cyccommute 

I’ll agree that a drum brake is the way to go. That said, there doesn’t seem to be a reaction arm anchor on the left chainstay (see red arrow below). That is somewhat problematic. Coaster brakes use a simple, thin metal strap for the reaction arm but that might not be up to snuf for this kind of brake. You will need something stronger than the 1/16” thick steel strap that coaster brakes typically use.

Rick

I have the no longer made Magura HS66 and a non modified Araya drum brake on my 1990 Burly Bongo Tandem. The Magura brakes are much better stoppers than my Paul Klampers on my touring bike. Both the Tandem and Touring bike have 26" wheels. I would like to see some pictures of your brakes.

PromptCritical 

I put a Magura HS33 on the front of our tandem, replacing the cantilever brake that looked and sounded like it was made when Truman was president. I’m amazed at how well the bike stops. Arguably as well as my road bike, and definitely well enough to get “counseled” by my stoker…..

Duragrouch

Regarding braking on long descents, so a drag brake... the problem with bike discs is partly the small amount of heat sink of the disc because they are so thin, though I still like that better than heating up aluminum rims to the point of popping spokes and worse. Bike drum brakes, beside having more mass to the drum, if it is similar to car drum brakes, I'm betting it has long front and rear shoes (pads) so it has 20X (wild-@ss guess) the pad surface as disc pads, thus much longer life.

When cars went to discs, the discs were thick and vented (USA iron), but it was still a challenge at first to be as powerful as drums, because the wheel diameters were small and thus with the caliper and clearance needed, the disc was not as large as desired. Audi for their V8 model made a "hat" disc where the caliper was on the inside and the disc nearly as large as the wheel diameter, huge increase in swept area, but expensive, and shops weren't set up to reface them, so those didn't last. Now, with huge wheels on factory cars, and cars being lighter, sufficient size discs are easy. On really high performance cars, they use carbon pads against carbon discs ("carbon-carbon") which has tremendous coefficient of friction and great stopping power. Expensive.

But getting back to bikes... My folding bike is near perfect after all my mods, but I want discs, long decents will pop spokes on the 20" rims. Rear disc mounts would involve framebuilding, if they were willing, expensive. I looked for disc fork, didn't see one. Plus I would need new wheels. I decided it was cheaper to put the money toward a disc folder in the future.

cyccommute 

I have never heard of anyone popping spokes because of rim heat nor have I even heard someone suggest that is possible. The tire would fail long before any heat build up could cause structural damage to the rim and/or spoke. Even tire failure due to heat is something that is rare. Additionally, rubber brake pads would melt before even that occurs. Further, the mass…and area…of a rim brake is so large and under flow of air that building up the kind of heat you are talking about would be almost impossible.

You are correct that bicycle disc rotors are small in terms of being a heat sink but they aren’t too small for the job.

If you are popping spokes on descents, I would look for some other cause. Heat from braking isn’t the culprit.

I would also suggest that you try a different approach to braking. Brakes…in a bike or car…shouldn’t be applied at the top of a hill and never released until the bottom. That’s a poor use of brakes. In a car, you should downshift and let the engine do most of the work. I do a lot…a lot…of mountain driving and I can go down 7% grades for miles without touching my brakes. If I do apply brakes I do so in short burst and then get off the brake to let the engine do the work.

On a bike, I similarly use my brakes sparingly, although I don’t have the engine compression to use. Sitting up and catching wind helps but my main strategy is to apply the brakes hard for only a few seconds, then get off them and repeat as needed. There is significant cooling of any brake surface in between braking events and no heat build up because there is no friction on the rim. I’ve ridden 10s of thousands of miles in mountain situations, both on and off road, and never had a brake surface even get hot, much less overheated.

Avoiding brake heating isn’t about the type of brake but how the brake is used.

Duragrouch

I do all of the things you mentioned, standing up for drag, not braking continuously. This was on a 220 foot descent over not a long distance and with turns, so a good amount of braking energy. My wheels are trued extremely meticulously, dead-on and even tension. But as I have explained before, a confluence of bad, there is not getting around:
- Aluminum has a high coefficient of thermal expansion. A bike shop decades ago told me that a prime advantage of discs was rims not expanding due to heat, which stresses both the rims at the spoke holes and the spokes, they said disc rims last longer due to that alone.
- Aluminum has a high specific heat, so will absorb more heat and cool more slowly.
- My 20" rims have less circumference and thus less heat sink than larger wheels, so get hotter for the same heat energy.
- My 20" wheel spokes are short and constant section, so have less elasticity than spokes on larger wheels, and especially if double-butted.
- 20" rims have greater "hoop/arch stiffness" than larger diameter rims for the same rim section, so have less radial elasticity.
- The 20" rear uses a standard 130mm cassette hub, so for the same wheel dish, the non-drive-side spokes are at a higher lateral angle, requiring a greater difference in tension between the NDS and DS spokes, the drive side spokes are higher tension, and more than on a larger wheel.
- On the day I snapped the spoke, I was using all rear brake, as the front rim was getting more concave sidewalls from years of wear and I was trying to baby it. So all heat going into rear rim, plus drive side spokes higher tension, and all the above factors, *pop*. The rim wasn't hot enough to burst the tube, but hot enough for significant growth. Maybe that spoke was fatigued and ready to pop, but nevertheless, the rim growth pushed it over the edge. When it popped, I was at the bottom of the hill going slow so even less air cooling, and had just let off the brake, no bump or other event. And it was a summer day.

