Braking on hills: scrub off speed vs riding the brakes?

[Jason Knight]

this is why Shimano makes ice tech rotors aluminum in the core and fix to help cool them

Now THAT is a smart design!

And sadly smart design seems to be really rare in bicycle tech. Far too often it feels like the mass manufacture stuff is ice skating uphill in a mix of either completely failing to grasp basic physics, or simply cost cutting because "It's just a bicycle"

 

[Jason Knight]

Almost forgot:

This is why sports cars and race cars have huge brake rotors. Its also why things like alloy hats on 2-piece rotors decrease the severe-duty effectiveness of a brake rotor because they reduce the mass that can absorb heat (their benefit is reducing rotating mass).

Uhm... no. The larger rotors increase leverage for more powerful -- and controllable -- braking, with less friction, for less heat, whilst increasing the surface area to allow for faster cooling. The surfaces on truly high performance rotors also get thinner to improve heat dissipation which is why such vehicles burn through them faster. Basically applying the same amount of total energy over a larger area, resulting in lower overall temperatures.

Blindly dumping more heat in whilst reducing its ability to cool is just stupid, nonsensical, and reeks of "that's not how this works, that's not how any of this works!"

So I don't know where you got that idea, but it's utter poppycock.

Did a little more research, and the steel ring with aluminum sink and/or hub seems to really hit its peak with Shimano and Dura Ace's offerings.

Shaping them like fan blades to drive more air through? That's exactly what I'd have done if I were designing these things. Ok, so at least SOMEBODY making bike parts is thinking.

 

[m@Robertson]

I really have to question if one square inch -- or less -- of ceramic impregnated resin is "highly insulating" given they seem to get just as hot.

I don't know what to tell ya. Open air vs. none. Its an obvious issue that only goes one way in terms of effect. If you do some research you're going to see its a common issue addressed both in cycling - particularly on MTBs - and *big time* in performance / track automobile use.

In a material that has absolute garbage heat absorbing, storage, and dissipation. Irregardless of the material though, the thinner the rotor the more surface area per mass, thus the faster they can cool. Bigger thicker rotors cool slower because of their reduced surface to mass ratio. Just as thicker rotors should be more prone to warping as they cool akin to a bad temper, such as the difference between a nail and a shoe. I was a farrier's apprentice for a bit, so I know a wee bit about metalworking, forging, etc.

You are mistaking cooling capacity for braking capacity. A braking system works by converting forward momentum into heat. A brake rotor is the heat sink. More mass in the brake rotor means more capacity to absorb heat. More mass makes a rotor better at its job. Cooling is an entirely secondary function and to enhance cooling you do not want to reduce the capacity of your braking system. Cooling features are additional to increased rotor mass. On an automobile, you see this done with, say, post vanes between rotor surfaces vs. a solid rotor. Or directional vanes to take that a step further up. But when doing that (cooling vanes inside of a rotor) you do not make the rotor thinner. You beef it up. You make it larger so there is still enough mass to perform the braking function.

I wasn't a farmer's apprentice but I did field an amateur race car for about four years and I went through about three generations of brake redesigns, working in conjunction with Wilwood and one of their major resellers to try and get a 4000 lb streetable car to be competitive (4000 lbs is ridiculous for a track weight and nobody knew how to stop the thing until we worked it out the hard way). I also work in an industry in partnership with a major sports car manufacturer and I've had the chance to sit down with one of their engineers for an extended discussion about exactly what they are thinking when they design their track-ready braking systems. So... while this is not about bicycles it does give me a bit of background in how this sort of thing actually works vs. what is nonsense (ask me sometime how much cryogenic treatments on brake rotors improve their performance. Spoiler alert its zero).

Have you tried torching it to relax the metal?

Its a $25 rotor so if it was compromised I would just replace it. As it stands, the warp only spans the gap in the pads, is not audible and does not cause a hot spot when brakes are disengaged so far as I can tell. It offends my OCD though.

Though what you were quoting wasn't referring to "bedding them in" I was referring to during a normal everyday ride.

Yes and I think everyone (else) knows that. I mean come on... if you have a steep hill, obviously you lean hard on the brakes.

