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

[Stefan Mikes]

The brake modulation is the only way. The rotors need time to cool down a little. Riding the brakes is wrong.

 

[Jason Knight]

Thats actually a bad thing to do. It will induce a rotor warp. The rotors will cool unevenly as the area the hot pads are sitting over will stay hot.

I hear a lot of paranoia about that, but seriously if the steel used is that sensitive to heat expansion that it won't return to true, just how the devil is it ever able to survive normal everyday use. It's setting off my bullshit alarm since if you're hitting the 800 degrees+ needed to compromise what should be a tool grade steel, there's something WRONG.

Has anyone ever actually seen rotors warped from that, or is this another myth that people just blindly parrot? 'Cause to me it sounds as nonsensical as 911 "jet fuel can't melt steel" conspiracy nutjob BS. And with an overactive vmPFC, I have a real nose for recognizing BS.

The thing to do is to slowly, gently ride without touching the brakes and let the air cool the rotors.

Which does you not one blasted lick of good when you're halfway down a one mile 12 or steeper grade... since then how do you accomplish that in the first place.

But then I also wonder how one uses the front brake without the rear going downhill without going over the bars. That too sets off my bullshit alarm.

The only thing I have never seen transfer over is crazing (cracking), which is a good thing given how thin bike rotors are.

That thinness -- and smaller overall size -- is likely WHY they don't crack. The thicker it is the more prone heat expansion is to not return to true or find an imperfection to exploit. The larger mass cannot expand and contract as much because it's more prone to uneven heating.

In fact, that's part of WHY this "oh the warping" stuff sounds like bullshit to me. Given the performance regime I'd say you're more likely to screw up your rotors from tying up your bike on a public rack than you ever are from heating them up in normal use.

Cheap at $25 and maybe the heaviest duty rotors on the market. But they are so thick only a few brakesets on the market can fit them.

See, THEN I'd be worried about permanent damage from heating. The thicker the steel the more likely you are to have problems like that.

When I did my rotor swap on the front I was kind of worried about the fact that it was two pieces, the steel braking ring and the inner part being aluminum. But then it hit me, aluminum. A better conductor and dissipater of heat than steel! I mean it's no copper, but still. Assuming it's a relatively standard alloy it's anywhere from 160 to 240 wm/k as opposed to different flavors of steel ranging a pathetic 16 to 40 wm/k.

Which is why for laughs I sanded off the anodizing where it meets the steel, and added some thermal compound at the gap. Materials geek 101.

I have a friend who's an actual materials engineer. I should ask his expert opinion on if a bicycle brake disc would get hot enough for permanent warping to even be a thing.

What I know on the topic of steel says bullshit, but I'm open to being wrong about that. That's what research is for.
Might also come down to exactly what steel is being used. You get some cheap low carbon steel, it could be a problem. You get a high strength low ductile steel -- what I'd expect rotors to be made from -- worrying about heat warping should be utter nonsense until you get up into the temps where steel becomes a limp noodle.

Given how they seem to stay relatively shiny even places where there isn't wear, I'm wondering if austenitic steels are in use for bike rotors. Or maybe a duplex blend for strength and "the pretty?"

Seriously though, the way you folks are talking about warping from heat makes it sound like they're pig iron.

 

[AHicks]

There's a generally approved way to get down a hill without overheating your brakes. When speed becomes excessive, you get on your brakes HARD (BOTH of them) to haul the bike down to a reasonable speed as quickly as possible, then you get off of them (completely) to allow them to cool. The bike's motion and the design of the disk both allow that to happen pretty quickly. If speed gets out of hand again, get on the brakes again, hard! Then get off them and allow them to cool. Repeat as necessary. This is no different than handling a really steep hill in a motor vehicle.

