Brake pads and discs: material, wear, and the noises that mean something

A brake does one thing: it turns the car's motion into heat and throws the heat away. The original opens with the arithmetic and it is worth keeping because the number is genuinely startling.

A tonne of car at 130 km/h carries about 650 kilojoules of kinetic energy. Stop it firmly in four or five seconds and the brakes must dispose of roughly 150 kilowatts — more than the engine that got it there can produce. Do it repeatedly down a mountain and that power has nowhere to go but into the discs, which is why brakes fade, why discs crack, and why the friction material has to keep working at 300 to 800 °C.

Everything below follows from a component whose job is to be hot.

One correction to the opening sentence, because the two words get used interchangeably and they are different parts: a pad squeezes a disc, and a shoe presses outward against a drum. The original's first line has pads working against a drum. Most cars now have discs at the front and either at the rear.

What a pad is made of, and why it matters to you

The original's material section is the best part of it and is unusual in a consumer article, so it is kept. A pad is fibres for strength, fillers for wear resistance, friction modifiers to tune the grip, and a resin binder holding it together — and the binder is the part that decides whether the pad survives being hot, because a poor one lets the pad come apart under heat and pressure.

The five families it lists are real and the choice between them is a real trade-off:

The practical version: an organic pad in a taxi is the wrong pad, and a semi-metallic pad on a car that only does short cold journeys is also the wrong pad. The original makes exactly this point about city vehicles and it is right.

And since heat is the whole subject — a pad's ability to shed heat is part of its life. The organic pads that transfer heat poorly run hotter and therefore wear faster, which is why the same pad lasts differently on two identical cars driven differently.

The noises, and which ones mean anything

This is what the title promises and what neither source provides.

The grinding row is the one that matters and the corpus states it plainly: grinding means metal on metal, and every second of it is cutting into a disc that was going to be reusable. A pad change becomes a pads-and-discs change, quickly.

The squeal that is the wear indicator deserves respect for a design reason: it is a deliberate warning, engineered to make an unpleasant noise while there is still pad left. Silencing it by fitting new pads without looking at what caused the wear is how the same noise returns in six months.

Discs almost never warp

Brake judder is universally described as warped discs, and it usually is not. A cast-iron disc clamped to a hub does not bend from heat in normal use.

What actually happens is disc thickness variation. Pad material transfers onto the disc unevenly — a deposit builds in some places and not others — and the pad then rides over a surface that is a few hundredths of a millimetre thicker in patches. The caliper is pushed back and forth once per revolution, and you feel it in the pedal.

The distinction is worth having because it changes the cause and the cure. Deposits build up from holding the pedal hard at a standstill after heavy braking, which parks a hot pad on one spot of a hot disc, and from new pads that were never bedded in. And it means a disc that judders can sometimes be recovered by proper bedding rather than replaced.

The other genuine cause of judder is runout — the disc not sitting flat on the hub, usually because of rust or dirt on the mounting face, or an unevenly tightened wheel. Wire-brush the hub face before a new disc goes on and tighten the wheel bolts in a diagonal sequence to the specified torque. Those two minutes prevent the complaint.

Wear, and when to change

• Pads have a minimum thickness — commonly around 3 mm of friction material — and it is measurable through most wheels with a torch.
• Discs carry a minimum thickness figure stamped on the disc itself, usually on the hub face or the edge. A disc thinner than that has lost the heat capacity it was designed with, whatever it looks like.
• A lip at the outer edge is how much has been worn away from the rest. A pronounced one means the disc is near its limit even if the surface looks fine.
• Always across an axle. Two wheels on the same axle with different friction is a car that pulls under braking, which is worse than two equally worn sides.

The fault that causes most uneven wear is a sticking caliper slide pin. The caliper has to float sideways to press both pads equally; when its pins seize with corrosion and dried grease, one pad does most of the work, wears out early, and often drags. The symptoms are one pad far thinner than its partner, a wheel that is hot after a drive, a pull under braking or a smell. Cleaning and regreasing the pins is a cheap job and it is the first thing to check when one corner has worn oddly.

Bedding new pads in

New friction material has to transfer an even layer onto the disc before it works properly, and the first few hundred kilometres decide whether the brake judders for the rest of its life.

The manufacturer's procedure comes with the pads and should be followed. The general shape is a series of moderate stops from moderate speed, with cooling in between, and not a hard stop followed by sitting at a junction with the pedal pressed — which is precisely how an uneven deposit gets laid down on a hot disc.

Expect reduced bite for the first few stops and leave more room accordingly.

Three things that ruin brakes

Oil or grease on a friction surface. Handle pads by the backing plate and clean new discs of their protective coating with brake cleaner before fitting. A contaminated pad cannot be washed and has to be replaced.

Riding the pedal downhill. Continuous light braking keeps everything at temperature with no chance to cool, which is how fade arrives. Use a lower gear and let the engine do the work.

The cheapest pads available. This is the one place where the price difference buys something measurable. The binder resin decides whether a pad holds together hot, and it is the most expensive ingredient — which is exactly what a very cheap pad economises on. It will stop the car on a test drive and let go on a long descent.

And what the codes will not tell you

Worth stating because it surprises people: pad wear and disc condition set no fault codes on most cars. The brake-related codes in the corpus concern the electrical system around the brakes — the pedal switch, the pressure sensor, the wheel speed sensors that ABS depends on — and a car can be scanned clean with the pads down to the backing plates.

The brake warning light on the dashboard is a fluid-level and handbrake light, not a pad light, unless the car has electrical wear sensors wired to it. The instrument that reports pad wear on most cars is your ear, which is why the table above is the useful part of this article.

Video guides

Video, for the parts of this that are easier watched than read.