Re: Grant Peterson's Thoughts on Disc Brakes

On 9/25/2024 3:34 PM, Frank Krygowski wrote:

On 9/25/2024 4:20 PM, AMuzi wrote:

On 9/25/2024 2:58 PM, sms wrote:

<https://bikeretrogrouch.blogspot.com/2016/06/gps- thoughts- on-disc- brakes.html>
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If I could summarize Petersen's view on disc brakes, it is that there's nothing exactly wrong with them, but they aren't the vast and remarkable improvement over good rim brakes that the industry has been pushing for the last few years. In other words, they don't make bikes with rim brakes obsolete, and shouldn't.
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Petersen points out some of the benefits of disc brakes -- notably the fact that they are less affected by mud and water, and that they don't heat up rims to the point of tire blowout on scary-fast descents. Then again, for the majority of cyclists and conditions, those benefits are over-sold. And on the down-side, as he adds, the leverage of a disc brake concentrates a lot of force near the hub, putting a lot more stress on seat-stays, and on fork blades far from the crown. As a result, frames need to be beefed up in those areas, which can affect compliance and comfort. He also mentions how those braking forces concentrated out at the frame ends have been enough to overwhelm quick releases and even "lawyer tabs" on front forks, necessitating the move to through- axles. To wit, he asks the question, "Which is better -- a mechanical system that localizes stress on a small area, then bullies it into submission with bulk and beef, or one that disperses stress and spreads it out?"
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He goes on to say, "Disc brakes are fine, but if the bike could speak for itself, it might request a rim brake. . . The fact is, rim brakes are getting pounded these days, but it's a kind of artificial pounding by fashion and commerce."
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I would have to agree with that. I mean, if I were looking at a new bike on the showroom floor and the bike I wanted came with disc brakes, I certainly wouldn't reject it for that reason. But at the same time, I wouldn't be drawn to a particular bike because it had discs. And if there were another bike basically the same but with rim brakes, and selling for a lower price, I'd probably choose to save the money.
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But there's another point to be made that resonates with me. And that is regarding the simplicity of a traditional rim brake. Everything is out in the open, easy to see and easy to maintain, while potential problems are easy to diagnose and solve. Some would likely point out that once a modern hydraulic disc brake system is set up properly, it needs little maintenance. To which I would respond that getting it set up properly is a lot more likely to be something that requires an experienced mechanic, and if something actually goes wrong (and things do, indeed, go wrong - even on the best of systems) it can be a lot harder to diagnose or solve. This is something I wrote about last year when a bike reviewer for BikeRadar had a pretty scary disc brake failure on a test ride. In that case, the brake components were sent back to Shimano for inspection, but ultimately, even they couldn't adequately explain how or why the failure occurred.
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Petersen describes it like this: "Ultimately, you can expect the bicycle of the immediate future to become more of a high tech black box, with cables being replaced by hydraulics, and the visible levers and pulleys and other simple machines that combine into bicycle magic being hidden or replaced by electronics. The bicycle of the future will, absolutely, be shrouded in mystery and sold on reputation and faith, like a Samsung flat-screen TV."
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He continues, "There's a tendency to trust mechanisms you can't see more than those you can, because when you see how something works, you see also the potential for failure. . . If you're mechanically adept you might be more attracted to something you can figure out and fix, but more people aren't that than are."
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That really nails it for me, and it's something I've touched on again and again in this blog - whether it's electronic shifting, or disc brakes, or integrated/ connected dashboards and other electronic gewgaws -- all that stuff makes the bike more of a "black box" (I like that description, so I'm using it) and takes it further from the simplicity that I value in a bicycle. Fly-by- wire electronic and hydraulic systems, for efficiency, comfort, and safety, all controlled by a state-of-the-art computer is fine for my car. But what makes a bicycle special is that it demonstrably doesn't need any of that to make it any better.
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People do seem to like push-button/touch-screen convenience, which oddly enough seems so simple, but only because the far greater complexity is kept hidden, and only accessible by those who are specially trained and certified to look behind the plastic covers. That illusory simplicity is great when everything works as it should, but vanishes into the ether when something goes wrong. It's like a microwave oven. If something goes wrong, it ends up costing more to fix it than to just scrap it and buy a new one. Bikes and bike components shouldn't be that.
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In the end of Petersen's blug post, he concludes by saying "Don't dis the rim brake. It's beautiful and it works, and today's rim brakes are better than ever."
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Couldn't agree more.

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I'm OK with most of that.
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But braking force against the hub is the same, AEBE.  Any increase is at the margin, when a disc generates more braking force than a rim brake- a rare event given rear wheel lift.

 Braking force against the hub is the same, but the force that fights the twisting moment is, for a front disc brake, applied at the bottom of the fork. There's greater stress on the fork, so it has to be beefier. Same thing at the rear stays, although I don't think it's as critical there.
 

Agreed on fork/frame leverage.  And home 'disc conversions' with lightweight stays/blades do indeed snap off.
--
Andrew Muzi
am@yellowjersey.org
Open every day since 1 April, 1971