Brake disc skimming or replacement?

Skimming is the machining of a used brake disc on a lathe. A cutting tool takes a thin layer of cast iron off both braking surfaces, until the surface is flat, smooth and the same thickness all round. This is done either on a bench lathe, with the disc removed from the car, or with an on-car lathe that is coupled to the wheel hub. The aim is the same in each case: to remove grooves, a rust lip, the imprint of a pad that has stuck, or a thickness variation, without fitting a new disc.

The idea dates from the time when brake discs were thick and heavy and had plenty of reserve above the minimum thickness. Modern brake discs are designed lighter and thinner and, from new, often have only a few millimetres of wear reserve down to the minimum thickness. Every pass of the cutting tool removes part of that, and on both sides of the disc.

Skimming is only possible if the disc stays well above the manufacturer’s minimum thickness after skimming and is free of cracks, heat spots and deep pitting. The minimum thickness is stamped on the disc. Brake manufacturer Brembo explains that this value is marked on the disc as MIN TH and that a disc below it has to be replaced. HELLA writes that the minimum thickness in millimetres is stamped on the outer edge or in the hat of the disc and that the disc must not go below it. Why does that limit matter so much? According to Brembo, a thinner disc loses its ability to dissipate heat, and also its mechanical strength. A disc below the minimum thickness can no longer deliver the correct braking performance.

Based on the technical explanations of Brembo (minimum brake disc thickness, noises and vibrations), HELLA (minimum thickness) and Ferodo (brake judder).

There is something else. According to Ferodo, a vibration when braking is usually caused by a thickness variation of the disc, and that variation often arises from something other than the disc itself: rust or dirt between the disc and the hub, wheel bolts tightened to the wrong or uneven torque, overheating, or transfer of pad material onto the disc. If a disc is skimmed without that cause being removed, the vibration comes back. That is why every assessment at our workshop starts with measuring and with the hub, the caliper and the slide pins, not with the disc alone. You can read more in our article about a car that shakes when braking.

We do not skim brake discs. A disc that is too thin, warped or pitted, we replace. With modern discs that is usually better value than skimming, and you know for certain that the thickness and braking performance are right again. A skimmed disc is by definition thinner than before, keeps wearing in the same way and therefore reaches the minimum thickness sooner. A new disc starts with the full thickness and the full wear reserve.

1. We measure first: the thickness of each disc with a vernier caliper at several points around the disc, where the lowest value counts, and the run-out on the hub. We compare that with the minimum thickness stamped on the disc.
2. We only replace what is worn: if a disc is still thick enough, flat and free of pitting, it stays on. Then we replace only the brake pads, if those are worn.
3. Per axle, with new pads: if a disc is worn, we replace both discs on that axle together with new brake pads. Brembo writes that discs must be replaced on both sides, even if only one of the two is below the minimum, and HELLA that discs are replaced per axle and in combination with new brake pads.
4. Clean hub and the correct torque: we clean the contact surface between the hub and the disc and tighten the wheels with a torque wrench, so the new disc runs true.
5. Then we test: on the brake tester and with a short test drive. You can see the old parts if you wish.

We tell you one price in advance, based on your registration number (kenteken), including parts, labour, testing and disposal of the old parts. If anything extra comes up, we tell you that first. Which amounts the ANWB, the Dutch motoring organisation, gives as a national indication and what determines the price, you can read in our article about the cost of brake replacement in the Netherlands. Everything about how we work is on our page about brake replacement in Rotterdam.

Yes, usually. A disc with light grooves or a narrow rust lip is usually still good and does not need skimming or replacing. A brake disc does not have to be mirror-smooth to brake well. Light marks in the direction of rotation are normal. Three things are decisive: the thickness compared with MIN TH, the run-out and the condition of the braking surface. The ANWB puts it plainly: the garage can measure the discs, and if they have become too thin they must be replaced. If they are still thick enough, new pads can be fitted when the old pads are worn. HELLA adds that new pads may only be fitted if the disc is still above the minimum thickness.

If you are looking for a garage to skim your brake discs in Rotterdam, first have someone measure what is really going on. In many cases the disc is still good and the cause of a vibration or noise lies in the hub, the caliper, the slide pins or the brake pads. Then neither skimming nor replacement is needed. If the disc is worn or pitted, a new disc per axle with new pads is the solution that lets you know for certain that the thickness and the braking performance are right again.

Not sure? Call us on +31 10 479 00 00 and we will schedule a short brake check. If nothing needs doing, we tell you that too. We check the brakes at every service and every APK (MOT), the compulsory annual vehicle inspection in the Netherlands. We do this for all brands, including hybrid and electric cars, and for vans and motorhomes up to 3,500 kg.

No. We do not skim brake discs. A disc that is too thin, warped or pitted, we replace. With modern discs that is usually better value than skimming, and you know for certain that the thickness and braking performance are right again. If a disc is still good, it stays on and we replace only what is worn.

The minimum thickness is stamped on the disc, usually marked MIN TH followed by a number in millimetres. A garage measures the actual thickness with a vernier caliper at several points and compares it with that number. If the disc is below it, it has to be replaced. Measuring it yourself rarely works well, because the wear lip on the outer edge gives a false reading.

No, usually not. That differs per car and per situation. Skimming requires removal, machine time and usually new brake pads afterwards, and the disc is left with less wear reserve. With modern discs, replacement is therefore usually better value. With us, you are told one price in advance, based on your registration number, for replacing discs and pads per axle.

Yes, sometimes: if enough thickness is left and the cause of the warping has been removed. According to Ferodo, a vibration is usually caused by a thickness variation, which often arises from rust on the hub, the wrong torque on the wheel bolts or overheating. If that is not fixed, the vibration returns after skimming. We remove the cause and replace the disc if it is no longer flat.

Yes. New discs go together with new brake pads. Old pads have taken on the shape of the old disc and do not sit flat on the new braking surface. The ANWB and HELLA both write that new brake pads must be fitted when brake discs are replaced.

No, brake discs are replaced per axle, left and right at the same time. Brembo writes that discs must be replaced on both sides, even if only one of the two is below the minimum thickness. Otherwise the car brakes unevenly, and at the APK the difference in braking force between left and right on one axle may not exceed 30 percent.

Yes, if the car brakes normally and does not shake, grind or pull to one side. Light grooves are normal. Do have the discs measured at the next service or sooner. If you hear a grinding noise or feel a vibration that persists, come by sooner and avoid long journeys with a heavy load.

• Brembo: Minimum brake disc thickness (MIN TH, measure at 4 points, replace per axle)
• HELLA Tech World: Testing the minimum thickness of a brake disc
• Brembo: Noises and vibrations (run-out at most 0.10 mm, clean the hub, torque)
• Ferodo: Brake Judder (thickness variation, causes, blue spots)
• ANWB: Remschijven: opmeten, vervangen per as en nieuwe remblokken (brake discs, in Dutch)
• RDW: APK manual for passenger cars, service brake deceleration (left and right difference at most 30 percent, in Dutch)