Subject: [Yak] disks not rims

[John S. Allen]

At 05:01 PM 9/15/2008, Tony Raven wrote:

>On the one hand you can consider rim brakes as disk brakes with a 
>very large diameter rotor.  But the big difference, apart from the 
>contact surface being away from the mud and wet on a disk brake, is 
>that you do not have to cope with the rim being out of true and you 
>can optimise the disk/pad materials for braking.  The mechanical 
>advantage you could get with a perfectly true rim is lost because 
>you need to be able to take up the brake-rim space left to cope with 
>out of true before you can apply braking force.  Disk pads run very 
>close to a solid steel rotor so you can run high mechanical 
>advantage to maximise the clamping force on the rotor and you can 
>optimise the rotor and pad material.

True :-) enough. Another problem is to keep the brake shoes centered 
so one doesn't drag on the rim. Modern dual-pivot brakes 
automatically do this; most linear-pull brakes have spring tension 
adjusters -- so this situation has improved in recent years. I'll 
admit that I have an unfair advantage with my rim brakes, because I 
keep my rims very true. BF does a good job with this, too, and the 
small wheels are sturdier than big wheels.

Decades ago, Dr. David Gordon Wilson, author of the book Bicycling 
Science, invented something even better: a rim brake which 
automatically increased mechanical advantage when the shoes came into 
contact with the rim. Unfortunately, the industry wasn't interested.

compared with disk brakes, rim brakes are simpler, lighter, and place 
less stress on the frame and fork, also allowing lighter construction 
of these. Rim brakes will continue to be preferred in applications 
(road racing, performance-oriented recreational road riding, 
moderately-priced bicycles etc.) where these advantages are valued.

John S. Allen

Regional Director for New York and New England, League of American Bicyclists
League Cycling Instructor #77-C and Member of the League's Education Committee

http://bikexprt.com