To Brake or not to Brake: The Descender’s Dilemma
 

While maintaining a cautious 25mph or so on a long, exceptionally steep descent (1 mile, 1300ft), I found my brakes beginning to fade. What to do? Braking less hard seemed a slightly risky option, but it would let me catch up with others in my group ahead. Slowing down to a crawl seemed less chancy but also less attractive. I compromised by rolling on at the same prudent speed, using firmer braking as needed. Not far from the bottom a front spoke snapped and the wheel immediately developed a major wobble, which brought me to a halt rather awkwardly.
When rim heating is the main concern, the best speeds are the two extremes of coasting and crawling. So the correct choice between speeding up and slowing down depends on whether you are already going faster or slower than the worst case speed. If you are going faster, it will help to go faster still. If you are going slower, it will help to slow down even more. So: just what is the worst speed to descend at? Maybe half the coasting speed? But the exact answer will depend on how fast the wheel and pads dissipate heat.
Other questions arise: can thermal expansion of an aluminum rim really be enough to cause spoke or nipple breakage? (For a 50deg centigrade increase, I figure the expansion is about a full turn of a spoke wrench.) And is the main breakage risk on steep descents thermal, or mechanical? (I doubt that it’s mechanical, since only very extreme gradients change the magnitude of the forces exerted on the rim by a big factor.)

On long descents where I don't want to tailgate or pass other cyclists I'll sit up straight to let air drag slow me down. Getting out of their draft line helps too.

If you're wearing the right clothing, unzipping a jersey or jacket can help catch some air drag. But watch for tricky cross winds.

Those tricks worked on some long downhill areas in California. Here in Texas we don't have many long fast downhill runs. Usually I try to enjoy the short downhill runs as fast as I can go because they don't last long enough to worry about brake fade.

I suppose water or mist on the wheels could help, but I wouldn't want to take a hand off the bar to try it.

Texas is a big place, so you must be speaking for your part of Texas - for instance, if you live in Dallas, my house is 1200' higher than you

Is it really 1300 feet in a mile? That's an average of 24%, extremely steep.

On really steep downhills, sitting up doesn't do much to slow me down. I'll alternate front and back brakes, braking hard, then letting the brake off completely. That seems to be better for heat management than holding the brakes on continuously.

The front brake is way better when I need to slow down for a sharp curve, it has a lot more stopping power.

On a 15% or higher downhill, I'd stop periodically to let the rims cool. And give my fingers a rest too.

Don't think I've been on a hill that steep for that long. There are a couple steep ones near me though, and my approach is to alternate between braking hard and not braking at all. My speed oscillates between slower than I'd like and faster than I'd like, and I try to avoid riding the brakes all the way down.

Let 'er rip and brake hard for the corners. As you've found 25mph requires about the maximum amount of energy to be dissipated by your brakes. I have a graph somewhere showing power vs speed while descending and my recollection is it peaked around 25mph. Alternating brakes doesn't really help if you try and maintain the same speed as the same amount of power still needs to be removed from your wheels.

For a 6% grade, 18mph is the worst possible speed. On your steep hill 25mph is likely very close to optimally bad.

With rim brakes, the main risk is tire blow out due to the increased air pressure. The pressure goes up and pushes through the rim strip.

diamacleod, I've always just alternated my brakes if I need to maintain speed on a lengthy downhill. I don't apply the brake for very long on a particular wheel and if I really need to slow down quickly, both brakes.

Brad

Other than the fact that 18 mph is an extremely slow speed for a 6% grade, I fail to see how it's the "worst possible speed".

If you are using rubber rim strips, maybe. But the increase in air pressure from heating has less of an effect than most people think. If you have a tire at 100 psi at 70°F and you raise the temperature to 270°F, the pressure rises 37psi. That might be enough to push into a rubber rim strip but if you use a fabric rim strip...which you should...it won't be a problem.

Probably not. A piece of aluminum that is 1" long at 70°C is 1.0119" long at 122°F (50°C). Brass expands even less.

In order to not make the post long, here's a link to an article I wrote about braking technique:
Braking technique - Cycle Gremlin

Now to answer the topic.

1) Speed
Never go faster than you can see. If you have a clear view of 100 meters in front of yourself, make sure you can stop in those 100 meters in case the road is blocked.

