onespeedbiker

At this point I wouldn't worry about the rims over heating. I'm guessing few of the posters ever dealt with a rider really afraid of speed; 25-30 mph is very frightening; as discussed earlier, it will come in time. Obviously it's better to go with the flow and take what the hill will give you, but the amount of heat is dependent on weight and grade. Whether you brake all the way down or pulse the same amount of heat is generated, but pulsing gives the rims more time to cool down. With the OP's GF, it's doubtful she would be able to generate enough heat to be a problem.

It does remind me however of an all day bike ride that had 3 six thousand foot climbs and 3 six thousand foot descents. My disc brakes would get so hot they were squealing like some inappropriate comparison. The ride sponsors were giving out some fruit flavored powder for the water. I soon learned that if I squirted some water on the discs they would shut up for a while. Of course it was fruit flavored water so every time I cooled down the disc I was treated to fruit odored steam.

I also have a tandem that my wife and I ride and I am amazed how well the V brakes work. I have ridden tandems with dual pivot brakes and they aren't even close to the stoppers my Avid Arch Rivals are (too bad they stopped making them). Speaking of hub brakes, with tandems they are used a consistent drag not to stop the bike.

DannoXYZ 

You need to calculate thermal-conductivity. This is a time & distance function. Slamming on the brakes at the last-second going into the corner will generate higher delta-T between the surface of the rim and the surrounding air. Yet, due to the shorter amount of time this heat is applied, less of it will sink into the interior of the rim (thermal-conductivity of rim is constant). The higher delta-T also means the rim will shed heat to the surrounding air faster and less of it gets into the rim.

Auto and motorcycle racers only know this too well. The really high-end stuff uses carbon & ceramic braking surfaces. These have extremely low thermal-conductivity and these discs stay very cool internally. Yet their surfaces will be red-hot to transfer heat away to the air as fast as possible. Discolouring the paint on the wheels from heat isn't uncommon.

And with maximum-braking in the last 1-2sec before going into the corner, you increase the amount of time off the brakes on the next straightaway to cool the rims. It's a duty-cycle thing, if you can spend 5% of the time braking and 95% of the time cooling, your rims will be cooler than someone who's braking 95% of the time and only giving it 5% cooling.

Test it with a torche, lighter or candle under your hand. Woud you rather hold it over the flame for 5-seconds, then take it off for 95-seconds before repeating? Or hold it over for 95-seconds with only 5-seconds breaks in between?

cyccommute 

Precisely. That's why rekmeyata's "brake stabbing" technique works for all brakes, not just drum brakes. No brake... disc, drum, spoon or feet dragging... works well if applied all the time. Here in Colorado, for example, cars are required to stop at Glen Cove on Pikes Peak for a brake check. People who are unfamiliar with mountain driving will ride their brakes the entire 6 miles from the top to Glenn Cove and brakes...even disc...have failed. Experienced mountain drivers (and bike riders) know not to apply their brakes at the top of the hill and not stop. A motorist can use engine compression but a bicyclist has to use their brakes. It's best to let the bike run, apply the brakes to decelerate quickly and then get off them. This is a problem for people who want to go down any hill at 15 mph but even they could benefit from short duty-cycle braking.

onespeedbiker

As a police officer (before I retired) I took pursuit training where they taught 7-8-9 braking. The idea being, as is being said here, that the temperature of the brakes will increase dramatically but cool off quickly due to depth of the heating and more time to cool. An interesting side note is most vehicle pursuits that end with a collision is the result of the suspect loosing their vehicles braking ability. Your average vehicle on the road is simply not designed for high speed driving/braking; the engine and brakes overheat and the brakes fail rather quickly.

JPMacG

OK Thanks. I get it. By pulsing the brakes you create a higher temperature at the surface of the rims, which allows higher heat transfer by convection. You're trying to maximize convection in relation to conduction.

FBinNY 

Yes that's a key part of the process. Pulsing also reduces total heat transfer to the shoes, which in fact have comparatively poor heat tolerance.

Delmarva

How long are the descents the new rider is on? If you are not riding loaded or the downhills are not looong I would not worry about heat buildup. Just let her get used to riding and reduced braking will come as she gains confidence. Also you can help by setting a moderate pace and restraining the urge to max out on the downhills.

dabac

Ballpark value is that one meter of steel will grow one millimeter for each 100 deg C it heats up. I'd worry more about the rubber going soft than the actual stretch.

rydabent

This is why disc brakes are a better and more elegant engineering idea. With discs you are not wearing out a vital part of the bike the wheels. Bolting on a new disc and pads is quick and easy compared to replacing a rim and relacing the spokes. I have seen several reports where cross country cyclist have worn thru and cracked a rim due to wear.

rekmeyata

While that can happen, it's the exception and not the rule. The touring world has been using rim brakes forever, all over the world, in mountainous areas a lot steeper then the United States has. The reason why some tourists had that issue was due to inferior rims to begin with, or rims that were partially worn before they even started the trip! A fully loaded touring rig with new rims designed for touring will last at least 15,000 miles of heavy touring use.

Read this page for a quick study on the two different types of brakes: https://www.rodbikes.com/articles/brakes.html

Please don't misread into what I'm saying, I'm not saying disk brakes are a bad idea.

FBinNY 

+1, discs have advantages, but the rim wear issue is exaggerated. In over 100,000 miles, a decent amount of which was touring, and using rims with a wall thickness of only 1mm (300gr tubulars) I've never worn out a rim. And I'm not an exception. Rim wear only became a meaningful concern with the advent of mountain biking. Mtn bike brake shoes are always full of grit and can cut through a rim fairly quickly. That combined with the higher side stress of large section HP tires led to the adoption of wear standards and wear depth indicators.

Even my commuter which has about 20,000 all weather stop & go miles, is still on the original rims, though the brake wear is very obvious, and it's likely that I will wear a rim out eventually.

Otherwise, for road use, brake wear shouldn't be a serious factor in whether to go to disc brakes. But disc's do have other advantages, especially for all weather riders, so it's an option to consider.

rekmeyata

My oldest set of wheels are Torelli Master Series rims and they have 35,000 miles on them and are still not worn out, but they aren't toured on either.