Question about disc rotor design?
 

I have several bikes with disc brakes, and looking at all/most of them, none have a rotor that I would have designed.

Look at motorcycles and cars. The pad is in full contact with the rotor under braking. As in, 100 percent of the pad surface is contacting the rotor top to bottom. The higher performance vehicles have cooling/lightening holes in the rotors.

Now look at bicycle rotors. They have friggen windows in the braking surface. Some have small holes but a lot have gaps built in. The width of the rotor sometimes goes from narrow to wide in some models (Hope). The contact is opposite what motorized vehicles have. 100 percent of the rotor surface under the pad is in contact but the pad is rarely contacting the rotor with it's entire surface area. On some setups, this sounds like a light chatter while braking, like the sound of a grasshopper flying. On other setups, the rider won't hear or feel anything different. In either case, it's not terrible. Maybe the pads wear out quicker but maybe all the opening keep the pads from getting glazed.

Performance wise, my little lizard brain seems to think that more rotor touching the pad will mean better braking. Lightening holes would be the only modification for heat dissipation and extra bite. That's not what the selection of rotors available to purchase would seem to indicate. It's safe to assume to that every component manufacturer has done far more research into the efficacy of their rotors than I have. I'm not making a statement about how all the companies are doing it wrong. I've never had anything less than excellent disc brake performance. I'm just curious how they arrived at a design that seems counter-intuitive.

A simple google image search will give you examples of exactly what I mean.

I'm not here to re-litigate whether or not you like disc brakes. Save your cries for attention for your therapist.

I'm not an engineer; but my guess would be that there's a difference in rotor designs because there's such a difference in work loads. A bicycle disc brake has to stop a few hundred pounds at most, while an automotive disc brake has to stop a ton or two.

You can ponder all sorts of things about them. But if you go out and try any of them, I'm sure they work quite well. Just make sure it's the rotor for your brakes and the type pads you use.

I'd bet the bike rotors are designed to be lighter as well, that's not a consideration on a motor vehicle.

Ive never been one to simply accept. It’s a different kind of life philosophy but it serves me well.

I don't simply accept things either. But when there are all sorts of brake rotors out there as you describe. And no one of them in particular being flamed with bad reviews or recalls and such, then I tend to think the designers must have done their due diligence and my musings are just that, musings.

Bike brakes have to be thinner and lighter than other applications, so that likely has something to do with the design. They also do not have air space between the braking surfaces to aid in cooling so they have to get their cooling air elsewhere. Here is a picture of a car rotor that shows those details.

https://cimg6.ibsrv.net/gimg/bikefor...1c724eed4.jpeg
I don't know jack**** about this stuff, but can see the design differences and what the designers try to accomplish.

They stop my lead ass just fine when going down the backside of alpine climbs... and I ride the brakes a bit. 20 mile long downhills...

Car disks are ventilated between the surfaces but bike disks are a single layer. They need cutouts for ventilation and cooling.

The shape of the cutouts is not very important as long as it's regularly spaced within the size of the pad, and scrapes the entire pad surface about the same amount. You could accomplish that with straight channels or a series of holes. So then it's down to styling. You should not dismiss styling as a good reason when there's little engineering advantage or disadvantage. It's really important in selling stuff to you and your enjoyment of it.

Your bicycle disk rotors are not going to get heated up to 500 degrees in normal use, or as high as 1000 degrees in competition. Motor vehicle disk brakes are an entirely different animal.

I question those numbers. I’ve melted a hole in my gloves, more than once, when I reached down to see if my brakes were hot after a long descent.

Nylon melts at 515 F.

I’m 140lb and average to average + in my aggressiveness. I’m sure there are people over 200 who are absolutely sending it. Probably on the same brakes I’ve got.

I don’t think it’s unreasonable for a bike rotor to be close to 1000 degrees in the right conditions.

A race car rotor is probably even hotter though.

Indeed. Incandescent discs:

https://cimg1.ibsrv.net/gimg/bikefor...35433d699d.jpg

- while 600 - 700°F may be time to re-evaluate what you've got bolted to those tires between you and everything else you're seeing pass by your eyes....

https://cimg6.ibsrv.net/gimg/bikefor...558e4e7aa.jpeg
https://cimg9.ibsrv.net/gimg/bikefor...f69263b7e.jpeg

On cars, the disc brake rotors are in slight contact with the brake pads even when the brakes are not applied at all, to keep the braking interface relatively clean, at the expense of some brake drag.
On bikes, the disc brake rotors are not in contact with the brake pads when the brakes are not applied, to avoid brake drag, and thus have gaps to remove surface contaminants on the brake pads.

Can be as low as 375°F.

Too, most of the bike disc brake rotors I've encountered are aluminum so they 'lose' heat very, very rapidly. Motor vehicle rotors aren't the same animal, more likely iron or ferrous alloy composition, slower to lose heat to air, or carbon / ceramic, not something common to too many bikes.

Bicycles have a steel rotor, usually riveted to an aluminum frame to mount to the hub. I recon they’d last a few days at most of the braking surface was aluminum. Totally different forces than rim brakes.

