Brake pads: when to really change them

The pads are the part of the braking system that wears by design: they are made to wear themselves instead of the disc. The question is not whether to change them, but when — and the answer is not a mileage.

The thickness, which is the number that counts

The thickness of a new pad varies from model to model. The minimum thickness is given by the manufacturer, usually around 2-3 mm, and below 3 mm it is already worth replacing them: the remaining material heats up much faster, and a long downhill braking can take it beyond the temperature it was made for.

The measurement is taken at the outer edge, looking through the spokes of the wheel. If the wheel is solid, it has to come off. If you have experience you can measure it yourself; if in doubt, go to a workshop: brakes are a safety system.

The signs that come first

1. The metallic squeal under light braking. It is not a defect: many pads have a tab that touches the disc when the friction material is almost gone. It is made to be heard.
2. The warning light on the dashboard, if the car has a wear sensor. The sensor is a wire that wears together with the pad: when the light comes on you have reached the limit, and the pads must be replaced.

A noise that appears only when cold, in the first braking of the morning, is often not wear but a film of rust on the disc. It disappears after a few stops. If it stays, then it is something else.

A long or spongy pedal is not wear

Why they are changed in pairs on the same axle

Pads are replaced always together on the same axle. A new pad on the right and a worn one on the left means different braking between the two wheels, that is, a car that pulls to one side exactly when it needs to go straight.

Front and rear, on the other hand, wear at different rates: the front does most of the braking work, so the front pads usually last less. There is no need to change them together, but when you change the front ones it is worth checking the rear ones too.

Electric parking brake

On cars with an electric parking brake, the rear caliper pistons are retracted with the diagnostic tool (service mode), not by hand: forcing them can damage the caliper motor.

What to check while they are off

• The brake disc. The minimum thickness is stamped on the disc: if the measured thickness is below that value, or if there is a lip on the edge you can feel with a fingernail, the disc is changed together with the pads.
• The caliper guide pins. They must move by hand. If they are stiff, the caliper stays partly applied and the new pads wear sooner.
• The brake fluid level, which rises when the pistons are pushed back. If it was already at maximum, fluid must be removed before, not after.

Bedding in

New pads need a bedding-in period on the disc, which the pad manufacturer specifies. During that time braking is slightly less sharp, and long, hard braking should be avoided: the material can glaze and become noisy.

This advice is general: for the values and procedures for your car, refer to the manufacturer's handbook.