Why Wheel Bearings Are Important, And What Happens When They Fail

Wheel bearings play an important role within the drivetrain of any vehicle, as they provide the first connection between the moving and static areas of the car. A bearing – in its simplest form – is a friction reduction device placed within something like a wheel to aid efficiency of rotation. This is achieved as rolling produces much less friction force than sliding.

A car’s wheel bearing does this by using small metal balls that roll between two smooth rings of metal. Along with grease, the bearing rotates in-tandem with the wheel’s rotation, the rolling motion of the balls allowing the wheel hub to rotate as freely as possible.

The wheel bearing is housed within the hub assembly, providing the static connection with the hub carrier through an outer ring or ‘racer’. The driveshaft travels from the transmission and passes through the centre of the wheel bearing through the inner ring, creating the rotational partnership. Roller bearings are used generally on the driven wheels of a vehicle, while tapered bearings are predominantly used on the non-driven wheels.

Like all mechanical parts on a car that rotate, rub and roll, they tend to wear out. And the tell-tale sign of a wheel bearing that’s had its day is a constant drone or hum from whichever wheel is affected at various speeds. There are three general causes of a dodgy bearing:

• Incorrect alignment due to poor installation
• A damaged or leaking bearing seal
• An inward collision due to a side impact

The most common mechanical failure is due to a seal leak, which allows the lubricating grease to exit the bearing, and water and grit managing to worm its way in. This all amounts to the degradation of the balls and the outer and inner housing of the bearing, often creating that gritty, skimming sound as the wheel is rotating. Looking inside a worn-out bearing, there will often be small specs of metal that have broken off from inside the bearing itself and are grinding their way around the racers, causing more damage.

Considering wheel bearings are generally made from hardened steel, they can stand a serious amount of rotational abuse. But apply heat (through lack of lubrication) and water (broken seal) and all hell can break loose.

To replace a wheel bearing properly, it has to be popped out of the hub assembly, preferably using a hydraulic press. Problems can arise with the re-installation of the new bearing, as the alignment must be accurate to avoid the bearing wearing out rapidly and causing further pain. Wheel bearings should last tens of thousands of miles, but many mechanics will simply hammer the bearing back into the hub assembly instead of precisely aligning it and using the aforementioned press. This will inevitably lead to the bearing not rotating in the perpendicular fashion that it was designed for, accelerating wear and damage to the part.

Most online parts stores will now sell the entire hub assembly with the wheel bearing pressed into it, saving the hassle and potential scope for mistakes within the specific bearing assembly.

If left to wear down further, a wheel bearing could seize and lock the axle, so think carefully about your timing if the classic wheel bearing drone starts. However, considering the forces that a drivetrain has to counteract during its lifetime of usage, it is impressive how long wheel bearings can last if inserted properly and looked after.

Although it can be a labour-intensive job if one was to fail, the shift to selling entire hub assemblies should make a bad wheel bearing fixable from home, with a decent tool kit and a bit of know-how. So do not fret the next time that horrid whir begins to creep into the cabin, as a fix may be only a ratchet spanner away.