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Why are disk brakes not 100% round? Why is the outside edge "rough"?

Here's a pic of what I'm describing: enter image description here

  • There are some which are circular or this. – Batman Sep 18 '16 at 20:59
  • +1 Welcome to Bicycles SE. Good first question. I suspect it has something to do with heat dissipation, but I'm not sure. – BSO rider Sep 18 '16 at 21:03
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For the brakes to self clean and remain effective, the surface of the pad must transition from metal to void at least once every revolution across the entire pad (i.e. no continuous line of metal). The 'sawtooth' rim achieves this. Some very cheap disks do not achieve this.

You may find the very good answer by @trailmax in this question useful.

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    That can't be true, or car disk brakes wouldn't work. They have contiguous rotor surfaces. – Fake Name Sep 19 '16 at 4:31
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    @FakeName really? A quick image search shows a lot with holes and a few without. I suspect that this might be another case where bicycles are not the same as car? – Móż Sep 19 '16 at 4:47
  • Car brakes build up a lot more heat (drying effect) and power assisted callipers. The disks are more protected from mud and water compared to bike discs. Motorbikes also have holes in discs, but not as many as bikes as the the energy involved in motorbikes (therefore strength and heat build up issues) is much more. – mattnz Sep 19 '16 at 4:54
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    @Móż - The great, great majority of car disk brakes are not drilled/slotted (a.k.a. "vented"). Drilling/slotting is done on performance brakes, generally to improve heat dissipation, and a few other reasons (see the wikipedia page on disk brakes. It's worth noting I've never heard of the holes or slots being for "self cleaning". Wikipedia on bike disk breaks also has nothing about the pad cleaning action. – Fake Name Sep 19 '16 at 5:38
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    AFICT from a brief overview, holes are done to : 1. Improve cooling, 2. Reduce Weight, 3. Improve performance in wet conditions by allowing any water/mud/etc to leave the gap between the pad and rotor more easily, and that's about it. – Fake Name Sep 19 '16 at 5:43
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I'm relatively confident in saying that bike disk brakes have such outlines because it makes them look "cool" or "fast".

The great, great majority of car disk brakes have no holes, and they work fine. "Performance" car disk brakes have holes and/or slots, but they are absolutely not required.

The general reasons for holes or slots in disk rotors are:

  1. Improvement of cooling, by increasing surface area and breaking up laminar airflow, as well as (for car rotors) adding an additional airflow path.
  2. It can serve to reduce the rotating weight of the brake disk.
  3. In wet or dirty conditions, it helps water or dirt escape from the brake-pad brake-disk interface, which improves braking performance in such conditions.

As such, while holes or other shapes cut out of the actual brake surface are reasonable, I can't think of any good reason for the non-circular outline, except perhaps some minor improvement in cooling due to it acting like a blower fan a little tiny bit.

Note that this is not a vented rotor, but rather a cross-drilled rotor. Car disk brakes do have internal ventilation, but it runs parallel to the brake surface, rather then through it.

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  • Car disc brakes are typically vented in the front, even on low end cars. Rear may be drums on lower end cars, followed by solid or vented discs. – Batman Sep 19 '16 at 18:38
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    @Batman - Vented or cross drilled? Because most car disk brakes have internal vents, but are not cross drilled. The disk brake in the OP has cross-drilled (milled) slots, but it's not vented in the way card disk brakes are. – Fake Name Sep 19 '16 at 21:13

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