Theres always that one guy on a group ride. Thhht Cht Thhht Cht Thhhht Cht…

Race car rotors are sometimes made from carbon-carbon or from carbon-SiC, both aerospace composite materials that are pretty fancy. It’s hard to call them high tech now, as they have been around since the 1970’s, but they are beyond anything used in the mechanical components of a bicycle.

I did the math a while back. The surface area of 160mm disc rotor v/s a 622mm diameter brake track (Left and right side)...The rotor had about ⅐-⅛-ish the area with which to dissipate the same workload as the rim brake. Consequently disc brake cooling is a much bigger concern. All the holes/cutouts, etc...are additional area to dissipate heat. The inside edge of each hole is a radiating surface as thick as the rotor.

Within reason it doesn't really matter so much how quickly or how much the rotor can accept heat. After all, we have no intention of storing it or utilizing it elsewhere :foo:. The only performance a bicycle is interested in is how quickly it can shed it. We can always grab the brakes harder if needed and/or enjoy the nice progressive response curve between lever and action due to the multiplication afforded by hydraulic forces that get the same work out of ⅐ the surface to do the work on. Higher performance rotors tend to be more spider-webby &/or be combined with cooling fins, heat conductive materials and other heat shedding/mitigating strategies to that end. (Obviously total capacity matters. But it's a lot like filling a bathtub without the drain plug. The better the heat shedding, the bigger the energy drain hole out of the system is.)

The waviness of the rotors outer edge in all it's stylized forms are to help sweep mud from inside the caliper so it remains clear in filthy conditions.

OP: I've wondered about the same things.

Early bike discs were similar to performance autos; lots of cooling holes, and if slots, like they do on cars, the slots are angled to be like a spiral, not perpindicular to the pad surface. On recent discs I have seen, the cooling holes (polygons) seem excessively large, with edges close to radial, so abrupt transition with the pads. I would think that wears pads fast, as it's not only friction, but possibly some shearing.

Regarding outer periphery shape not engaging with pads: That could be just styling (which happens more than you think on what should be engineered components), or additional non-swept heat sink, or there to prevent resonance; Treaded car tires, have the tread blocks and spaces angled different to one another by a tiny amount, to not have them all generating the same noise and vibration frequency when rolling at high speeds, to reduce perceived noise.

This guy did some tests.

He got a whopping 180 degrees F max temp on fairly hard descents.

And the temps dropped to ambient within 20 seconds.

Even if his data is off by a factor of 3, or someone is bombing 2-3x harder down the hill - over 500 degrees seems unlikely.

Not sure its any concern for us regular folk.

My bicycle shipped with 160 mm diameter X 1.8 mm thick TRP rotors. The front brake had excessive squawking when heating up going down steep grades. When asked the bicycle manufacture told me I could run the largest rotors that would fit with my bicycle. I put a Magura ebike rotor that is 2 mm thick and 180 mm diameter on the front. Because of a tapered fork that is the largest that will fit in case the rotor has any bend. I put a Magura Storm Rohloff rotor on the rear. It is 2 mm thick and 203 mm diameter. That was 10 years ago, I was looking at parts today and saw rotors that are 2.3 mm thick. I no longer have the squawking. I have better more confident braking. I have frame couplers, so mechanical disks are used on my bicycle. Compared to other bicycles I have owned, the rim brakes were better performers. I changed from The TRP Spyre to the TRP HY/RD. They worked better until they didn't, meaning they couldn't take the heat going down grades and blew out the cylinders. I currently have the Paul Klampers. They stop better than the HY/RDs when they were working. So when I replace my rotors It will be with the thicker rotors. When it comes to disk brakes on bicycles light weight just means under engineered to me.

Ok, that’s fine. It’s wrong but it’s fine.

Water boils at 212F. Even if you’re in Leadville CO, it’s going to boil at 194F. I’ve heard water sizzle on rotors when squirted. I hear people squirting their rotors all the time after descents. The sizzle is real.

I googled that nylon melts at 515. The other guy said 375. I don’t know what my gloves are made of but it’s not wax, butter, or popsicles. And the rotor melted a hole clean through them.

This was in Colorado from Leadville to FairPlay on rough roads. At least my rear rotor was 140, I can’t remember the front. This was on SRAM Rival brakes. It got hot!

The same bike now has 160 front and rear and Campagnolo brakes, it stops so much better that I wouldn’t expect it get as hot if I did it again.


Last fall on the east side of Bend, OR I melted the edge of a glove after a very white knuckle off road descent (I also fractured my collarbone on the same ride, but that’s probably unrelated). That was on 180’s and Magura’s.

I wouldn’t be surprised if it takes more than 175 degrees to pop a tube on a rim brake. Do you remember those days? I do.

Acknowledging the thread drift, I have a cycling friend with a burn scar on his leg in the detailed image of his mtn bike disc rotor. No idea how long between stopping and coming into contact, assume not long to cause the 2nd degree burn. Hot? Youbetcha.

Have three disc brake bikes and they all do a fine job, each using a different rotor design. I don't favor one over another from an efficiency or noise standpoint. Happy they don't generate copious dust like some car brakes I've used, but that's pad compound and not the rotor. Light is good, so long as they don't warp—the two-piece ones with an aluminum center and SS rotor seem on par with one-piece steel while reducing weight. If replacing one worn thin, I get the lightest I can find.