 

[retiredNH]

There's a generally approved way to get down a hill without overheating your brakes. When speed becomes excessive, you get on your brakes HARD (BOTH of them) to haul the bike down to a reasonable speed as quickly as possible, then you get off of them (completely) to allow them to cool. The bike's motion and the design of the disk both allow that to happen pretty quickly. If speed gets out of hand again, get on the brakes again, hard! Then get off them and allow them to cool. Repeat as necessary. This is no different than handling a really steep hill in a motor vehicle.

Generally approved by whom? This has no basis in science or actual field testing.
Much better to not get an excessive speed to begin with. And, what do you do when you release the brakes to cool? Hold on for dear life?
Seems like we've gone the full circle...

 

[AHicks]

Generally approved by whom? This has no basis in science or actual field testing.
Much better to not get an excessive speed to begin with. And, what do you do when you release the brakes to cool? Hold on for dear life?
Seems like we've gone the full circle...

On the bold, that's total BS you've pulled from your ..... Suggest you educate yourself a bit before making a.... Oh, never mind....

No clue who it was approved by, but I wouldn't knock it until you've tried it. Suggest you do a little research. I'll make it easy for you. See below-

Please share how you are going to avoid "excessive speed" completely on a steep down hill run? That's nonsense. Let's keep it realistic. Have you ever even ridden in big hills?

When you've pulled the bike down to what, maybe 10mph and let go of the brakes to cool, it's going to take a while to get back up to the speed that you're concerned with. That whole time your brakes are cooling. If you're dragging the brakes, the brakes just keep getting hotter and hotter until they fail.....

This is related to cars, but the idea/dynamics regarding hills and brakes are no different-

Which braking method in driving downhill could reduce heat, separated hard braking or continuous soft braking?

Answer (1 of 16): Use fewer harder braking events to reduce heat buildup. Better still is to shift into a lower gear and let the engine do the braking. This is possible on most automatics and all manual transmission vehicles.

www.quora.com

"Which braking method in downhill could reduce heat? Separated hard braking vs Continuous soft braking?

The amount of heat overall is the same, but the interrupted breaking allows the rotors to cool down between applications of clamping force on the rotor sides with the brake pads"

Vk Gopalakrishnan
Mechanical Engineering Design Consultant Author has 142 answers and 486.4K answer views2y
Don Sutton has given good comments on the above question. I would like to add the following points regarding the four valid questions that he has raised in his comments.

1. The amount of energy is likely to be relatively less with separated hard braking, and hence the heat generated would be less.
2. If the interval between consecutive hard brake applications is not too large, and the vehicle speed is low, there may not be sufficient cooling to cause cracking of rotors/drums. Hence short heavy braking can be applied.
3. It is possible that short heavy braking allows brake dust to escape and make the next application more efficient, but the effect may not be very significant.
4. Long light braking certainly promotes glazing of the brake linings, and lead to ineffective braking after a while. This is a very unsafe condition, and hence must be avoided while driving downhill.

There's more here, and pretty sure there's a LOT more available on the topic. This is just enough to prove others are thinking along the same lines I am. "No basis" what a putz....

 

[fooferdoggie]

Generally approved by whom? This has no basis in science or actual field testing.
Much better to not get an excessive speed to begin with. And, what do you do when you release the brakes to cool? Hold on for dear life?
Seems like we've gone the full circle...

my tandem group has talked about this method as the best as it gives the brakes plenty of time to cool plus they are not used long enough to over heat. but I dont like going above 30 on the road so this does not always work to keep the speed down.

 

[Tars Tarkas]

Second, it should not matter on a bike whether one steadily brakes on a long decline or brakes hard then releases for a time. The energy that the brakes must dissipate is the same in either case (ignoring very minor contributions by wind resistance etc, that are speed related).

With steady, unrelenting braking the pads just get hotter and hotter with no opportunity for them to cool down. This should be avoided. The method I use is to use the front and rear brakes alternately. Use the front brake while the rear one cools off, and vice versa.

TT

 

[Tars Tarkas]

But then I also wonder how one uses the front brake without the rear going downhill without going over the bars. That too sets off my bullshit alarm.

