Following the thread with interest.
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- A rim brake works directly on the rim and the attached tyre; a disc brake applies a potentially large torque moment at the hub. The latter has two main disadvantages:
- The torque moment must be transmitted to the tyre through the wheel components: flanges, spokes, nipples, and rim spoke bed. An engineered disc brake would reduce weight by not having most of the metal rim components
- A front disc brake places a bending moment on the fork between the caliper anchor points and the tip of the dropout. In order to counter this moment and to support the anchor points and weight of the caliper, the fork must be of a certain size (most likely heavier).
https://en.wikipedia.org/wiki/Bicycle_brake#Disc_brakes
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I think that the critical difference between rim brakes and disc brakes, that the addition was trying to convey, is that with disc brakes the torque of braking occurs between the hub and the rim,
while with rim brakes the only time torque occurs between the hub and the rim is at the rear hub during pedaling. It is true that rim brakes can work with radial spoke lacing on the front wheel, while disc brakes require cross-lacing (usually 3x). So while the addition was somewhat inarticulately expressed, it is indeed based in fact.
Matt Gies 17:25, 11 May 2006 (UTC)
https://en.wikipedia.org/wiki/Talk%3...kes_and_spokes
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With rim brakes, forks are loaded in bending at the fork crown that similarly supports road shock, while a disc brake places an equal bending torque at the tip of the fork and only on one blade. A fork can only be slender and light because it carries no bending loads at the dropout. With disc brakes, forks would require a substantial increase in cross section (and weight) and the brake would be heavier.
https://www.sheldonbrown.com/brandt/brakes.html
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