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

(Example: Framebuilders:Tony Beek)

From: Jerry & Liz Moos <jerrymoos@sbcglobal.net>
To: "Sheldon Brown" <CaptBike@sheldonbrown.com>
Cc: <classicrendezvous@bikelist.org>
References: <14.20b673b1.2970d595@aol.com> <001a01c19b04$573e9c20$6e615cd1@YOUNGC> <v04210107b86564aa3898@[10.0.1.13]> <3C3FE35C.42FC@adnc.com> <v04210112b8660b4a5e9c@[10.0.1.13]>
Subject: Re: [CR]Why normal reach calipers on short reach frames?
Date: Sat, 12 Jan 2002 03:45:09 -0600


Very good article, Sheldon, but focused on analyzing cantilever brakes. Have you done a similar analysis for conventional centerpulls, conventional sidepulls, or dual pivots?

Regards,

Jerry Moos
Houston, TX


----- Original Message -----
From: Sheldon Brown
To: rocklube@adnc.com
Cc: classicrendezvous@bikelist.org
Sent: Saturday, January 12, 2002 10:16 AM
Subject: Re: [CR]Why normal reach calipers on short reach frames?



> 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 |
> +------------------------------------------------------+
> 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