Gently, with a modicum of common sense. Ain't no biggie. No BS.

TT

 

[m@Robertson]

I hear a lot of paranoia about that, but seriously if the steel used is that sensitive to heat expansion that it won't return to true, just how the devil is it ever able to survive normal everyday use. It's setting off my bullshit alarm since if you're hitting the 800 degrees+ needed to compromise what should be a tool grade steel, there's something WRONG.

You should be able to figure out just by using your brain. If you have a rotor in open air, it cools one way. If you surround it with something that is highly insulating that is also almost touching it... then duh of course the air will not cool it as evenly. No need to google that to see if its true. I hope. This is standard training for high performance (i.e. race track) driving and the problem is worse, not better, on a bicycle because the rotors are soooo thin the heat sink that is a brake rotor has far less material to work with.

Has anyone ever actually seen rotors warped from that, or is this another myth that people just blindly parrot?

oh hell yes. I've got a front rotor right now I can't fix because of it. I try not to run my mouth on the internet unless its something I have actually done or directly experienced. Which pisses people off but too bad. Get out there and ride or drive or whatever and then talk.

Which does you not one blasted lick of good when you're halfway down a one mile 12 or steeper grade... since then how do you accomplish that in the first place.

If you have to stop on a steep downgrade (like at a stoplight), you reach your 'stop' point a few cars back. Then you slowly let the bike roll forward during the red light until you get to the crosswalk. Do it right and you are still a car or two back when it goes green. Works just as well on flat ground. Probably not necessary unless the circumstances are extreme. Riding cargo bikes, I'm more used to those extremes being the norm.

But then I also wonder how one uses the front brake without the rear going downhill without going over the bars. That too sets off my bullshit alarm.

Thats because you are all worked up and not reading carefully. I said such a technique was used when bedding the rotors. Anyone with any brains is bedding their brakes on a sleepy neighborhood road thats not an alpine descent.

See, THEN I'd be worried about permanent damage from heating. The thicker the steel the more likely you are to have problems like that.

The bicycle industry disagrees with you. Downhill rotors are a thing whether you like it or not. And the reason is straightforward: Brake rotors are heat sinks. More material in the rotors means more heat can be sunk into them. Ergo they are more effective under extreme conditions. 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).

Given how they seem to stay relatively shiny even places where there isn't wear, I'm wondering if austenitic steels are in use for bike rotors. Or maybe a duplex blend for strength and "the pretty?"

Bicycle rotors will rust. I keep some bikes at the coast and the progression is slow but its there.

 

[Rexlion]

Regardless of the warping question, the pads are subject to damage or fast wear from overheating. Since continual braking is known to generate more heat than sporadic, hard braking, the latter is the preferred method.

I realize that automotive braking is done at higher speeds and involves different brake composition, but it is worth noting that any trucker worth his salt knows better than to ride the brakes down a long grade; it's a good way to wind up dead near the bottom (or if lucky, on the runaway ramp) when the brakes stop braking. People who tow large rv trailers and 5th wheels are in the same situation. Drivers of lighter weight vehicles can usually (but not always) get away with constant braking. Bike riders might get away with it 99 times out of 100 too, but that 1 time is all it takes to put the rider in a world of hurt, right?

 

[retiredNH]

With steady, unrelenting braking the pads just get hotter and hotter with no opportunity for them to cool down. This should be avoided. The method I use is to use the front and rear brakes alternately. Use the front brake while the rear one cools off, and vice versa.

TT

Urban legend...as I said before, the energy the rotors must dissipate is the same either way. Your way means the rotors will get even hotter when you intermittently use them.
There's not a lot of decent, non-anecdotal studies done on bike rotor warping, but the consensus seems to be that uneven cooling on stopping may be the main cause, so intermittent braking is probably more likely to cause warping, since when you finally stop (if you do, such as at a stop light at the bottom of a steep hill) your brakes will be hotter.
It's interesting how the same BS keeps getting tossed around the internet and boards like this, with no basis in fact. I'd love it, for example, if someone would link to a rationally designed study that compares these two approaches, but I doubt that one exists...

