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


Example: Racing:Roger de Vlaeminck

In-Reply-To: <3C3FE35C.42FC@adnc.com>
References: <14.20b673b1.2970d595@aol.com> <001a01c19b04$573e9c20$6e615cd1@YOUNGC> <v04210107b86564aa3898@[10.0.1.13]>
Date: Sat, 12 Jan 2002 11:16:48 -0500
To: rocklube@adnc.com
From: Sheldon Brown <CaptBike@sheldonbrown.com>
Subject: Re: [CR]Why normal reach calipers on short reach frames?
Cc: classicrendezvous@bikelist.org


Brian Baylis wrote:
>Sheldon,
>
>I was refering to the concept of minimum clearance and shorter lengths
>resulting in stiffer parts/frames or what-have-you. My understanding of
>the purpose of designing a frame to give minimum brake drop was to give
>the brakes the least amount of flex in the caliper.

That wasn't actually the point I was making about the brakes. While it is true that there is less flex with a shorter arm, I was referring to the difference in mechanical advantage, which results in the pad being squished harder against the rim for the same amount of cable tension.

The issue of flex in brake systems is, in my opinion, less important than mechanical advantage or friction in the mechanisms. Flex doesn't actually have any direct effect on the power of a brake, that's just determined by the geometry and frictional losses.

Flex does consume some _travel_ so a flexible brake system has to be adjusted a bit tighter to make up for it. Since there's only a limited amount of travel available, determined by the average length of the riders' fingers, flex does enter into the design aspect of a brake system. Even so, it's more a matter of providing a good "feel" than of actually improving stopping power.

People often misjudge braking systems based on flex-related feel issues.

A braking system with low mechanical advantage will not transmit enough force to the caliper/cantilever to cause much flex, or to squish the pad very much. Such a system will have a very firm "feel" at the lever, once the pads hit the rim, the lever will suddenly stop moving. This is often interpreted as a sign of wonderfulness in brakes, but more often it is a sign of a braking system that will require a lot of finger strength to stop even in favorable conditions.

The classic Campagnolo single-pivot sidepulls are examples of this, and you also run into this issue with cantilevers that have been set up incorrectly.

For more on this issue, see: http://sheldonbrown.com/cantilever-geometry.html#mechanical
>Shorter tubes also
>gives the same results in the frame. I suppose I should have explained
>the concept I was focusing on which is not brakes per se, but shorter is
>stiffer. Comming from a racing background and a racing frame heritage it
>is something I am aware of as a framebuilder. I guess I should also have
>mentioned that I just don't see the point of track bikes built with road
>clearances or with road fork blades for that matter; assuming we are
>talking about track bikes for racing as opposed to some perversion
>thereof. Make sense, or no?

Hmm, perversion... ;-)

I'm not sure I buy "shorter is stiffer" in the areas that relate to brake reach.

In the front, the total length of the fork is related to how high the handlebars need to be. Reducing the vertical clearance by shortening the blades results in a corresponding lengthening of the steerer. I don't believe that results in a stiffer fork.

I do agree about the road blades vs track blades. The round blades used on track sprint bikes give greater lateral stiffness, which is much more important on a sprint bike. Round blades give less fore/aft stiffness, which is mainly a braking issue, so it doesn't apply to track bikes.

In the back, I refuse to believe that moving the seatstay bridge up or down a centimeter has any effect whatever in the performance of a bike.

I, and many other fans of classic frame design, continue to decry the useless shortening of clearances for what I perceive to be purely cosmetic reasons, because it gains nothing and creates a bicycle that is less useful once its racing career is over.

Sheldon "Pervert" Brown Newtonville, Massachusetts +------------------------------------------------------+ | It were not best that we should all think alike; | | it is difference of opinion that makes horse-races. | | -- Mark Twain | +------------------------------------------------------+
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