Do disc die-cut designs drastically decrease drag?

[rain dogs]

A less alliterative way of asking what I am curious about is: if all else is equal (diameter, material, construction), how to the cutouts on the rotor influence stopping power?

More cutouts = more braking power?
More cutouts = same braking power?
More cutouts = less braking power?

Friction isn't increased with surface area due to the reduction of pressure over the larger surface area, but perhaps the cutouts do something?

Maybe they only increase cooling and nothing more?
Is it just weight savings?
Durability?

Nobody seems to talk about this. There are disc rotors like these on the market. What's the stopping power difference if any?

[batman1425]

The friction v. surface area normalization breaks (no pun intended) down as contact area gets small. I'm not sure how small is small enough for that to occur though. This might still be well with in the realm.

More slots will likely improve cooling, but there is also less material to act as a heat sink to draw energy away from the pads, calipers, and fluid. At bicycling speeds, limited heat capacity of the rotor may play a bigger role in brake performance than the benefits of better air cooling. Shimano seem to have taken a firm stance on this with their icetech stuff - favoring heat transfer away from the caliper with extra material and fins to dump that heat more efficiently.

[benb]

I think it's all weight weenie...

There are no rotors like this in other applications.

I am not a fan of changing rotors away from what the system was designed for personally. Shimano or whoever set up the calipers + pads & piston ratios for a specific rotor design for a reason. If the rotors change it is most beneficial to consider the other aspects of the design as well.

Bikes have a really high requirement for light weight. Not a lot of requirement for heat dissipation because we are very light weight and can't accelerate to great speed repeatedly. Not a huge requirement for large brake forces because of the light weight. So we can get away with rotors with more material drilled away.

Hard to say if these designs have better heat dissipation than others.. lots of air for more cooling but there are likely other aspects to a good heat dissipation design.

edit: The rotor design also apparently influences the amount of noise the brake makes, and the rotor design supposedly needs to match the intended pad material.

One of the rotors in the OPs post (all silver) looks like it's the Hayes rotors that came with my MTB's brake system, that is a sintered/semi-metallic setup so different than most Shimano designs. IIRC when they're designing a system for sintered or semi-metallic pads they want the rotor to heat up faster to get to the best operating range for those pads. Organic pads work best at a lower temperature I think.

One thing I noted a few years back when road disc started to be a big new thing.. a lot of the "The sky is falling, discs suck!" articles from bike journos involved them assembling their own systems with calipers from brand A + pads from brand B + rotors from brand C, seemingly in an attempt to go weight weenie but they built a mismatched system that had not been designed/tested as a unit.. then they had all kinds of problems with overheating, etc... I don't think that helped anything.. when you get a pre-assembled system on a bike they don't do that and there are way less surprises. This is mostly bike insanity. You would not swap stuff like this willy nilly on a car.

[MikeD]

Don't know why rotors have cutouts. Seems to me they would act to chew up your pads, like a cheese grater grates cheese. I don't see holes or cutouts on my car rotors.

[Mark McM]

Originally Posted by MikeD

Don't know why rotors have cutouts. Seems to me they would act to chew up your pads, like a cheese grater grates cheese. I don't see holes or cutouts on my car rotors.

There's one school of thought that rotors with cutouts result in faster pad wear in muddy, gritty conditions. It is thought that the cutouts can trap mud and crud, which grinds down the pads.

There's another school of though that the cutouts improve braking under high brake heat. It is thought that pads can out gas under high temperature, creating a thin layer of gas between pad and rotor, reducing the effectiveness of the pad/rotor friction. The cutouts are thought to help vent any layer of gas.

I have no idea if either of this is true.

[David Kirk]

As Mark states above some of it is to allow grit a passage away from the pad/rotor interface.....and it also allows for the pads to gas-out and not float on the hot gases reducing grip.

It also means less mass at the braking surface....so it cools off faster. Now it also heats up faster and higher so it might be a net zero deal.

The curved arms are designed to allow the disc to swell when it gets hot while still running true. Some rotors are prone to running clean when cool but then coming out of true and rubbing when they get hot.

Lastly - one can not overlook the huge factor of aesthetics....I can only imagine the back room battles between marketing and engineering with stuff like this. The simple fact of the matter is that much of the bike biz has become a fashion business over time and true function is not the primary goal. I get it - companies need to move the SKU's and people buy stuff based on the looks more than anything else. Sell the sizzle not the steak.

dave

[NHAero]

I had some KCNC very lightweight, lacy rotors on my Anderson when it still had mechanical calipers, and the front, which was 160 and does most of the braking, was really noticeably poor. A more robust slightly heavier rotor made a world of difference. Now that I have hydros, I'm tempted to re-install it and see if it works better - it would match the 140 that I left on the rear.

[kingpin75s]

I ran the Ashima ones for awhile about 8 years ago on a few bikes.

Replaced them all with the stock BB7 rotors after awhile.

Do not remember the specific issues and do not think they were significant, just did not like them as much as the standard rotors.

The weight savings and cool looks were not worth the performance change for me. I think the standard rotors just braked better for me. I do not have long sustained downhills in the midwest so heat dissipation is not an issue. We also close our trails after the rains so I only ride in the mud when I traveling for races.

[Buzz Killington]

I’m not answering the question here, but sharing this:
I’ve been told that one of the reasons rotors warp is due to the difference in temps between the disc where it’s clamped by the pads and where it is not. For example, you do a loooong downhill on your bike. At the bottom, you stop and keep the brakes squeezed for whatever reason. The disc part under the pads will cool less quickly than the disc exposed to air. That difference in cooling will cause the disc to warp. Or so I’ve been told.

[Davist]

Originally Posted by benb

I think it's all weight weenie...

There are no rotors like this in other applications.

I am not a fan of changing rotors away from what the system was designed for personally. Shimano or whoever set up the calipers + pads & piston ratios for a specific rotor design for a reason. If the rotors change it is most beneficial to consider the other aspects of the design as well.

Bikes have a really high requirement for light weight. Not a lot of requirement for heat dissipation because we are very light weight and can't accelerate to great speed repeatedly. Not a huge requirement for large brake forces because of the light weight. So we can get away with rotors with more material drilled away.

Hard to say if these designs have better heat dissipation than others.. lots of air for more cooling but there are likely other aspects to a good heat dissipation design.

edit: The rotor design also apparently influences the amount of noise the brake makes, and the rotor design supposedly needs to match the intended pad material.

One of the rotors in the OPs post (all silver) looks like it's the Hayes rotors that came with my MTB's brake system, that is a sintered/semi-metallic setup so different than most Shimano designs. IIRC when they're designing a system for sintered or semi-metallic pads they want the rotor to heat up faster to get to the best operating range for those pads. Organic pads work best at a lower temperature I think.

One thing I noted a few years back when road disc started to be a big new thing.. a lot of the "The sky is falling, discs suck!" articles from bike journos involved them assembling their own systems with calipers from brand A + pads from brand B + rotors from brand C, seemingly in an attempt to go weight weenie but they built a mismatched system that had not been designed/tested as a unit.. then they had all kinds of problems with overheating, etc... I don't think that helped anything.. when you get a pre-assembled system on a bike they don't do that and there are way less surprises. This is mostly bike insanity. You would not swap stuff like this willy nilly on a car.

Motocross rotors look very similar to me, agreed on your other points, though.. I heard about "glazing" of the pads from heat from moto days, I had thought that's the reason around some of the vents.