 

[retiredNH]

Regardless of the warping question, the pads are subject to damage or fast wear from overheating. Since continual braking is known to generate more heat than sporadic, hard braking, the latter is the preferred method.

O really? Did you ever take a physics course? The total energy to be dissipated on a downhill run is independent of how you brake, so the heat created is the same. In one case, you have steady state heat generation. In the other case, you alternate between higher temperatures and lower temps.

I'll leave this discussion now, because it's obvious that the physics behind this topic eludes too many of the posters.

 

[AHicks]

Urban legend...as I said before, the energy the rotors must dissipate is the same either way. Your way means the rotors will get even hotter when you intermittently use them.
There's not a lot of decent, non-anecdotal studies done on bike rotor warping, but the consensus seems to be that uneven cooling on stopping may be the main cause, so intermittent braking is probably more likely to cause warping, since when you finally stop (if you do, such as at a stop light at the bottom of a steep hill) your brakes will be hotter.
It's interesting how the same BS keeps getting tossed around the internet and boards like this, with no basis in fact. I'd love it, for example, if someone would link to a rationally designed study that compares these two approaches, but I doubt that one exists...

Urban Legend? Get your head out of your A$$!
Go back up to post 25 and read what a Mechanical Engineering Design Consultant has to say on the topic.

Think you're smarter/above all that? Let's see some documentation. Put up or shut up....

 

[fooferdoggie]

oh hell yes. I've got a front rotor right now I can't fix because of it. I try not to run my mouth on the internet unless its something I have actually done or directly experienced. Which pisses people off but too bad. Get out there and ride or drive or whatever and then talk.

though I have not seen the issue on any of my e bikes. though our tandem the front rotor would warp a bit on hard stops then straighten out. cable actuated brakes can cause it because of uneven pressure. I used to get warpage on my recumbent it was a pain.

 

[AHicks]

though I have not seen the issue on any of my e bikes. though our tandem the front rotor would warp a bit on hard stops then straighten out. cable actuated brakes can cause it because of uneven pressure. I used to get warpage on my recumbent it was a pain.

I've had that happen too (warp, then straighten as it cools). A reminder to do it right.... -Al

 

[Tars Tarkas]

Back to the original question, you almost certainly are not going to get in trouble with your brakes on a 3/4 mile descent no matter what. Overheating issues are only likely to kick in on something like a long alpine descent. There you might want larger rotors with higher tech heat dissipation and maybe other mods.

If you're worried about hot brakes the best thing to do is to stop for a while and let them cool off.

TT

 

[AHicks]

Back to the original question, you almost certainly are not going to get in trouble with your brakes on a 3/4 mile descent no matter what. Overheating issues are only likely to kick in on something like a long alpine descent. There you might want larger rotors with higher tech heat dissipation and maybe other mods.

If you're worried about hot brakes the best thing to do is to stop for a while and let them cool off.

TT

Sorry, I would need to differ. There are several hills locally that have me paying attention. One is only maybe a couple hundred yards long, ending at a stop sign....

 

[Tars Tarkas]

Sorry, I would need to differ. There are several hills locally that have me paying attention. One is only maybe a couple hundred yards long, ending at a stop sign....

Well, you're an exceptionally large person on a bike. You're right though, taking that into consideration.

TT

 

[TrevorB]

To add a level of discussion I just thought of, there's also the potential for said wife to be using rear brake only.....

My wife was using rear brake most of time as I discovered hard way. She said had poor braking, rear pads were totally gone (metal on metal) while front had hardly been used. I've asked her to use both brakes together, think she is now based on wear.

 

[6zfshdb]

What you need is regen braking. That 3/4 mile hill would put quite a bit of juice back into your battery.