For a generation or at most a generation and a half, people have become used to the idea of purchasing computer and car and photo and music and some other kinds of performance via improved technology. And to an extent that can happen with bikes, but … that’s the topic here.
Immature, emerging, nascent, and futuristic widgets are the ones most in need of and most likely to benefit from radical changes. Bikes have been pretty good for at least twenty of the past 154 years they’ve been around, but the whole headline-making machinery behind the popular bike demands constant change, and this situation creates another situation in which change is automatically positive, and anybody who questions it is automatically an anchor in the mud living in the past.
Let’s try to be objective about brakes. I won’t be able to be fully objective, but I’ll try, at least.
The bigger diameter the disc, the more leverage it has against the rotating force of the wheel. If you could somehow apply brakes well above the wheel, where now there’s only air or maybe a bee, it would take less force to slow that wheel. That’s currently impossible, so the next logical place to brake is at the rim. I’m not advocating the ancient spoon-brakes that pushed down on top of the tire; just talking ‘bout the rim.
The rim IS a disc, the biggest practical disc on the wheel, and mechanically it has a theoretical advantage over a hub-mounted brake. Hub-disc brakes compensate nicely by providing more contact area, which also dissipates heat well. But there’s a ton of force on that disc, and the force is transferred to the frame (seat stays) and fork and spokes, which is why frames have to be overbuilt to withstand the braking forces, and spokes on disc wheels break more often.
Hub-disc-ers point out that a rim designed for rim brakes has to compromise its two functions (braking surface, holding the tire), and that also is a theoretical point against rim brakes. But it so happens that the compromises are nearly invisible. The taller braking surface on a rim-brake rim also adds strength. Why don’t hub disc-brake rims have this? I can’t see the advantage to a low, curved, brakepad-incompatible sidewall, unless it’s to shave grams. There are better places to shave grams.
There’s room for all kinds of technology and detail in bicycles. Thank god not all bikes look like ours, or else ours wouldn’t be special and we’d go under in a month. But the idea that hub-disc brakes are an advancement or even desirable for general purpose riding…is nonsense. For special purpose riding, they must have their place. They belong in the panoply, for those super slimy gritty conditions that characterize downhill races on the slopes of volcanoes in the rain…and even maybe on the greasy trails you ride. But when they’re tossed onto $700 commute bikes and then sold (on the sales floor) as a desirable technology bump, that’s where it gets weird. For most riding (maybe not the extreme-condition riding that people fantasize about, but for most riding, in all seasons and over most terrain) rim-disc brakes remain our favorite.
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