2) When descending, if you want to slow down, go as fast as it is safe. :) Yes, let the wind drag do the braking for you. The faster you go, the more energy is absorbed by wind drag. Sit up straight, use your torso as a parachute to slow you down. But always make sure you follow the rule 1)

3) When braking, it is usually best done before corners. Make braking as short and as hard as safe (you should always feel safe and in control, but make the braking hard within those limits). Rim heating is mostly a matter of time - braking vs cooling down time (brakes off). So feathering the brake for 30 seconds doesn't heat the rim any less than 3 seconds of really hard braking - quite the contrary. That's why you should make the heating count - by using short and hard braking, then letting the brakes go, enabling the rims to cool down.

Braking down to almost a stop, then letting the bike speed up, using the wind as energy that slows you down and cools the brakes, then another hard brake and so on.

It's the speed at which the most power (heat) is dumped into your braking. If you go faster, aerodynamic drag takes away more of the energy. If you go slower, the rate at which you need to dissipate energy is reduced because you're descending more slowly.

I was looking at carbon rims, so I put together a spreadsheet that let me calculate the braking energy required to maintain a constant speed on a downhill. The input power is simply the vertical descent rate (mgh), and then subtract aerodynamic drag, the difference is the power that your brakes require.

It's a crude approximation, but it gives you a general sense of what the trends and things are. It's also about the speed that a timid rider might descend a curvy mountain road, and have issues with melted carbon rims. A pro rider will fly down hardly touching his brakes.

I have this problem, as I live in a very steep hilly area, and am borderline clinically paranoid after a nasty ankle break. My solution was to get Shimano hydraulic road disc brakes. This really helps, and although I blow through a lot of pads and rotors, I've never had a problem controlling the bike. (I've also regained some confidence so am a bit less heavily reliant upon these brakes, but it is good to know they are there when I need them, or get paranoid.)

You are making a wrong assumption and coupling two things together that shouldn't be coupled. How much heat is generated converting kinetic energy to heat energy doesn't depend on the aerodynamic drag. Yes, you can get some benefit from sitting up and using aerodynamic drag to slow you but that doesn't have any influence on the heat you generate at the rim. That's just a function of the friction on the rim, pads and speed.

Carbon rims, by the way, are a special case. The carbon fiber matrix...not only the fiber but the epoxy holding it together...is an insulating material with respect to heat. Heat built up in the material stays in the material and isn't radiated out as quickly as aluminum can radiate it.

I also doubt that the carbon wheels will actually "melt". The epoxy has a high melt temperature as does the fiber itself. To reclaim carbon fiber, it has to be pyrolized at very high temperatures...600°C or higher. The rubber pads have a much lower melt temperature and would melt before the wheels will.

If you have a long descend, and want to stay, say below 80 km/h, it is better to wait until you reach the 80 km/h before braking. If you keep braking before reaching 40 km/h, most of the work will be done by the brakes, effectively heating the rims up more. But if you keep going to 80 km/h, a lot more energy will be absorbed by the air drag, as it increases with speed increase, exponentially.

Not sure if I've explained it correctly, but it's basic physics. Using the air to do braking, along with the brakes.

cyccommute: In maintaining a steady speed, the heat generated, and the clamping force at the pads, both approach zero as the speed approaches the coasting speed. If you go down at a crawl, the clamping force and the total heat generated are both maximal, but the heat generated per unit time (and the rim temperature elevation) again goes toward zero. In between these cases, there is a 'worst speed' to descend in the sense that rim temperature is maximal.

Scientific Answer - I hope
 

Ok, so I don't have a lot of experience on long descents (I'm in the midwest. Biggest descents tend to be bike path overpasses), but I'm reasonably decent with physics. The thing in question in an absolute value, which is the work needed to bring cyclist to a stop from a descent, and where the necessary work comes from.

There are 2 equations that relate to this.

1) force = mass x acceleration

2) work = force x distance

Let's get our values straight to begin with. Our gentleman thread poster says he has a 1300 ft descent. That is 396 meters. Let's assume he weighs 100kg. Now, the force of gravity is 9.8 meters/second squared.

So again, force = mass x acceleration, or

f=ma
force = 100kg x 9.8m/sec2
force = 980 Newtons

So now we use the equation to figure out the work needed to stop the cyclist. Now, this is assuming a 100% grade. 9.8m/sec2 is the force of gravity straight downward. For the purposes of a bicycle descent, however, the distinction is meaningless. Slope will have an effect on the power at any given time, but not on total work performed.

work = force x distance,
work = 980newtons x 396meters
work = 388,080 joules

The best way to look at this is once you've gotten to the bottom of the descent, if you've put in 388,080 joules to slow yourself, you've stopped. If you haven't, you've hit whatever was at the bottom of the hill.

