In short. For a car driven on the road, the brake makers we read do not promise a large difference between the two. Brembo writes that cross-drilled and slotted discs "offer comparable benefits in terms of performance", and that either can wear pads faster. The real differences are durability, pad wear and noise: AP Racing says grooved (slotted) discs last longer than cross-drilled ones and that drilled discs are less resistant to cracking, while a plain disc is what it lists for road cars where low noise matters. Fine surface cracks around the holes of a drilled disc can be normal; a crack that runs to the edge of the braking surface or from one hole to the next is not. Have it checked before you drive again.
What the holes and the slots are for
Both designs cut into the braking surface of a cast iron disc (called a rotor in the US). Brembo lists four effects for both, in its own support answer: they keep the pad surface clean by scraping off old friction material; they break up the film of water on the disc, so the brakes respond from the first press in the wet; they let out the gases the pad gives off when very hot, which otherwise reduce friction (fade); and they help the disc shed heat (Brembo, Max or Xtra). On its slotted Max disc, Brembo puts the fade point concretely: above about 800 °C, as on a long mountain descent, the resins in the pad burn and give off gas between the pad and the disc (Brembo Max).
AP Racing, which makes racing and road brakes, gives the same purpose in one line: grooves and sometimes cross-drilling "clean the surface of the pad and allow gases produced to escape", and in doing so change the friction behaviour, so the best pattern depends on the use (AP Racing, disc face types).
None of these effects is unique to one design, so the question is which costs less in cracking, pads and noise for the way you drive.
What the brake makers say, side by side
| Source | Drilled (cross-drilled, perforated) | Slotted (grooved) |
|---|---|---|
| Brembo, support FAQ | Comparable benefits to slotted; more common on sporty road cars because "not as harsh" on pads; machined holes can weaken the disc, so design and testing matter | Comparable benefits to drilled; more common in motorsport for water and gas expulsion; can also wear pads faster in some conditions |
| Brembo, Xtra and Max pages | On the most stressed applications, blind holes (dimples) instead of through-holes, for greater resistance to thermomechanical stress | One groove on the Max disc is a wear indicator: when it has gone, the disc is at minimum thickness |
| AP Racing | Cross-drilled "can compromise disc life"; radius drilled discs are "less resistant to cracking than grooved or plain discs" | "Grooved discs have a longer life than cross-drilled discs"; grooves give more consistent performance; recommended for track days |
| EBC Brakes | Slotted discs resist the cracking it calls common with through-drilled discs | Slots keep pad wear flat and parallel; wide slots make some air noise under heavy braking |
| BMW (M compound brakes, service bulletins) | Humming from ventilated, perforated discs at high speed is normal, with no safety risk; hairline surface cracks are normal, replace past 13 mm | Not covered |
Sources: Brembo FAQ, Brembo Xtra, Brembo Max, AP Racing, disc face types and disc choice, EBC, daily drivers (17 February 2026) and EBC FAQs, BMW bulletins SI B34 04 14 (2015) and SIB 34 10 19 (2021).
Each maker describes its own range: EBC's sport discs are slotted, Brembo sells both. Where they agree, the agreement carries weight. None publishes a stopping distance comparison between the two designs on a road car, and we found none, so we do not claim one stops shorter.
Cracking: what is normal and what is not
A disc heats and cools every time you brake, and holes and slots are machined into it; Brembo says they can weaken it, and AP Racing ranks drilled discs below grooved and plain ones for crack resistance. Brembo's answer is engineering per application: the number, size and position of each hole or slot are set for each disc, and on some of its most stressed discs it uses blind holes (dimples) rather than through-holes (Brembo, dimples).
