Got a technical question? Ask TRW’s expert

TRW’s expert Brian Newell talks about the possible causes of disc vibration and judder.

Q: When I replaced the brake discs for a driver who complained of brake vibration and judder, one side of the discs had turned blue and had blistered. Why?

A: This all points to the brake discs overheating. We commonly call this ‘blueing’ due to the changing colour of the metal. Overheating in turn, can lead to brake vibration and judder. It can happen for a couple of reasons:

A ‘sticking’ caliper piston would result in the brake pads and disc being in constant contact; causing overheating. If only one side has blistered and turned blue, you can assume that the caliper on the other side is releasing correctly.

A more likely reason for overheating is corrosion or dirt in the carrier ears. Again, this would cause the brake pad to stick, and the constant contact could cause the disc to overheat.
Excessive heating changes the properties of the cast iron of the disc and causes the blue glazed surface effect; which can be rubbed off leaving the area underneath looking rusty.

To prevent overheating:

• When changing brake pads, clean the area with a wire brush to allow the pads to slide, relatively freely
• Check the caliper. Look for undue corrosion, stiffness in the guide pins and movement of the carrier
• Check the seals on the piston. Perfect seals mean no water or dirt can enter and cause premature failure