Like I said, a confluence of bad. Most problems are a combination of factors, not just one.

Back when engine head gaskets were not as good as now, well-designed piston engines (like aircraft engines), instead of having short head bolts, had very long head bolts from the head to the lower skirt of the block, greater than the cylinder length, this gave much greater elasticity and kept the head gasket tight over a wider temperature range, and to compensate for gasket compression over time. Longer spokes do the same on bike wheels, greater elasticity.

cyccommute 

One should never trust a bike shop too much when it comes to materials. They traffic is a lot of bad information.

Yes, aluminum does have a high coefficient of thermal expansion…about twice that of steel but more on that just a little bit later.


Yes, aluminum has a higher specific heat than steel, 0.9J/g-°C vs 0.4J/g-°C. However, aluminum has a much higher thermal conductivity than steel… 210 W/m-K vs 75 W/m-K, respectively. What that means is that aluminum will absorb more heat than steel but it is much more efficient at getting rid of that heat. Aluminum is used a lot as a heat sink because it sheds heat so well. So while aluminum can absorb a lot of heat, it moves the heat very efficiently.




That’s one of those “yabut” things. Ya, but the amount of heat put into a rim brake is not all that much and heat input only occurs at a very small contact patch. Radiation of the heat occurs over a much larger area. They’re simply not enough heat being put into rim to overheat it to the point of damaging spokes or rims, even on a small rim like yours.

Some of that is true but the shorter spokes make for less leverage on the spoke resulting in a stronger wheel because the head of the spoke flexes less.

That could contribute to more spoke breakage but it is due to the construction of the wheel and has nothing to do with heat. That’s a large part of the problem with narrower hubs independent of rim size and much the reason that modern wheels are going to wider and wider hubs. At around 145mm, dish is no longer a problem.

Too many factors to blame it on heat. There are plenty of other explanations that are more reasonable to blame spoke breakage on heat. Or, I should say, overheating of the rim is way down on the list of possibilities. People don’t report spokes breaking while braking…even under very hard braking. Additionally, I would expect spokes to be more susceptible to breakage with disc rotors because the rotor is putting twisting force on only one side of the hub. I’m not saying that disc mounted hubs have a problem with spoke breakage but, if they did, it would be due to uneven forces on the hub.

Cheese to chalk. The temperatures involve in an internal combustion engine are two orders of magnitude higher than the temperatures encountered in bicycling. Heating a rim to much over 100°C is going to take a lot of effort and, frankly, foolishness. Getting up to 50°C would be difficult. There’s a whole lot of air moving past the bike that is going to sweep the heat away constantly.

Duragrouch

(above) The part about conductivity is true, but again, for the same rim section and material, a larger rim has more heat sink so will not heat up as much as a smaller rim with less heat sink..

I don't seem to have any credibility with you, so I'm gonna wait for one of the folks here who have had a lot of pro wrenching time on bikes and can reply to both my points and your points. Or someone who's done mountain descents and can attest to rims overheating and popped spokes, but again, larger rims have several things going for them that my 20" rims don't.

Andrew R Stewart 

I've got to support Stuart's understanding of the various aspects we'll talking about just now. I'll add that disk brake equipped bikes do break spokes on front wheels. We've replace more front wheel spokes then years ago and I can't remember the last time a rim brake ft wheel spoke broke that wasn't from an incident.

Do rims heat up during long downhills, sure and there are many stories about blowing off tires from increasing pressures, pads glazing and/or melting the contact surface (rubber is a poor heat conductor). But I can't ever remember a claim of spoke breakage from downhill braking with rim brakes. I have felt my own rims many times with my fingers, usually when I was curious about how hot they actually felt. This includes tandeming on the Blue Ridge and many tours with over 100lbs of bike and gear on my single (and with that kind of load you don't really go too fast but still have a huge momentum that loads the brakes). I have never had rims so hot that I couldn't touch them. Sometimes they were uncomfortable but not painful at all.

Now I have seen the results of disk rotor being way too hot. beautiful colors... Andy

Duragrouch

I believe you, but all I can say is, long steep descent using rear brake only, just as I got to the bottom, *ding* on the drive side. Nothing else was going on except the braking, I had just slowed, smooth road. I think the combination of the 20" rims, short stiff spokes, greater increase in drive side tension due to bigger angle on the NDS, were just enough combined factors to make it more heat sensitive. Oh, also not mentioned, my rims have long since polished smooth, and I'm not using the best compound brake pads, I would probably have better braking with less heat with softer pads, and longer V-brake pads instead of the cartridge road pads I use (for ease of removing the pads for cleaning). Even after a good pad and rim cleaning, I can't lock up the brakes on pavement unless it's rear only and a steep downhill and I'm standing, that happened once, needed to sit and use both brakes.