Fine cracks on the surface are not, on their own, a fault. A BMW service bulletin for its M cars says visible cracks come from high thermal stress during rapid heating and cooling, that braking hard from speed on cold brakes makes them more likely, and that such hairline cracks "have no negative influence on the durability or braking performance". The same bulletin gives a limit: at a routine pad change, replace the discs if surface cracks are longer than 13 mm (about half an inch), or if the disc is below minimum thickness, always in pairs per axle and with new pads (BMW SI B34 04 14, September 2015). That figure is BMW's, for the models listed in the bulletin; for your car, the maker's service documentation decides.
The UN regulation on replacement discs (Regulation No. 90) shows what engineers count as damage. In its thermal fatigue test for passenger car discs, these end the test (UN R90, Annex 11, 4.1.1.1.3):
- radial cracks on the friction surface longer than two thirds of the radial height of the friction surface;
- cracks that reach the inner or outer diameter of the friction surface;
- a crack through any friction ring;
- any structural damage or crack outside the friction surface.
Our arithmetic on the same test: 150 cycles of two stops each, at 5.0 m/s² from the car's top speed down to 20 km/h, one stop every 70 seconds, each cycle starting at 100 °C, is 300 hard stops in 350 minutes, almost six hours of braking and cooling. A disc passes at 150 cycles without damage; below that, it is retested or compared with the original part. The test is the same for a drilled, slotted or plain disc.
As a driver: short hairline cracks are the kind the BMW bulletin calls normal. A crack that joins two holes, reaches the outer or inner edge of the braking surface, or shows on the hat or the vanes is the kind R90 counts as damage. Do not drive on it: have the disc checked before you take the car back on the road.
Pad wear and noise
Brembo says that, depending on the pad design and friction material, both drilled and slotted discs "may lead to faster brake pad wear". It adds that slotted discs are the ones used more in motorsport, while drilled discs are used more on sporty road cars because they are gentler on the pad material (Brembo FAQ). AP Racing makes a related choice on its own road kits: where it drills a road disc, the drilling is more restrained than on its race discs, "as pad longevity is more important on a road car than weight saving" (AP Racing, disc choice).
EBC states the counterpoint for slots: they keep pad wear flat and parallel and avoid the grooves that form on a plain disc (EBC). Wear rates in miles were not published by any of the three, so we cannot put a number on the difference.
Noise is the cost a daily driver notices first. EBC says its wide-slot discs make some air noise under heavy braking; it moved from five slots to three to reduce it, and says the noise drops over up to 2,000 miles of use and then stays (EBC FAQs). BMW tells its dealers that a humming or droning from ventilated, perforated discs when braking from high speed is normal and needs no repair (BMW SIB 34 10 19, 2021). AP Racing lists the plain disc as the one "mainly used on road applications where low noise is vital".
How much braking surface each design removes
Neither Brembo nor AP Racing publishes the surface lost to holes or slots, so here is the arithmetic on an assumed disc. These are invented dimensions to show proportions, not the figures of any car or product.
| Assumption | Calculation | Result |
|---|---|---|
| Braking ring from 330 mm to 230 mm diameter | π/4 × (330² − 230²) | 43,982 mm² per face |
| 48 through-holes, 8 mm diameter | 48 × π × 4² | 2,413 mm², 5.5 % of the face |
| 6 slots, 50 mm long, 2 mm wide | 6 × 50 × 2 | 600 mm², 1.4 % of the face |
With these assumptions the holes take about four times the face area of the slots, and they go right through the friction plate, while a slot only removes a shallow groove. Removed metal is also removed mass that would otherwise store heat. That fits what the makers say about durability. It does not show that one design stops worse: we have no measurement linking a few per cent of area to stopping distance.
What carmakers fit
Carmakers fit all three. Porsche introduced cross-drilled discs on the 911 Carrera 3.0 in 1974, saying the holes clear pad dust and water and improve response in the wet (Porsche Newsroom). BMW's 2021 bulletin, which covers M models from the M2 to the X6 M with M compound brakes as standard, has an entry on noise from their ventilated and perforated discs. AP Racing, for its part, describes the plain disc as the one mainly used on road applications.
Brembo's own warning applies to any replacement: because holes and slots can weaken a disc, choose a maker with proven experience in designing them.
