When is it appropriate to grind a brake disc?

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When is it appropriate to grind a brake disc?

A stuttering, jerky brake can be due to ripple wear of the brake discs. This does not necessarily call for a new set of brake discs. Under certain circumstances brake discs can be made fully functional with a simple, quick and cheap solution.

wear limit of a brake discEvery braking manoeuvre puts the material under high strain and this always causes some abrasion. As a result brake discs can wear unevenly with potentially fatal consequences: the braking distance becomes longer and when braking strongly, vibrations of the car and steering wheel are clearly felt.

Why grinding a brake disc?

Why grinding a brake discTo grind or not to grind, is not a question but a simple equation:

No disassembly is required for grinding the brake disc. Garages offering this service generally have the right equipment, allowing for treatment of the brake discs without any need for their removal.

Only the wheel and brake caliper should be removed. A professional surface grinder costs ca. EUR 10.000 EUR, (± £8,800), yet the service fee starts at EUR 50 (± £44). Not even the cheapest brake discs can compete with this, not to mention the additional working fee for replacement.

Depending on the type of car a new set of brake discs could become very expensive. In compact and family cars, simple brake discs are available at EUR 60 (± £53) per set. For heavy, high-HP vehicles new brake discs could set you back several hundreds of pounds. That is why grinding deserves consideration in case of repairable damage of the brake discs.

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Repairable brake disc damage

brake disc constructionA brake disc consists of a brake bell and a brake ring. The brake bell is the central part of the brake disc which is pulled over the wheel hub and screwed tight. The brake ring is the part carrying the brake linings.

Repairable brake disc damageBrake discs consist of grey cast steel, relatively soft and yet very tough. Strong friction forces impact on the brake disc, which are transmitted with a high shear stress on the part where brake bell and brake ring meet. Therefore it is important for the material to have a certain elasticity.

brake disc proneness to rustThe price for this toughness is its high proneness to rust.

Only three days of leaving the car standing in the rain causes a visible rust film on the disc, which the first braking manoeuvre will succeed in scouring away.

If the car remains standing for a longer period of time, rust will rapidly expand.
Braking clean” a strongly corroded brake disc does not really make sense as the rust particles act like a scouring agent on the brake disc and brake linings.

The earlier mentioned rippling effect is therefore the consequence.

To put it in a nutshell:

Surface grinding can be applied in case of corrosion and rippling on condition that the minimum brake disc thickness is not exceeded.

Irreparable brake disc damage

Irreparable brake disc damageIt is amazing, how even heavily corroded brake discs can be made fully functional with this simple and quick method. Including wheel and brake caliper disassembly and assembly, the entire brake disc grinding procedure only takes 10 minutes per wheel. Nevertheless, the treatment is subject to clear-cut limits, such as:

– minimal thickness
– material damage

The minimum thickness of the brake discThe minimum thickness of the brake disc is determined by its manufacturer and embossed on the brake bell. It does not specify the brake failure limit. It merely says: “Up to this size it is o.k. to install a new set of brake pads. This is all meant to ensure the safest result of brake system maintenance.

Depending on the brake disc damage, this minimum thickness could be accidentally exceeded by grinding. In that case all the work has been in vain. Therefore be sure to thoroughly inspect the disc prior to treatment.

Inspecting the brake disc automatically includes checking for cracks. These can occur on the edges, at the point where ring and bell meet as well as on the drilling holes. If there is just the tiniest crack, the disc can no longer be used. This means the end for the opposite component as well. Brake discs are principally replaced per axle.

Watch out for the blues

brake disc turned blueGenerally, a brake disc turning blue is repairable as long as the minimum thickness has not been exceeded. Nevertheless, a blue glaze of the disc is an indication that there is something wrong with the brake system. Excessive overheating causes the brake disc to turn blue.

Normal braking manoeuvres should not have this effect. If, for example, the brake pistons are jammed and the brake pads no longer disconnect from the brake disc, this is exactly what happens: the brake pads permanently grind on the disc with slight pressure. The friction generates a constant heating of the brake disc, ultimately colouring it blue.
In this case, the entire functioning of the brake should be checked prior to grinding.

What else must be done

brake discs with strong ripplingWhen brake discs have developed strong rippling, the brake pads should be replaced. As the brake caliper has been removed for the grinding anyway, this means just one additional measure.

Brake pads are cheap wear components. Their replacement is included in the service offered by most grinding service providers. Worn brake linings would otherwise cause similar wear on the brake disc, and the whole work would be futile.

Often, cars are left standing outside for a long time. In that case, flash rust of the brake disc is unavoidable. As in most cases, it is sufficient only to grind the brake disc surface. The brake linings should be checked for size and that should suffice.
АТЕ Ceramic brake pads for front and rear axlebrake pistons jamNevertheless, the brake piston might jam when the car has been standing for a long time. The disassembled brake caliper provides the ideal occasion for making the brake piston fully functional again. To do so, the brake linings are removed and the brake is applied.

Now, the brake piston is put in its original position with a return tool for brake pistons. At EUR 15 – 50 (± £13 – £43) this tool is quite cheap. Nevertheless, checking and repairing brake pistons should be left to a garage. If it is not included in the service package for surface grinding, the option should be added. It does not make the repair much more expensive and restores full safety.
inspect the steering mechanism of the front axleWhen the wheel is disassembled and the brake caliper hung aside, this is an excellent occasion for inspecting the steering mechanism of the front axle. Repairing other damage now makes the car safer and saves additional costs. Pay special attention to the following points:

– tightness of the axle sleeves
– condition of the ball joint
– condition of the suspension joint
– noise development in the wheel bearings
– functioning and condition of the shock absorber, coil spring and strut bearing
– condition of wishbone and stabiliser rod.

All these components are relatively easy to replace in a car which has been disassembled this far. The occasion should be made the best use of. A freshly ground brake disc is of little use if the other components of the car are used up beyond their wear limit. Investing a few bob more now re-establishes full driving safety. This should be worth it.

Foto: Chesky, BELL KA PANG, H K Singh, Sista Vongjintanaruks, Let Geo Create, Mikhail Gnatkovskiy, AOF_SNIPER, Rat007 / shutterstock.com

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