Wheel Hubs, Bearings & Steering Knuckles

Use cornering to load each side. A worn bearing hums or roars in proportion to road speed, and the noise changes as the load moves between the wheels, so on a quiet road a gentle sweep to the left loads the right hand bearing and usually makes that side louder. Swap the direction and confirm. Then check for play by raising the wheel and rocking it top to bottom, which is more conclusive than sound. Rule out tyres first, since worn tread produces a similar roar.

It depends on how the bearing is fitted. Where the hub unit bolts to the knuckle, the unit alone is the obvious purchase and the job is straightforward. Where the bearing is pressed into the knuckle, the knuckle has to come off and go to a press regardless, and a knuckle supplied complete with the hub already fitted can cost less once labour is counted and avoids the risk of damaging a new bearing during pressing. This category lists both routes.

Because the magnetic encoder that the wheel speed sensor reads is built into the hub unit seal on most current vehicles. When the bearing wears and the seal is damaged, or when the encoder is scratched or demagnetised during fitting, the signal becomes erratic and the control unit logs a wheel speed fault. That is why a rumbling bearing and an antilock warning often arrive together, and why the encoder face must never be struck, cleaned with a magnet or fitted the wrong way round.

No. A modern hub unit is sealed and its bearing preload is set at the factory, with no adjustment, no grease nipple and no way to replace the seals. Once it has become noisy the races are already damaged and the grease contaminated. Replace the complete unit, torque the hub nut to the specified figure with a new nut, and avoid the temptation to reuse a nut that looks fine, since most are designed to stretch and are single use.

Critical, and it is the step most often rushed. On many designs the hub nut sets the clamp load through the bearing, so under torquing leaves the bearing loose and it fails within months, while over torquing crushes it. The nut is usually a stretch or staked type intended for a single use. Use a torque wrench and the manufacturer's figure, tighten with the vehicle's weight off the wheel unless instructed otherwise, and stake the nut where the design requires it.

A bare bearing is the cheapest route and a bolt on hub unit costs more, but the hub unit usually wins on total cost because it needs no press work. Front units generally cost more than rear ones, since they carry the drive spline and often the encoder ring. A complete steering knuckle, especially an aluminium one supplied with the bearing already fitted, is the top of this group. Today's low and high sit in the summary strip above. Include the hub nut, which is single use.

For a short distance at moderate speed, and it is not something to postpone. A bearing that has begun to roar is already breaking down internally, and continued use turns a hum into play, then into heat, and in the worst case into a wheel that seizes or separates from the hub. The antilock and stability systems also lose accurate speed data from that corner. Book the repair promptly, and avoid long motorway journeys with a bearing that is audibly failing.

A bolted hub unit is a realistic job: remove the wheel, the caliper and carrier, the disc, the wheel speed sensor and the hub nut, then unbolt the unit from the knuckle. Budget one to two hours. A pressed bearing is a different proposition, needing the knuckle removed and a hydraulic press with the correct drifts, and taking three hours or more even with the tools. Improvised pressing damages the new bearing and the encoder. Know which type you have before starting.

After impact damage, when a bearing bore has been worn or damaged by a spinning race, when a ball joint or press fit housing has torn, or when corrosion has thinned the casting. A knuckle is what sets camber and caster at that corner, so any distortion shows up as geometry that cannot be brought into specification on the alignment rig. Have the corner measured before ordering, and book an alignment after fitting whichever part you choose.

Usually, and considerably. Front units are larger, carry steering loads and usually have a splined bore for the driveshaft, while rear units on a non driven axle may be a plain flanged unit with no bore at all. Encoder types can differ between axles as well. That gives at least two distinct references per vehicle, and often four where left and right differ. Match the OE number and the position stated in the listing rather than the appearance.

Only where the number agrees. Bearing size follows vehicle weight, the bolt pattern and stud count follow the wheel, the bore follows the driveshaft, and the encoder type follows the sensor. Two units that look identical can differ in any of those. Battery electric versions of a body are often heavier and use a larger bearing. Read the number from the old unit, and where the reference brings back nothing, send it through the quote form so the catalogue is checked.

Yes. Hub units and knuckles are handed and differ between driven and undriven axles, so the less common corners are the ones without a figure. Send the number and say which corner of the car it belongs to. Check whether your bearing is a bolt on hub unit or a press fit bearing before ordering, because the two are entirely different parts and the press fit version needs a shop press and a bearing puller. Order a new hub nut with either, and where the bearing carries an encoder ring, handle it away from magnets and swarf.