Hello Paul, my 2006 Mercedes ML 320 CDi has squealing brakes which vibrate when I brake suddenly. We have just replaced the pads but the discs seem to be the problem. One mechanic has suggested we cut a layer off the brake discs and another says we out-rightly replace the discs. I would like to know the advantage and disadvantage of the alternatives and what you would recommend. Robinah Mugisha.
Hello Robinah, you need to consider outright replacement of the brake discs or rotors. When you apply brakes to stop your car, the brake pads are hydraulically and mechanically engaged to both sides of the discs or friction plates by callipers to stop the car.
Brake discs help to slow the car by converting energy from kinetic, generated when moving fast, to thermal energy through friction.
The contact of brake pads and disc rotors during braking involves a lot of friction and heat which, depending on the material and quality of brake pads, can eventually cause damage to the discs in the form of warping, ridges, spotting, pitting, grooving and excessive run out or uneven wear of the disc surface.
The damaged disc surface will cause reduced braking performance, vibration and squeaking noise when you brake. Cutting or skimming a layer off the discs using a lathe machine is the cheaper option to get rid of the above damage and improve brake disc performance.
Whereas this is often done with the thicker single layer discs on some vehicle types, there is a minimum disc width that should be observed beyond which the disc becomes too thin. This challenge is greater with the vented double layered discs on Mercedes Benz ML.
Skimming these kinds of discs creates a risk of making them thinner than recommended, which affects braking performance (how fast you are able to slow the car) and in extreme situations can compromise road safety when the discs crack or break into pieces during emergency braking.
Most European car manufacturers like Mercedes Benz do not recommend disc skimming. The price of discs or rotors is cheaper today and they are readily available.