dynaryder

Harder materials. Disc rotors are stainless,wheel rims are(usually) aluminum. Disc pads are metal or organic particles suspended in resin,rim pads are rubber. Discs also don't usually get as much debris on them.

dynaryder

Materials. See above.

I'm going to guess two reasons. First,there's alot more clamping force involved with disc brakes. The braking surface is considerably smaller and much closer to the center of the wheel's rotation,so the calipers need to clamp harder to achieve the same level of braking performance as rims. This harder clamping prolly helps strip any ice build-up off. Second,disc rotors hold heat better than rims. Smaller surface area to radiate heat,plus it's partially blocked from the airflow by the rim/tire,and stainless holds heat in better than alum. All I know is,I've ridden in freezing rain/sleet and never had any probs with my discs.

nmanhipot

There's also several torque factors involved. I believe disc brakes tend to modulate better because the moment arm is much smaller from the center of the hub axle to the center line of the disc brake pad, as compared to the distance to the center line of a rim pad. This requires more torque to be applied to the rotor as compared to a rim. This also leads to a wider range of dynamic frictional coefficients. Of course, there's also a dampening effects from the spokes, since the breaking force occurs in the hub and radiates out to the rim through the spokes. Rim brakes obviously have a more direct impact on the forward motion of the contact surface than hub-based brakes and for this reason I've noticed that caliper brakes tend to pulse more than disc brakes, especially in wet weather.

Beacuse of the shorter moment arm, comparatively higher torque required, and spoke dampening, disc brakes have superior modulation than rim brakes. When I ride my Dura-Ace equipped road bike, the braking feels a little less smooth and predictable than my disc-brake equipped commuter. I get used to the difference pretty quickly, but there is a noticeable difference in performance, especially down the steep hills in my 'hood. The pulsing generally occurs with the front wheel and is dampened by the fork and transfers the energy to the frame and the rider, but there's definitely a discernable difference and much more movement in the fork.

For your reading pleasure: https://en.wikipedia.org/wiki/Torque

ItsJustMe

You might, but there are both tighter clearances and a lot more force involved. If you have ice on the rims, and apply the brakes, you're pushing a round-shouldered, rubber pad against the ice, and there's a fair amount of springiness in the system. It just slips over the top.

If you have ice on the brake rotor and apply the brakes, you're pushing a square-edged, very hard metallic composite pad into ice that's on a thin hard stainless steel rotor. It'll tear the ice off in a single revolution.

Also, there's very little clearance between the pad and rotor; mine are adjusted to where I can barely see daylight between them, so probably 1/32". If there's more ice than that, it just gets ripped off while I'm riding, BEFORE I hit the brakes.

Also, rims have a lot more thermal mass to dissipate heat than a rotor does. Obviously the same amount of heat is being put INTO either the rim or the rotor to stop a given mass from a given speed. But that heat will make the rotor much hotter than the rim, so it'll stay above freezing longer. Since freezing rain really only occurs within a few degrees of 0*C, you don't have to use your disc brakes very many times an hour to keep the rotor temp above freezing, as opposed to rim brakes.

vrkelley

Ho man...is this thread still going? thanks for the great replies

DannoXYZ 

Just a clarification, the "flying over the bars" sensation is based upon Newton's laws. What's actually happening is that the braking/deceleration force is pushing back on the bike at the front-tyre's contact patch. The body on top still wants to continue in a straight line at the same speed as before. You end up with a torque around the front contact patch.

This torque has ONLY to do with the amount of deceleration, regardless of the type of brake you use. ALL brakes pretty much has enough power to generate enough deceleration to send you over the bars regardless if it's calipers, cantilevers, V-brakes or discs. The faster you decelerate, the more force your bike will be pushing backwards and the more "over the bars" force you'll feel.

The only way to fight this is to not brake so hard, and/or to scoot your center-of-gravity backwards and lower. This lowers the roll-over torque given the same amount of deceleration/braking force.

The ultimate limit on deceleration/braking force is front-tyre traction. At some point, even with scooting back and down, you'll have so much roll-over torque that the rear-wheel will be unweighted and you'll fly over. This typically happens well before you overcome the traction limits of the front-tyre (locked and sliding). With sufficient training, it may be possible to lock up the front-tyre and get higher maximum-deceleration/braking force right before it slides, but I don't know of many people with this kind of training.