Re: [CR] here we go with another can o' worms (was brake flex)


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Date: Tue, 4 Jan 2011 17:51:04 -0800
To: "Mark Petry" <mark@petry.org>, <classicrendezvous@bikelist.org>
From: "Jan Heine" <heine94@earthlink.net>
Subject: Re: [CR] here we go with another can o' worms (was brake flex)


At 2:05 PM -0800 1/4/11, Mark Petry wrote:
> But a centerpull has several additional pieces when compared to
>a sidepull, and they all flex under braking loads.

Centerpulls and cantilevers with brazed-on pivots are more powerful than sidepulls because they place the pivots much closer to the rim. This means that you have only less than an inch of brake arm to flex, plus a very beefy pivot that doesn't flex much itself.

On a sidepull, you have a tiny pivot (5 mm centerbolt) all the way on top of the tire, and then an aluminum brake arch that wraps around the tire. It's not hard to flex that. The longer the brake reach, the more flex (on a sidepull), hence you get more brake power with short-reach brakes. Disc brakes are ultra-short reach brakes, and have the most power.

On a cantilever, the part above the pivot on a cantilever is just a cable stressed in tension, and on a centerpull, the upper arm stressed only in one plane, so it can be thin and tall, like the Mafac brakes. It doesn't flex much in a detrimental way. That is why centerpull can be made so much lighter than a sidepull.

For example, a "standard reach" Mafac Competition weighs about 20 g less than a Campy sidepull with the yoke. If you braze on the pivots, you reduce the weight by another 50 or so grams.

For those interested in the history and technology of brakes, I recommend Bicycle Quarterly "Brake Special" (Vol. 7, No. 2). We basically did a "Dancing Chain" for brakes, with hundreds of Rebour drawings, studio photos, and explanations how they work and why some work better than others. You can see a snippet at

http://www.bikequarterly.com/BQ72.html

Jan Heine Editor Bicycle Quarterly 2116 Western Ave. Seattle WA 98121 http://www.bikequarterly.com

Follow our blog at http://janheine.wordpress.com/