So, once you know the work that needs to be done (388,080 joules), you can start accounting for it. There are really only 4 things that do the work.

mechanical friction (really just hubs)
tire rolling resistance
brakes
wind resistance

Mechanical friction at speed I think is mostly negligible
tire rolling resistance is very real, but I believe scales linearly
brake power and heat creation believe scales linearly as well, though I could be wrong about that
wind resistance force is the SQUARE of speed. This means that if you increase average speed during the descent, the proportion of work done relative to the 388,080 needed to bring you to a stop INCREASES, meaning everything else MUST decrease, including the brakes.

Wow this went on longer than I intended lol....I hope it is sort of helpful. This is a long way of saying that a faster speed when safe will result in less of a chance of overheating wheels.

Absolutely. Also true for motor vehicles.

A lot depends on the hill, road surface, corners, and traffic.

A couple of weekends ago I was out on a loaded tour.

1 lane, very steep road, with blind corners, no pullouts, and switchbacks, and a heavy load. I got my rims quite hot.

2 lane, steep, but gentle corners (although the road surface was a bit rough), road closed to cars. I was barrelling down the hills at 40 MPH.

On a previous ride, I hit 49 MPH on a hill that was absolutely straight, good view top to bottom, and relatively short. I have to go back there and see if I can hit 50+.

Make sure you use both front and rear brakes. In general, I tend to drag more on the rear, and reserve the front for more rapid braking.

This question of the worst descent speed for brake heating came up before so I'm cutting and pasting my answer from then:
"Let's calculate the speed at which the most power is being dissipated by the brakes. This is the difference between the drag power on the bike (assume all air drag for a fast descent) and the power exerted by gravity as the bike/rider descends the slope.

So let's take a bike/rider of mass m descending a slope of grade s at velocity v. The power exerted by gravity will be:
Pg = mgsv
and that of the air resistance will be (with 'c' a constant based on frontal area and shape):
Pa = cv^3
Terminal velocity (vT), or 'max. coasting speed' occurs when these two are equal, so
mgs(vT) = c(vT)^3, therefore c = mgs/(vT)^2
Therefore, the power dissipated by the brakes (Pb) will be:
Pb = Pg - Pa = mgsv - cv^3 = mgsv - mgsv^3/(vT)^2 = mgs[v - v^3/(vT)^2] = mgs(vT)[{(v/(vT) - v/(vT)**^3]
What we want to know is what value of v/(vT) gives the most heat dissipation by the brakes, so we can
substitute x = v/(vT):
Pb = mgs(vT)(x - x^3)
to find an extremum we differentiate and set equal to 0:
1 - 3x^2 = 0, so the maximum heat dissipation will be when x = sqrt(1/3) = v/(vT), and
the worst velocity v = vT/sqrt(3) or about 58% of the terminal velocity."

This isn't quite right since the brakes will be cooled by airflow over the pads and rims which will be more effective at higher speeds. So I'd expect the actual worst speed to be a little lower than calculated above.

:love: my disc brakes....

I was using melt somewhat colloquially.
Many of the early carbon fiber rims had relatively low Tg (glass transition temperature), some as low at ~60C (cheap Chinese). If the epoxy resin gets to Tg, it softens significantly, and a clincher rim bead fails due to the tire pressure.

Road Bike Action | Tech Report: The Real Story Behind Carbon Clinchers

Newer rims and carbon specific brake pads have essentially solved the issue.

I noticed your sig; Do you have a cheap internal gear hub you could recommend for a winter bike? I picked up an old trek mountain bike for this winter with click shifters. There werent working at all and I WD40'd the heck out of the inside of them and they SORT of work now, but I'm thinking just doing away with derailleurs altogether for the winter.

Thanks in advance for any suggestions.

1300 feet per mile isn't a hill, it's a ski run. I hear the "Goofy yell" just thinking about it.

I would coast until I didn't like the speed, then slow way down and do it again. If my brakes started to fade, that would be a good time to stop for a minute or two and admire the view.

In #21, prathmann gives a definite answer with a physical justification. To restate his argument: the power (energy per unit time) to be converted into heat by braking has a gravity-generated component proportional to speed X, but is reduced by the loss due to air resistance which prathmann takes to be proportional to the cube of the speed, X^3. If the speed X is expressed as a fraction of coasting speed, those cancel for X =1, so the net braking power must be simply proportional to X – X^3, which is maximal at X = .58. I would add that if we assume, more simply, that air resistance increases as the square rather than the cube of speed, the braking power is X – X^2 and is greatest at X = .5, or half the coasting velocity.