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From: AMuzi <am@yellowjersey.org>
Newsgroups: rec.bicycles.tech
Subject: Re: Grant Peterson's Thoughts on Disc Brakes
Date: Wed, 25 Sep 2024 16:01:22 -0500
Organization: Yellow Jersey, Ltd.
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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>
>>>
>>> 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.
>>>
>>> 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?"
>>>
>>> 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."
>>>
>>> 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.
>>>
>>> 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.
>>>
>>> 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."
>>>
>>> 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."
>>>
>>> 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.
>>>
>>> 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.
>>>
>>> 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."
>>>
>>> Couldn't agree more.
>>
>> I'm OK with most of that.
>>
>> 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