Re: [CR]Why normal reach calipers on short reach frames?


Example: Production Builders:Peugeot:PY-10

In-Reply-To: <001a01c19b04$573e9c20$6e615cd1@YOUNGC>
References: <14.20b673b1.2970d595@aol.com>
Date: Fri, 11 Jan 2002 23:21:36 -0500
To: "Charles T. Young" <youngc@NetReach.Net>, <RaleighPro531@aol.com>
From: Sheldon Brown <CaptBike@sheldonbrown.com>
Subject: Re: [CR]Why normal reach calipers on short reach frames?
Cc: "Classic Rendezvous" <classicrendezvous@bikelist.org>


I wrote:
> > Short reach calipers generally have a reach range of 39-49. Long
>> reach is usually 47-57. (Talk of "normal" reach is likely to be
>> confusing, and I'd urge everybody to purge this ambiguous term from
>> their vocabulary.)
>>
>> A caliper brake has its greatest mechanical advantage when the pads
>> are high up the arms, close to the pivot. Back when long reach was
>> the "norm" a conscientious frame builder building a high performance
>> bike would locate the bridge and crown to maximize the braking
>> mechanical advantage. Especially, the old Campag single pivots had
>> such poor braking that this was a very nice touch.
>>
>> You may also see this on bikes made to work with both 630 mm (27
>> inch) clinchers and 622 (700c) tubulars. When the larger wheels were
> > installed, the shoes should be near the tops of the slots.

Brian Baylis wrote:
>
>Pin a silver star on Mr. Sheldon Brown! That is exactly the reason for
>minimizing the brake reach on a racing bike. The same principal applies
>to track frames and tire clearance between the fork crown and the seat
>stay bridge.

Huh? How does this apply to track frames? They aren't made for brakes.

Charles T. Young wrote:
>As long as the brake pads are the same distance from the pivot point (and
>the arms have equal amounts of flex), it shouldn't matter. The braking force
>will be solely a function of the length of a straight line from the pivot to
>the pad.

That's a bit of an oversimplification. With a single-pivot sidepull, it's a function of that line _and_ of the distance from the pivot to the cable. The ratio between these two distances is the mechanical advantage of the caliper. Calipers with longer lower arms generally have longer upper arms to maintain the desired mechanical advantage.

The _system_ mechanical advantage is the product of the caliper's M.A. and that of the lever.

The mechanical advantage or "power" of a caliper has nothing to do with it's quality or cost, you can make a caliper with any desired mechanical advantage for the same cost. Making them both light and stiff does cost money, but stiffness is mainly a matter of "feel" not function.

Douglas R. Brooks wrote:
>I understand Sheldon's point entirely but my question is that
>given the power of contemporary brakes, such that long reach brakes
>like RX100s have stopping power comparable to, say, Dura Ace brakes,
>are the vast majority of modern brakes made short reach to create
>_that much more mechanical advantage_? Alas, I see no advantage
>to the short reachers now that brakes are so much better!
>I guess race bikes don't need clearances, is that it?

I believe that's it. This is sort of like the disappearance of fender eyelets. Although it doesn't make the bike a jot faster, it says "this is a pure racing machine, useless for any practical purpose." It's basically a fashion statement.
>Classic content: Is there any reason why classic centerpulls have
>not made a comeback?

I've long thought that they were due onacountta the cyclical nature of bicycle technology.
>They have many advantages as I see it: easy
>set up, pulling from the center is mechanically simple and distributes
>the power efficiently, lots of clearance for fenders, so much so that
>on my Rivendell it is the fork crown, not the brakes that create the limits.

Centerpulls, like dual pivots, are a way of getting the pivot closer to the rim and thus shortening the lower arm, increasing mechanical advantage while allowing plenty of tire clearance.
>With Mathauser pads, even the low end Mafacs work as well as any
>modern brake I use (DA, Record, Mavic, etc.)

That's true. The much superior design of modern brake levers also helps. My '61 Paramount still has the original Weinmann centerpulls, but with low-end Ergo brifters they work great, in fact better than any single-pivot Campagnolo setup I've ever tried.

Sheldon "Freins" Brown Newtonville, Massachusetts +------------------------------------+ | Experience is a hard teacher, | | because she gives the test first, | | the lesson after. -- Vernon Law | +------------------------------------+
      Harris Cyclery, West Newton, Massachusetts Phone 617-244-9772, 617-244-1040, FAX 617-244-1041
            http://harriscyclery.com
       Hard-to-find parts shipped Worldwide
            http://captainbike.com
    Useful articles about bicycles and cycling
            http://sheldonbrown.com