How brake pads, shoes and discs are made
A brake pad is a block of friction material fixed to a steel back plate. The friction mix combines fibres, fillers, abrasives, lubricants and a resin binder; low-copper and copper-free formulations replace copper with other materials. The mix is weighed, preformed and hot-pressed onto a back plate that has been shot-blasted and coated with adhesive, or that carries a mechanical retention layer. Pads are then cured in ovens, ground to thickness, slotted and chamfered, painted or powder-coated, and fitted with shims, clips and wear indicators. Drum brake shoes follow a similar route, with the lining bonded or riveted to a steel shoe.
Brake discs and drums are grey iron castings. They are machined on both braking faces, drilled for wheel studs, sometimes balanced, and given an anti-corrosion coating on the non-braking areas. Because the casting and the friction material come from different processes, many pad makers buy back plates and many disc makers buy rough castings. Ask who makes each part, and see our page on iron castings for the foundry side.
What we check on brake parts
| Check | How it is checked | What fails |
|---|---|---|
| Application and profile | Pad outline compared with the buyer's template or drawing; back-plate tabs, holes and abutments gauged | Outline or tab position that will not seat in the caliper |
| Thickness | Total and friction-material thickness measured with a micrometer at several points; back plate flatness on a surface plate | Outside drawing tolerance; tapered pads not ordered as tapered |
| Bond and retention | Visual check of the bond line; the factory's shear test witnessed on pads we pick, with the force recorded | Gaps at the bond line; shear result below the agreed minimum |
| Friction surface | Cracks, chips, delamination, foreign particles, slots and chamfers to drawing | Cracks across the block; chipped edges beyond the agreed size |
| Accessories | Shims, clips, wear indicators and sensors present, correctly fitted and of the right type for each pair | Missing or wrong wear indicator; shims loose |
| Disc and drum dimensions | Thickness, diameter, centre bore, stud holes and height; drum inside diameter | Bore or stud pattern off; disc below nominal thickness |
| Runout and thickness variation | Disc mounted on the factory's fixture and turned under a dial indicator; thickness at several points around the face | Lateral runout or thickness variation above the drawing limit (causes pedal pulsation) |
| Casting soundness and hardness | Faces and vent passages examined for porosity and cracks; Brinell hardness witnessed on the factory's tester | Porosity on braking faces; cracks; hardness outside the specified range |
| Marking | Approval mark, batch or date, edge code and LeafMark read on sampled parts and compared with the certificates | Mark missing, unreadable, or not backed by a certificate for this formulation |
Common defects in brake parts
| Defect | Why it matters | Class |
|---|---|---|
| Friction material separated or partly unbonded | Pad can break up during braking | Critical |
| Crack through a disc or drum | Can fail under heat and load | Critical |
| Wrong part in a box labelled for another vehicle | Wrong pads fitted to a braking system | Critical |
| Approval or copper marking not backed by a certificate | Goods cannot be sold legally in that market | Critical |
| Disc runout or thickness variation over the limit | Vibration and customer returns | Major |
| Missing shims, clips or wear indicator | Noise; no wear warning | Major |
| Rust on disc braking faces | Needs cleaning before fitting; complaints | Major |
| Chipped pad edge within the agreed size; paint marks | Appearance only | Minor |
Market rules for replacement brake parts
UN Regulation No. 90. This regulation sets uniform provisions for approving replacement brake lining assemblies, drum brake linings, discs and drums. Its lining-assembly scope covers vehicle categories M, N, O and L; discs and drums cover M, N and O. Original parts fitted when the vehicle was built, and original replacement parts, are outside it. Approved linings must be sold in axle sets in sealed packs showing the approval mark and the vehicles covered, with fitting instructions. Each lining carries the approval mark and a date or batch. Each approved disc or drum is marked with the maker, approval number, traceability data, and the minimum disc thickness or maximum drum diameter. Approval is granted by a type-approval authority after tests. We do not approve parts; we check that marks match the approval documents.
US copper limits. Washington's Better Brakes Law (Chapter 70A.340 RCW) and California's Motor Vehicle Brake Friction Material Law limit asbestos, cadmium, chromium VI, lead and mercury, and require less than 0.5% copper by weight in pads from 1 January 2025. The industry LeafMark shows the level met: Level N (three leaves) meets all requirements, including the 2025 copper limit. Manufacturers certify their formulations using accredited lab tests. We check the mark and the certification against the formulation and edge code on the pads.
FMVSS 135. This US standard sets brake system requirements for light vehicles. It applies to the vehicle, which its maker certifies, not to a pad sold on its own. A supplier claiming "meets FMVSS 135" should explain which vehicle test that claim rests on.
Chemicals. In the EU, asbestos is banned under REACH Annex XVII. Origin marking follows the market's rules; see country of origin labelling.
What we witness and what needs a lab
Clear To Load does not own a brake dynamometer, a friction tester or a lab. On site, we witness the factory's shear test on pads we choose, its hardness test on discs, and its runout check, and we record the readings. We review the factory's compressibility, density and hardness records for the batch. Friction coefficient, fade and wear are type-test properties measured on a dynamometer by an approved test service or an accredited lab. Where you want independent results, we pull and seal pads or discs and send them through lab testing coordination. Our report says what we witnessed and does not present it as a type test.
Packing, transit and your purchase order
Discs rust within days in a humid container if the braking faces are bare. Check that faces carry the agreed rust preventive or that discs are packed in VCI bags, and that cartons are dry. Pads chip if boxes are loose. Heavy disc cartons crush lighter ones, so the inspector checks carton strength and the stacking plan.
Put these in your purchase order:
- Part numbers with vehicle application, drawing revision and the template or reference part.
- Friction formulation code, edge code and the approval and LeafMark documents for it.
- Thickness, runout, thickness-variation and hardness limits, and the minimum shear force.
- Shims, clips, wear indicators and hardware kits per box.
- Box artwork, approval marking, batch coding and origin marking.
- Rust prevention, pack per axle set, carton grade and pallet pattern.
- How many pads are to be shear tested in your inspector's presence.
Book an inspection anywhere in India.
Sources
- UN Regulation No. 90, replacement brake lining assemblies, drum brake linings, discs and drums (EUR-Lex)
- Washington State Department of Ecology: Better Brakes Law and LeafMark
- California Bureau of Automotive Repair: Motor Vehicle Brake Friction Materials
- 49 CFR 571.135, Light vehicle brake systems (eCFR)
- Department of Commerce, TradeStat: commodity-wise exports (HS 870830, FY2025-26 provisional)