You utterly, totally, and completely misunderstood what I was saying. Which seems to go hand in hand with your not seeming to understand much of anything I'm trying to say.
I was not referring to a normal stop, I'm referring to where you're halfway down a hill and HAVE to stop because your brakes are losing grip and/or blowing smoke. How the bloody hell are you supposed to continue going down "gently" to let things cool off when you're halfway down a hill and they're overheating? That's not just directed at you either. This "oh just slow down to let them cool" s*it doesn't mean anything in places where you're choices are stop completely or go 40+.

As I said before you are getting all worked up here and not reading carefully. You never said any of the things you just did above about extreme use. Go back and look. You just told people to pull over and let their brakes cool. Which is a bad idea. Duh of course if you have to lean on the brakes, you do. And look up I said that already.

I don't use the Shimano super whoopee 2-piece rotors for a simple reason: They are in my mind ridiculously expensive and I can get equivalent performance (at least) with simple $25 downhill rotors. This is the same reason I do not buy the new Magura 2-piece rotors that are also shiny and pretty: they cost stupid money.

 

[m@Robertson]

Almost forgot:

Uhm... no. The larger rotors increase leverage for more powerful -- and controllable -- braking, with less friction, for less heat, whilst increasing the surface area to allow for faster cooling. The surfaces on truly high performance rotors also get thinner to improve heat dissipation which is why such vehicles burn through them faster. Basically applying the same amount of total energy over a larger area, resulting in lower overall temperatures.

You are out of your league and don't know what you are talking about. Big rotors are there because they are big heat sinks (sure there's more to it than this but you obviously don't understand the core issue). This right here illustrates the difference between a keyboard expert and someone who is actually out there on the race track, learning what works, paying for what doesn't and improving as he goes along. Thats not you.

 

[The Brick]

Big rotors are there because they are big heat sinks (sure there's more to it than this but you obviously don't understand the core issue).

LOL. You're clueless.

 

[m@Robertson]

LOL. You're clueless.

No. Experienced. And intolerant of bench racers claiming nonsense. I will admit that what I'm the most adamant about being dead-silly wrong is in the automotive world. Thinner rotors being preferable for instance. That set me off as patently ridiculous and a clear sign of no actual experience backing up the words. Here's a quote that just touches on this:

Braking relies upon friction, which in turn increases the temperature of brake components, particularly the rotor and pads. According to Brembo, during heavy braking (i.e., during turns), rotors can get as hot as 1800° F so it is critical that they do not overheat to the point of failing. Thicker rotors with enlarged ventilation channels along with beefed-up calipers are braking system features customized to short track events.

Braking systems for road courses are similar to those employed on short tracks because the turns are sharp, and the straightaways (which allow for the air cooling of braking components) are shorter. However, because road courses have numerous turns and thus more straightaway portions, the ventilation channels on the rotors are typically smaller because more air cooling can be harnessed.

My experience is almost all on road courses. Mostly in California. Laguna Seca, Thunder Hill, Willow Springs, Auto Club Speedway, Buttonwillow, Sonoma. The recipe for an amateur racer putting big brakes on a streetable car was big rotors, 1-piece so they would survive more than a day or so and be replaceable for less than stupid money (I was using slotted rotors that cost only $95 each and lasted 3-4 days). alloy-hatted, cryo'd, lighter rotors died after Day 1 with no performance benefit. In fact the brakes got so hot the alloy hats couldn't take the heat and got wobbly.

So you use larger diameter rotors. Thicker not thinner because the thinner ones were proven to be unable to take the beat-down. This is what happens to a thinner, 2-piece, 'performance' $350+ rotor after one day (less than 2 hours, really) on Laguna Seca. And see the heat-scarred alloy hat up near the top? It wobbled. Temperature paint on the rotor at the root next to the hat showed I was hitting around 1250 degrees (1700 out at the edge) and thats too much for an alloy hat.

So 1 piece and thick (25 lbs each vs. about half that for the 2-piece). Big 6-piston calipers so big they can take the heat without deforming/spreading (tried the lighter ones and they couldn't take the punishment). Iron hats on a 1-piece rotor gave more mass to accept more heat. Titanium pad backing plates and phenolic-injected pistons to help limit the spread of heat into the caliper and fluid. And brake ducts facing only into the center of the rotor to impel air into the space between (center to edge, not pointing at the inner rotor face) were good for a 200 degree rotor temp reduction. With powered inline bilge fans force feeding more air, that benefit increased by an additional 150 degrees.

