Technical support

Material, process, tolerance

What the disc is made of, how it is processed, and the numbers we hold. Written for workshops, distributors and engineering teams who need to compare like for like rather than read a brochure.

1. Material

The friction ring and the hat do different jobs, so they are made from different materials and joined by floating bolts. The ring has to absorb heat and resist cracking; the hat has to stay stiff, locate the ring concentric with the hub, and keep heat away from the wheel bearing.

Friction ring

G3500 high-carbon grey cast iron. The graphite flakes damp vibration and blunt crack tips; the carbon content gives the ring thermal capacity, so it takes longer to reach the temperatures where pad fade starts.

Hat

6061-T6 aluminium, turned from billet and black anodised. Aluminium carries far less heat into the hub than a one-piece iron disc, and the anodised skin resists corrosion at the mating face.

Floating bolts

Carbon steel, coated. They locate the ring radially and carry drive torque while allowing it to expand as it heats.

Anti-rattle clips

Stainless spring steel, fitted between ring and hat to stop the ring chattering at low speed.

Coating

Geomet over the whole disc, neutral salt spray tested to 120 hours. It protects the part in transit and storage, and clears from the friction face once the brakes are used.

2. Heat treatment

Every disc is stress-relieved in a 650 °C furnace for 24 hours before any machining takes place. Castings arrive with internal stress from cooling; if that stress is released after the faces are turned, the ring moves — a few hundredths of a millimetre is enough to feel through the pedal.

Treating before machining means the metal is dimensionally stable when it is cut, and stays that way through heat cycles in service. The discs go through the furnace as a batch load, cool slowly, and are then machined, so the tolerances below are measured on a ring that has already finished moving.

3. Process

Casting

Ring cast in G3500, hat machined from 6061-T6 billet. Castings are cleaned and inspected for porosity before heat treatment.

Machining

Both friction faces turned in one operation to hold thickness variation; outer diameter, centre bore and bolt PCD cut; slot or drill pattern added; all edges chamfered.

Balancing

Each assembly is dynamically balanced, with correction made by drilling rather than by adding weights that can come off.

Assembly

Floating bolts torqued to the set figure with thread lock, anti-rattle clips fitted, hat-to-ring joint checked for full seating.

Inspection

Runout and thickness variation measured on the finished assembly; balance recorded.

Coating

Geomet applied after inspection, then packed.

4. Tolerance and inspection

Runout is measured on the friction face with a dial gauge, with the disc bolted to a hub that has itself been checked. It is the number that decides whether the driver feels a shake through the steering wheel, and it is the first thing ruined by dirt trapped between hub and hat.

Thickness variation is measured with a micrometer at eight points around the ring. Two friction faces that are not parallel pump the pads in and out once per revolution; the driver feels it as pedal pulsation and usually misdiagnoses it as a warped disc. Holding 0.0127 mm — five ten-thousandths of an inch — is what keeps that out of the car.

Balance is corrected on the finished assembly, not on the bare ring, because the hat and the bolts contribute. Values are checked on every unit rather than sampled. If a disc arrives outside these figures, stop before fitting and contact us: the numbers are the specification, not a target.

5. Packaging

Rotors are supplied as a handed pair, marked L and R on the carton and on each hat, with corner protection and the friction faces separated so the coating is not rubbed through in transit. Report any carton damage within 7 days of delivery, with photographs taken before the part is fitted.

6. Limits

These are road and occasional track discs, not a competition consumable. Repeated hard stops from high speed, a pad compound outside its temperature range, or machining a scored ring will all take the disc outside the numbers above — and none of them are covered by the warranty.