A brake rotor is the friction surface the caliper squeezes to slow the car.
What a rotor does
A rotor contains nothing that moves by itself; it simply rotates along with the wheel. Nearly every application uses steel. It normally sits at the wheel hub (older Jaguars, for example, place it inboard of the suspension), and either way its only function is to provide a surface for the caliper to grab.
Before casting, makers decide on the metal recipe. The rotor is then machined to tight tolerances, and every one has to be balanced, otherwise it vibrates. The faces are machined close to perfectly flat: a variation of only 0.05 mm puts it outside spec. The rotor must keep its balance and finish through repeated clamping and heating to over 600 degrees F (roughly 316 C).
Vented and non-vented rotors
The majority of modern rotors are vented, meaning a pair of friction surfaces connected by vanes, which move air through the rotor to cool it. Non-vented rotors are still built, but only for the back of smaller cars that are not performance models. Exotic cars built for the track use ceramic carbon composite rotors that are vented too, and they are far lighter than steel with far better thermal behavior, essentially removing brake fade.
Matching the rotor to the driving
A rotor that works for commuting may not cope with hard driving, so start by deciding how the car will be driven.
Performance and race use mean repeated hard stops at very high temperatures. When a rotor overheats, the friction surface distorts into a wavy shape. You feel it as steering wheel shake and pedal pulsing, along with unpleasant noises. Extreme cases can lead to cracking, which is dangerous to drive on. Thermal stability is therefore crucial.
Metal make-up and weight
The chemical make-up is the most important factor. Iron is the main ingredient of a steel rotor, mixed with other elements including carbon for durability. Heat leaves carbon more quickly than it leaves iron, so more carbon means better heat shedding, which guards against warping and reduces how much heat reaches the brake fluid. This is important since the fluid can boil, and boiling means lost brake pressure. Iron is softer than carbon, so a higher carbon content gives more wear resistance and a longer service life.
Weight and size are secondary to the metal make-up, though they still count. A rotor is unsprung weight. Cutting unsprung weight helps the car accelerate, corner and brake, and no downside is noted.
Aluminum hats and floating rotors
Most rotors are heavy cast steel alloy. One answer is a steel friction ring paired with an aluminum center section, called the hat, the middle part that fits over the hub, riveted to the ring. It adds only a little to the price compared with all-steel, and it is used by Mercedes-AMG and BMW M.
Specialist brake makers build two-piece floating rotors, and Brembo is one. Zimmermann is another. The hat can be reused, while the friction rings are the replaceable part. The rings are attached with fasteners that hold them firmly while still allowing a small amount of movement, hence the name floating.
The floating design brings several benefits:
- Less heat moves from the ring into the hat, so the hub and wheel bearing carry a lower heat load.
- The ring heats up and expands independently of the hat, which lowers the risk of warping.
- Depending on the brand, the ring is more open near its base, so the vanes can move more air.
- A worn ring can be replaced without the hat, which keeps the cost fairly close to that of an OE rotor.
Blank, slotted, drilled and combined surfaces
| Surface | What it is | Where it fits |
|---|---|---|
| Blank | Plain friction face, free of grooves and dimples. The type most often fitted at the factory, usually the cheapest, readily available and made with higher-carbon alloys. | Spirited driving on back roads and autocross. For regular track driving, better options exist. |
| Slotted | Grooves milled into the friction face, in straight or curved patterns that vary by maker, that vent built-up gas. | Paired with high-carbon metal, arguably the top budget pick for light track duty. |
| Drilled | Separate holes drilled through the friction face. Often found on road-going European sports cars, including the Porsche Carrera, BMW M, Audi RS and Mercedes-AMG. | An improvement over blank for repeated driving on back roads. |
| Slotted and drilled | Both patterns on one rotor, sold by certain aftermarket firms. | With good metal it outperforms blank on track; on the road it adds only looks. |
| Multi-piece | An aluminum hat bolted onto a ring that can be replaced. | The top choice for a car that sees regular time on track. Only carbon ceramic beats it for thermal performance and low weight. |
Wearing pads give off a film of gas between the pad and the rotor; this is called outgassing. It can be strong enough to lessen contact between pad and rotor and hurt braking effectiveness. Slots and holes give it a way out. Some makers also specify holes to help disperse water.
Drilled rotors suit spirited road driving and autocross, though under regular track use they usually wear out sooner than slotted ones. When very hot, the surface crazes, meaning micro-cracks appear between the holes. Caught early it is not a disaster, but leaving the cracks alone causes problems on track. Replace the rotor once cracks have formed.
Some aftermarket firms, EBC among them, offer rotors that are both slotted and drilled. StopTech is another such firm. The combination retains the advantages of each pattern, with nothing extra.
You cannot get multi-piece rotors for every car; they are likely limited to the popular performance sizes, M3 and Carrera being examples. On a car driven purely on the road, the benefit beyond lower weight will not show.
What slotted and drilled rotors change
By themselves, neither slotting nor drilling shortens braking distance. A rotor takes in the heat that the pads generate and then gets rid of it. A larger rotor can take a larger pad, but the parts around it have to change too. Slotted or drilled is worthwhile only when the situation is right and the supporting parts match.
Drivers who spend time on track, drive aggressively on back roads or run autocross will notice a difference from slotted or drilled rotors over blank ones, in thermal stability, water dispersal and brake feel. For a daily driver those advantages count for less, unless the car gets driven hard.
Choosing them for looks is fine, though service intervals may shorten: the pads may wear sooner and the brakes may be noisier. A slight cutting effect can scrub contamination and rust from the pad surface after the car has stood in wet conditions. If you can live with pads wearing sooner, they suit daily driving.
Choosing the right size
Bigger is not automatically better. A rotor that is too small struggles with repeated stops. One that is too big runs at low brake temperatures and adds rotating mass, which also hurts stopping.
Like engine oil, rotors and pads perform best inside a particular temperature window determined by what they are made of. Bigger rotors and pads provide extra capacity to cool the brakes under repeated hard braking. A lighter, less powerful car has less mass and speed to deal with, so it needs less braking force and produces less heat. Oversized rotors paired with multi-piston calipers can leave the brakes below the temperature where they work best.
Fitting a cooler-range pad is possible in theory, but the oversize parts' extra unsprung weight stays. Sizing the system correctly saves weight and lets the brakes operate where they work efficiently.
Key takeaways
- Settle how the car will be driven before choosing a rotor.
- Metal make-up is the biggest factor, and higher carbon helps shed heat and resist wear.
- Slotting and drilling alone will not shorten stopping distances.
- Drilled rotors can craze under regular track use; replace them once cracks form.
- Multi-piece rotors suit regular track time, but are not made for every car.
- Match the rotor size to the car, since a bigger rotor is not automatically better.
General information only. Always check the specification for your own vehicle.