Many of us forget that what stops a vehicle is friction, turning kinetic energy into heat when the driver applies the foot brake. With vehicle performance constantly advancing it comes as no surprise that our brakes have to work harder than ever.

It is important when carrying out a brake inspection to check the whole system for wear i.e. the brake disc, brake calliper, hydraulic brake lines and brake pads. After all it is the braking system that can make the difference between life and death.

Brake discs

Brake disc rotors are largely made of cast iron as it is relatively hard, resistant to wear and due to the high temperatures involved in braking can dissipate heat effectively without distorting. They more often than not have central venting to optimise cooling and reduce harmonics that can lead to the dreaded brake squeal.

Checking your brake discs regularly can avoid unnecessary repair work and ensure passenger safety is never compromised.

Essential measurements and checks

When undertaking brake repair work it is always advised that the brake pads and discs are measured against vehicle manufacturers wear limits using suitable measuring equipment.

Once this has been completed you can assess the disc for visible signs of:

• excess corrosion
• grooves worn into the surface
• localised hotspots

Checking an old disc for serviceability

The correct method of checking a brake disc should start with the disc mounted on the hub and the wheel nuts installed to the correct torque. The disc should then be checked with a micrometer at several points around its wear face 10mm in from the edge. These measurements should then be compared to vehicle manufacturers wear limits.

The disc should then be checked for excessive run out using a dial test indicator and compared to the vehicle manufacturers wear limits.

Results of fitting new pads to worn discs

Corrosion is often overlooked on the inner face of the disc due to the fact it is often hidden from view but remember that any corrosion on the braking surface reduces the overall brake surface area. This in turn affects the efficiency of the vehicles brakes.

Fitting new pads to worn discs is making the judgment that the disc has worn evenly over the whole surface. All that is happening is a reduction in braking efficiency while wearing a new pad into the shape of a worn disc.

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