Daily driving: which suits which use
Onyx Forged does not recommend a size or an offset, and does not recommend a brake disc either: the part is the one your car maker or a brake maker lists for your car. What the sources above say, use by use:
| How you drive | What the sources say | What to weigh |
|---|---|---|
| Commuting, town and motorway | Brembo: comparable benefits; AP Racing: plain discs where low noise is vital | Noise and pad wear count for more than fade at these temperatures |
| Lots of rain | Brembo: holes and outward slots both clear the water film | No published figure puts one design ahead |
| Long descents, heavy loads, towing | Brembo: both designs expel the gases behind fade, and slotted discs are the ones used more in motorsport; AP Racing: grooved discs last longer | Crack resistance ranks above looks here; the handbook gives the towing limits |
| Occasional track days | AP Racing, for its own kits: "For Track Days, Grooved discs are recommended" | Repeated heat is where the makers rank drilled discs lowest for cracking; pads and fluid matter as much |
| Mostly the look | Brembo: drilled discs give a sportier look | Open-spoke wheels show the disc; check the caliper still clears the wheel |
What the rules say, country by country
United Kingdom. The MOT manual has no item on drilled or slotted discs. Under 1.1.14 it fails a disc "significantly and obviously worn" (major) and a disc "insecure, fractured or otherwise likely to fail" (dangerous), and it tells testers that a disc must be significantly worn before rejection: being below the maker's limit is "not a reason in itself" (MOT inspection manual, updated 14 October 2026). In our reading, a crack that reaches the edge of the disc is the plainest case of "fractured"; the tester decides.
UK and Europe: approval of the disc. UN Regulation No. 90 approves replacement discs. It defines an "interchangeable" disc as one with the same interface dimensions as the original that "may differ from the original brake disc in terms of its design, material composition and mechanical properties" (UN R90, 2.3.3.5.1). In our reading, a drilled or slotted disc that replaces a plain original falls under that definition. Brembo states that its Xtra drilled discs carry R90 approval. EBC says the regulation applies legally within Europe, and that R90 requires the slotted disc itself to go through the tests, even when it is machined from a plain disc that already holds the approval (EBC, R90, updated June 2024). We did not find a UK government page giving the date from which discs, unlike pads, must carry the approval, so we leave that open.
United States. The federal brake standard for cars, FMVSS No. 135, is a vehicle standard: it "specifies requirements for service brake and associated parking brake systems" and applies to passenger cars built from 1 September 2000 (49 CFR 571.135). The list of standards in Part 571 has standards for brake hoses and brake fluid, and none for replacement rotors. Inspection is left to the states. Pennsylvania, for example, rejects a car when the disc is thinner than the minimum stamped on it or the maker's figure, when rotors are "scored deeper than .015 inch" (0.38 mm), or when there is "mechanical damage other than wear" (67 Pa. Code § 175.80, (7)). Holes and slots are not mentioned.
Australia. Western Australia's transport department states that modifications to the braking system "are permissible only where listed by the vehicle manufacturer as an option for the particular model", and refers substitute brake systems to Section LG of the national code VSB 14 (Transport WA, updated 25 February 2026). We could not confirm whether a drilled or slotted disc of the original size counts as a modification there; ask your state's registration authority before fitting.
Canada. We did not read a federal or provincial text on disc patterns for this guide. Provinces run their own safety inspections, so the provincial inspection standard is the document to check.
Checking drilled or slotted discs yourself
You can see a lot through the spokes of a wheel, but a proper look needs the wheel off. If you lift the car, it goes on axle stands on the jacking points in the handbook, never on the jack alone. Then:
- Look at every hole. Short hairline cracks can be normal. Lines that join holes, or reach the outer or inner edge of the braking surface, are cracks to have assessed before driving.
- Look at the slots. On a Brembo Max disc, one groove is a wear indicator: when it has gone, the disc has reached minimum thickness. On any disc, the minimum thickness is stamped on it or given by the maker.