So yes its a complicated subject, and I saw a profoundly flawed take on it, which I chose to rebut short/sweet. I think that was the right call as really we've gone way beyond the scope of the thread, crossed into useful side info ... and have now gone way past that. I'm out.

(whats missing in this pic of the engine bay? motorsport without a sponsor is not for the faint of heart)

 

[The Brick]

Wow. I'm impressed. You quoted Brembo.
Brake checking. Hardly a problem.
I raced at those tracks also, and wrecked some too, plus was an instructor, and was involved in a few of the professional series that visited some of those tracks. And, I've raced at other tracks.

Bigger rotors provide a greater moment arm. So, an equal amount of clamping force applied to larger diameter rotors, compared to smaller diameter rotors creates a larger moment (Force x moment arm). Heat dissipation is a factor but so is metallurgy, air flow, pad compound and other mechanical factors (cooling fans, brake bias, changing moment of inertia, caliper design, yada yada yada). But, you discounted the rotor diameter above.
Just because you think you're the only one who knows a small part of brake dynamics, it doesn't mean you know more than others.

 

[Jason Knight]

Thinner rotors being preferable for instance. That set me off as patently ridiculous and a clear sign of no actual experience backing up the words. Here's a quote that just touches on this:

And again that's you seemingly not understanding a blasted word I'm saying, and making garbage nonsense about what heat sinks are even for.

The rotor surfaces would be thinner, I think you're thinking the overall thickness between the two contact surfaces, not the two discs on each side of the porting.
That's what it's called when manufacturing them by the way. Those "vents" into the open space and/or internal fins (also to increase surface area) are called "porting". Much akin to what you can do to the barrel of a gun to add integral kick suppression.

And that's another point of contention, you have racing experience, I have manufacturing and engineering experience. We likely wouldn't agree on terminology, much less physics.

ASCII art time.

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I was talking A to C / D to B, you're talking A to B. Which means we actually kind of agree. In both cases were' talking about reducing the mass to surface ratio for better cooling. Making C to D bigger does give more surface area for better cooling in terms of porting and internal fins/pinning. But so does reducing A to B and D to C. You get better heat dissipation qualities making the two disks on each side thinner, irregardless of the total rotor width! Though yes, total width is better when you have porting and/or internal fin stacks.

The point across the board being to get RID of heat, not blindly let the thing soak it up. More mass without increasing the surface area isn't going to provide that! It is not about more mass to store it, it's about more surface area to get rid of it! ANYONE telling you otherwise is packing your fudge!

In the case of a bike rotor, we only have a single sheet, thus thinner is BETTER if you want to keep them cool and not have them overheat in the first place... the only limitation of that being how thin can you go before it fails structurally

And yeah, those Shimano's are expensive, but there are some aluminum sink ones in the sub $40 range. I mean FFS it's aluminum, just because crApple can charge a premium for using one of the cheapest metals doesn't make it fancy or expensive.

https://www.amazon.com/YBEKI-Brake-Dissipation-Stainless-Bicycle/dp/B08NSZV3CW

YBEKI sounds pretty sketchy as company names, but this is stuff any modern light machine shop could do from scratch easy-peasy.

Oh... my... God... YBEKI? Look at its rotor. It's so .... big.

Your tale of taking a 2 ton car to the track made me laugh. Reminded me of a neighbor who put Z badges on his crappy iron duke bitchin'c', then was pissed off when my even crappier el-cheapo little 1.6 liter four banger turbo slushbox Dodge Colt (a glorified Hyundai Excel) smoked him consistently the first half mile getting on the highway at exit 3 route 3 in Manomet, MA. Almost as if it was a three fifths his curb weight or something...

NOT that we ever illegally raced down the highway at 100+ in the '80's with no safety gear.

He was like a noughties kid slapping v-tec badges on stock model Civic's. The sticker makes it go faster.

 

[fooferdoggie]

And yeah, those Shimano's are expensive, but there are some aluminum sink ones in the sub $40 range. I mean FFS it's aluminum, just because crApple can charge a premium for using one of the cheapest metals doesn't make it fancy or expensive.

https://www.amazon.com/YBEKI-Brake-Dissipation-Stainless-Bicycle/dp/B08NSZV3CW

that thing looks noisy as hell and with so little surface area I bet you will loose some braking effectiveness. I found that o na name brand before I changed to 4 vision calipers I had more noise and less braking especially when wet. You can only reduce the braking surface so much before it becomes an issue.