New pads and discs: the bedding-in that changes everything

You have just fitted new pads, you pull out of the garage, and the front brake is soft. Not dangerous, but noticeably less bite than before you took the old ones out — and those were worn to the indicator. The classic reaction is to doubt the quality of the pads, or to suspect air in the system. In the vast majority of cases it is neither: the brake is working exactly as designed, it simply does not have a working surface yet.

A set of new pads laid against a disc touches that disc over a few per cent of its apparent surface. Both parts look flat to the eye and are rough under a microscope, and contact happens on the peaks. Bedding-in is not a manufacturer's precaution or a warranty clause: it is the physical operation that takes that contact area from a few per cent to nearly all of it, and that lays down on the disc the layer of material without which a modern brake does not bite.

What bedding-in actually does

Disc braking combines two mechanisms. The first is abrasive: the pad scrapes the disc, like sandpaper. That is the dominant mechanism on budget organic friction material and on a cold brake. The second is adhesive: the friction material lays a very thin film of itself onto the disc, and braking then happens pad against pad-film, with a far higher coefficient of friction. That material transfer is what gives modern sintered pads their bite.

So bedding-in serves two purposes at once.

• Conforming the surfaces. The high spots are flattened, and the pad takes on the real run-out and surface finish of the disc. That work is purely mechanical and takes a few dozen stops.
• Laying down the transfer film. This needs heat, but heat brought up gradually. That is where the difference is decided between a brake that bites and a brake that will stay mediocre for thousands of kilometres.
• Outgassing the friction material. New pads contain resins and binders that release gases the first few times they get hot. Those gases form a cushion between pad and disc and reduce contact until they are gone.

The received wisdom to correct: "you have to get it really hot straight away to bed it in". It is widespread because it is what you hear in racing, where specific materials and procedures are used. On the road, a violent temperature rise on pads that have not yet outgassed produces the opposite of what you want: the resin melts at the surface, glazes over, and leaves a glassy, smooth, shiny pad that will never bite properly again.

The procedure, in two phases

Do it on a clear road, with no traffic behind you, in the dry, and check the mirror before every stop. It takes about twenty minutes.

1. Check lever free play and travel first, bike stationary. Pump several times to bring the pistons up: after a rebuild the first pull is always long. If the lever is still spongy after ten pumps or so, you have a hydraulic problem, not a bedding-in problem.
2. Ride a few hundred metres at low speed braking very gently, to confirm the bike stops and that nothing is rubbing abnormally.
3. Phase 1, conforming: run 15 to 25 progressive stops, from about 60 to 30 km/h, never coming to a complete standstill and never braking hard. Let it roll between each one so things cool. By the end of this phase you should already feel the bite coming back.
4. Phase 2, transfer: run 5 to 10 firmer stops, from 100 to 50 km/h, building up in intensity. Firm braking but short of locking, and above all never down to a standstill.
5. Cool it down by riding, ten minutes without touching the brake, or as little as possible.
6. Never stop with the brake held on straight after a hard stop. Hold a pad against a very hot disc while stationary and you deposit a local patch of material in that exact spot. That raised area produces a pulsing at the lever or the pedal that many people then read as a warped disc. At a red light after a hard stop, roll forward ten centimetres or hold the bike on the rear brake.

Full potential arrives after 100 to 200 km of normal use. During that period, increase your safety margin: stopping distances really are longer.

The special case of a new disc

A new disc changes things, because it has no transfer film at all and often carries an anti-corrosion coating from the factory.

• Degrease the disc before fitting. Brake cleaner and a clean lint-free cloth, both sides, until the cloth comes away clean. Do not touch it with bare hands afterwards: the grease from your fingers is enough to create a dead spot.
• New disc and new pads together, always. Bedded pads on a new disc transfer the old imprint; new pads on a marked disc wear at an angle. The pair gets replaced as a pair.
• Extend phase 1. Count 25 to 35 gentle stops instead of 15 to 25 — a new disc needs more conforming.
• Torque the disc bolts, with medium-strength threadlock. The figures are low — in the region of twenty newton metres on M8 bolts — and guesswork tightening either distorts the disc or lets the bolt back out. The exact torque is manufacturer data, and the orders of magnitude by fastener type are summarised in the motorcycle torque settings table.
• On an ABS bike, check that the sensor tone ring has not been disturbed during the job and that no metal swarf has been left on the sensor.

Contamination: what cannot be recovered

This is the leading cause of new pads binned after 300 km, and it is nearly always avoidable.

Friction material is a porous material. When an oily liquid gets into it, it migrates deep and comes back out every time the pad gets hot. Sanding a millimetre off the surface gives the illusion of a result for two stops. On a pair of pads at £20 to £60, the arbitration is quickly made: you replace them.

Two habits are enough to avoid nearly every case. While bleeding, cover the caliper and disc with a cloth and immediately rinse off any brake fluid run — it attacks paint too. And never use multi-purpose penetrating degreaser anywhere near a brake: the right product is brake cleaner, which evaporates without leaving a film. A leaking fork seal or a weeping caliper piston has to be dealt with before new pads go in, otherwise you are sacrificing them — the method for freeing a seized piston is described in the article on the stuck caliper piston.

When the disc is the culprit

Before blaming the pads, check the disc. Three measurements answer almost every question.

• Thickness. The minimum value is engraved on the disc itself, usually near the hub. On a road bike a new disc is commonly 4.5 to 5.5 mm and the limit is about a millimetre below that: the total wear margin is small, which is why people go past it without noticing. Measure with a vernier at several points across the friction band, not just at the edge. Brake disc and pad condition is also part of what gets looked at during the annual MOT, which a motorcycle takes once it is three years old.
• Run-out. Dial gauge on a magnetic base, tip on the friction band, wheel turned by hand. Manufacturer tolerances often sit around 0.15 to 0.30 mm, to be confirmed in the documentation for your model. Beyond that, you feel a pulse at the lever.
• Surface condition. Deep circular scoring you can catch a fingernail in, a pronounced step at the outer edge, or areas blued by overheating condemn the disc even if the thickness is still good.

One important difference from cars: you do not skim a motorcycle disc. Motorcycle discs are too thin to survive machining, and many are floating, with the friction band mounted to a carrier on rivets or buttons — machining them destroys the assembly. When it is out of spec, it gets replaced. Budget £70 to £220 each in the aftermarket, more for an original part, and replace both front discs together to keep braking symmetrical.

The reassembly that governs everything

A successful bedding-in starts from a clean build. Clean the pad seats and the guide pins, check that the pistons go back without a tight spot — a partially seized piston makes one pad do all the work and glazes it within a few hundred kilometres. Push the pistons back slowly, with the master cylinder reservoir open and watched, because the fluid comes back up and overflows. Refit the pins and clips, all of them, the right way round.

If the lever is still long or spongy after ten pumps or so, that is not a bedding-in problem but air or old fluid, and it needs bleeding: the ordered procedure is in the article on bleeding motorcycle brakes.

In short, what you do after a change

Degrease the disc, build it clean, torque the fasteners, pump the lever until it goes hard, check at low speed in the yard. Then do phase 1 of gentle stops, phase 2 of firm stops, cool it down by riding, and do not stop with the brake held on while hot. Over the first 200 km, keep your margin. If the bite is still weak after 300 km, look at contamination or a piston that is not working, not at the brand of pad.

The precise values — minimum disc thickness, permissible run-out, torque settings for disc bolts and caliper, brake fluid type — depend on model and year. The L'Atelier technical assistant finds the manufacturer's data from the make, the model and the year of your bike.