- Check the side. AP Racing's kit discs are handed left and right; a disc marked for one side belongs on that side.
- Refit the wheel at the car maker's torque, in a crosswise order; the wheel nut torque guide explains why.
If the car shakes at a steady speed rather than only when braking, read the wheel balancing guide as well.
Do wheels change anything?
They change what you see, and sometimes what fits. An open-spoke wheel shows the disc face; a closed design hides it. If a disc change comes with a larger caliper, the wheel has to clear it: the brake caliper clearance guide explains how to check, and what size wheels fit my car covers the rest. Discs and wheels both count as unsprung mass; the unsprung weight guide puts that in proportion.
Frequently asked questions
Are drilled or slotted rotors better for daily driving?
For road use, Brembo says the two offer comparable benefits. The difference is in the costs: AP Racing says drilled discs are less resistant to cracking and grooved discs last longer; slots can be noisier, and both can wear pads faster. AP Racing lists plain discs for road cars where low noise is vital. The part for your car is the one its maker or a brake maker lists.
Do drilled rotors crack?
They are the design the brake makers rank lowest for crack resistance: AP Racing says so, and Brembo says machined holes can weaken a disc. Fine surface cracks around holes can be normal: BMW tells its dealers to replace M compound discs at a pad change only if surface cracks exceed 13 mm. A crack that joins holes or reaches an edge needs checking before you drive again.
Do slotted rotors wear out pads faster?
They can. Brembo says both slotted and drilled discs may wear pads faster depending on the pad and its compound. EBC says slots keep pad wear flat and parallel. None of them publishes a wear figure in miles, so the size of the difference is not known.
Do drilled or slotted rotors stop the car shorter?
We found no published stopping distance comparison on a road car. Brembo calls the benefits comparable.
Why do slotted or drilled rotors make a humming noise?
BMW puts it down to the air passages of ventilated, perforated discs. EBC says its wide slots make some air noise under heavy braking, falling over up to 2,000 miles; BMW calls the humming normal, with no repair needed.
Are drilled or slotted rotors legal, and will they pass the MOT?
The MOT manual does not mention holes or slots; it fails discs that are significantly worn, fractured or likely to fail. In Europe, replacement discs fall under UN Regulation No. 90. In the US, state inspections check thickness, scoring and damage. In Western Australia, brake modifications must be a maker's option for the model.
Do drilled and slotted rotors go on a particular side?
Some do: AP Racing's kit discs are handed left and right. Follow the marking on the disc or the maker's fitting sheet.
Wheels from Onyx Forged
Onyx Forged is a French brand of made-to-order forged wheels, designed to JWL/VIA standards. If you are changing discs or calipers at the same time as wheels, say so when you describe the car: the caliper is part of what the wheel has to clear. The designs are in the catalogue.
Sources
- Brembo Parts: slotted or perforated brake disc, which to choose; through-holes and dimples on Xtra discs; Max brake discs; Xtra brake discs.
- AP Racing: disc face types; disc choice for its road car brake kits.
- EBC Brakes: brake upgrades for daily drivers (17 February 2026); FAQs; ECE Regulation 90 (updated 21 June 2024).
- BMW of North America service bulletins, published by NHTSA: SI B34 04 14, compound brake complaints in M vehicles (September 2015); SIB 34 10 19, Motorsport vehicle brakes (8 September 2021).
- Porsche Newsroom, Porsche brake development.
- UN Regulation No. 90, consolidated text to Supplement 3 to the 02 series (unofficial consolidation) and Revision 3, Amendment 3 (22 February 2017).
- GOV.UK, MOT inspection manual, section 1: brakes, 1.1.14.
- 49 CFR 571.135, FMVSS No. 135, light vehicle brake systems; 67 Pa. Code § 175.80.
- Department of Transport, Western Australia, simple modifications.
All pages were read on 16 October 2026.
The Onyx Forged design office.