
Asymmetric wheel bearing hubs use different bearing row diameters to increase load capacity and overall rigidity. Outboard hub rows have larger diameters than inner rows and more rollers or balls. The hybrid asymmetrical design uses rollers and ball bearings in two rows.
The design increases the bearing load capacity and the unit is stiffer than conventional hubs of the same size. This stiffness prevents the brake rotor from flexing and tilting when the vehicle is cornering, and reduces brake caliper piston knockback that can cause a long brake pedal.
Installation
Axial and asymmetric bearings must be installed using the correct service or bearing manufacturer’s installation information. Hub units are usually pre-assembled and simply bolted to the suspension. This is a “maintenance free” and non-serviceable unit that is pre-set, pre-lubricated and pre-sealed.
Some applications use torque-to-yield bolts that stretch and maintain constant tension. This bolt can attach the hub assembly to the knuckle. In the case of axial ribbed hubs, the torque-to-yield bolts secure the hub to the axle. This bolt cannot be used twice.
While it may seem easier to use an impact wrench to tighten the axle nut or bolt, it is not recommended. The recommendation is to use a torque wrench. Impact wrenches can damage the axle nuts, threads and components.
