I explored disc brakes vs. rim brakes pretty thoroughly in my post, Putting on the Brakes –Part Two. In it, I pointed out that disc brakes offer somewhat better wet-condition braking, but somewhat inferior braking power under most other conditions, and much less modulation overall than good rim brakes. They are also much more prone to heat-induced fade and heat-warped rotors. But in the name of “progress,” the push to get them on more road bikes continues.
The attitude expressed by the component manufacturers is that bikes are behind the times. Cars and motorcycles have so much advanced technology, so why shouldn’t bikes follow suit? (see Bikes Aren’t Cars). One manufacturer even goes so far as to say that current braking technology for bicycles is from the “dark ages” (someone needs to study history better -- the Anglo-Saxons would've killed for some dual-pivot sidepulls). But beyond the belief that disc brakes are “new” so they must be “better,” is there anything else behind the trend?
I have a theory: Carbon fiber rims don’t make good brake surfaces. Ok, that’s not really a theory – that’s just a simple fact. But if the wheel manufacturers like Mavic, Enve, Reynolds, Zipp, HED, et. al. want to keep selling their $1000 - $5000 all-carbon wheelsets to weight-obsessed (and image-obsessed) roadies, then they need to do something about the crappy braking and shattering rims.
Fact: Braking on carbon-rimmed wheels is nowhere near as good as on aluminum rims. Braking on carbon-rimmed clincher wheels may be downright dangerous.
(reprinted with permission from Red Kite Prayer
Even with the special brake pads (required with carbon rims), braking distances are longer. (In the wet, they’re flat-out awful). Worse, the heat generated from braking on carbon rims builds up much faster than on aluminum rims. Combine that heat with the outward pressure (over 100 psi) on the rim’s sidewalls coming from clincher tires, and catastrophic (explosive!) failure ensues. Just urban legend? If only. Lots of carbon fans would like to ignore it, but the failures keep piling up. Many (including some of the manufacturers) would like to blame inexperienced riders, while others point fingers at one brand or another. But at last year’s Levi’s Gran Fondo in California, the organizers actually warned participants to please leave the carbon fiber clincher wheels at home because of the well-documented failures.
The all-carbon tubular wheels apparently aren’t as prone to these blowout failures because the pressure is better contained within the tire’s sewn up casing and not pushing outward on the rim sidewalls. But superheated rims aren't good for tubular tire glue, either. And the braking performance on these rims is still lousy, which isn’t much of a selling point – although it isn’t hurting sales so far, is it? Style and image trump safety and reliability (If you don't agree, people call you a retrogrouch! That's why I gladly embrace the name). And the life expectancy still has to be much shorter than on a good set of aluminum rims. Picture this: a tiny speck of grit gets lodged in a brake pad. Every time the brakes are applied, that little speck starts working a tiny gouge in the sidewall of the rim. If that rim is aluminum, one will probably notice it by the sound and clean the brake pad – but even if they don’t, it would take a long while before that grit could seriously damage the wheel. The damage would occur slowly and be much more likely to be caught before an actual failure would occur. But if the rim is carbon, that little speck of grit will do much more damage much more quickly. Putting a deep scratch into the carbon fiber will create a stress riser, leading to rapid delamination of the carbon in no time flat.
Enter disc brakes. Get the brakes off the rim, and then the rim won’t break.
So it isn’t really an issue where the new technology is actually superior to the old (as is the usual argument) but rather, the new technology is addressing problems caused by the previous new technology.
This is typical of the supposed “advancements” that drive the bicycle industry today. New technology supposedly improves this component or that one. The new “improvement” (note the quotes; in many cases it’s marginal) comes with side effects not previously experienced with the “old” technology. Maybe it's simply not the huge advancement that it's hyped as. Or maybe there are failures. So another new “improvement” comes along to address the side effects caused by the previous “improvement" which exposes another weak link or raises another previously nonexistent problem, and another and another and the process repeats. Of course, none of the ads describe it this way. Instead, they point to meaningless gains in "lateral stiffness" and "vertical compliance" or some other ridiculous marketing buzzwords. How else do you explain why headsets and fork steerers keep getting bigger? And bottom brackets? Handlebar diameters?
And so it is with wheels and brakes. Aluminum rims and rim brakes work fine; the rims are strong, last long, and fail rarely. But they’re “old” and we want “new.” Carbon fiber rims are introduced and the trend followers are swift to join in. But the brakes don’t work, the rims don’t last, and they fail. The brake manufacturers introduce new brake pad materials that help, but not much. The wheel manufacturers play with new “weaves” and finishes that help, but not enough. The carbon wheels keep failing. People point fingers, and in the meantime, cyclists with money to burn keep “upgrading” components that they just upgraded the previous season. Now the brake manufacturers are pushing disc brakes – not because they’re superior to rim brakes (as they claim) – but because they solve the problems that were created by pushing carbon rims onto the market. And the wheel manufacturers will get to sell new wheels that accommodate brake rotors (and the rims will probably be shaved even lighter now -- another big selling point to the target market). And the bike manufacturers get to sell new bikes because the new brakes can’t be installed on last year’s frame. And the whole industry is happy. It’s very symbiotic, really . . . kind of beautiful.
Is it any wonder there aren’t more Retrogrouches out there?