Motorcycle brake discs

A motorcycle brake disc, or rotor, is not a matter of taste: diameter, mounting and thickness have to match the part you are replacing, and almost every disc is made for one model, a set of years and a specific position. Here you will find Braking front and rear discs for scooters, naked bikes, sports bikes, adventure bikes, customs and off-road machines. This page explains how to tell if yours is worn and which choices really are yours to make.

When a disc needs replacing

A disc lasts much longer than a set of pads, but not forever. Check it every time you change them:

• Thickness: every disc has its minimum thickness stamped on it, often marked MIN TH. Measure it with a micrometer at several points on the braking surface; vernier callipers rest on the outer lip and read higher than the real figure.
• Warping: if the lever pulses when you brake hard, the disc may be buckled or worn unevenly.
• Surface: deep scoring, cracks between the holes or blue patches from overheating are reason enough to replace it before it reaches the thickness limit.

Floating or fixed: it depends on the bike

Sports bikes and many powerful nakeds use floating discs at the front, with the braking ring separate from the centre carrier, as they cope better with the heat of repeated hard stops. Scooters, 125s, adventure bikes, off-roaders and nearly all rear brakes use fixed one-piece discs, which are simpler and tougher against knocks. The sensible approach is to stick with the type the manufacturer fitted; a floating disc in place of a fixed one only makes sense if it keeps the same mounting and diameter.

Diameter, edge profile and approval

The diameter must match the original, because the caliper is positioned to work on a specific braking surface. A bigger, oversize disc needs a different caliper bracket and changes the braking system the bike was type-approved with, so in many countries, Spain among them, the modification has to be legalised for road use. The edge profile gives you more freedom: wavy discs, with their petal-shaped edge, and classic round discs do the same job, and the choice between them is mostly about looks.

When fitting them

• With twin front discs, replace both and fit new pads: old pads on a fresh surface brake less well and score it.
• Clean the hub mounting face, tighten the bolts to the torque in the manual with threadlocker, and degrease the braking surface before refitting the wheel.
• For the first few miles, brake gently and build up gradually so everything beds in.

If you have never worked on brakes, this is a job for a workshop. Brake pads have their own section, the code on each disc (W-FIX, W-FLO, R-FIX and so on) is explained under Braking brake discs, and the rest of the system is covered in motorcycle brakes.