NEW Disc Brakes

 

Short version followed by long version, if you want more of a deep dive:

Short:

Rims are rotors, are discs, and furthermore, they're large-diameter discs, which give them more leverage and power. From a finger-squeeze-to-rotor squeeze perspective, they're more effective. Hub-mounted disc brake have taken over because the perception is easy to sell: If cars and trucks have "disc" brakes, they must be better and we know they work. But no. Disc brakes have taken over on bicycles for fishy reasons:

  1. The bicycle market is pretty much full, and high-volume makers (who control the trends) need newness to get riders to replace bikes frequently.
  2. Carbon rims get trashed with rim brakes, and many high-end bikes have carbon rims, so they HAVE to have disc brakes. Cheaper bkes copy them.
  3. People hear "carbon" and drool in admiration and awe.

Disc brakes on traditional style fork ends (dropouts) jerk the front wheel down a nd forward out of the dropout. This is dangerous to riders and a liability to mfrs. This has led to "thru axles," which completely encircle the wheel's axle. But thru axles make it harder and slower to remove and install wheels. It can be argued that they're safer, but--seriously--they are purely a response to a need created by the wheel-ejecting force of disc brakes. It's a mess.

A rim is a disc, but it is far enough away from the axle to not create those problems.

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Long version:

All of our bikes have disc brakes

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.

- Grant