What kind of chemistry do you have with your car? Love it? Loathe it? Let us know and you could win a £300 John Lewis voucher | No thanks
Q

BRAKE DISCS AND PADS: Why do brake discs as well as pads need replacing so often? And why is it usually rear discs first?

FAQ added 10-3-2007 

A

Ever since asbestos was banned from brake pad material, and also to keep pace with the increased performance of modern cars, manufacturers have had to add metal to pad material and to make brake discs softer and sacrificial.

Rear disc brakes pose an additional problem. The front brakes do most of the actual retarding of the car. The rears merely balance this in extreme braking conditions.

Discs also rust all the time, especially overnight in a winter salt-laden, wet atmosphere. That's why the metallic content of the parking brake pads sometimes stick slightly to the discs when you release the parking brake first thing in the morning. They have effectively rusted together.

Your first brake of the morning cleans the surface rust off the front discs and you can often hear it as a grating sound. But unless you (first check your mirrors) and brake very hard, the rear pads take much longer to clean surface corrosion off the rear discs.

This gives oxidation a chance to eat deeper into the metal with the result that rusted rear discs start to adopt a grooved appearance. At this point braking efficiency is severely compromised and the car would probably fail an MOT.  

 

Ask Honest John

Value my car