Originally Posted by
AndreyT
Bicycle brake efficiency is capped by such objective limits as 1) wheel losing traction and beginning to skid, or 2) bicycle flipping over the front wheel. So, if your braking system can provide enough braking force to reach these objective limits, your braking system is obviously as efficient as any braking system can possibly be. Mechanical disc brakes easily reach these limits, which means that they are equally as effective as hydraulic ones.
Just to be clear, the wheel that loses traction is the rear wheel. The front wheel can’t skid on a bicycle. The rider will flip over the front wheel before that happens. Rim brakes can easily reach the same limit as any rim mounted disc brake.
Originally Posted by
AndreyT
The parameter that might differ here is not the efficiency, but rather the precision of the braking system, a.k.a. "modulation". I.e. how easy it is to precisely control intermediate braking forces from the braking lever. Hydraulic brakes usually offer better modulation than mechanical ones.
Another parameter that might differ is heat resistance. Mechanical brakes usually resist overheating better than hydraulic ones.
Your definition of modulation is correct but it’s not something that I have ever experienced with hydraulics. Hydraulics have always been powerful in my experience but it’s just raw power. Trying to control it in those places where intermediate braking is needed is not easy. Every hydraulic I’ve used has been grabby with control always on the edge of disaster. Mechanical (and rim brakes, for that matter) provide exactly the intermediate control as you describe it.
Originally Posted by
Chasboy
Hypothetically, if the disk and caliper were located out near the rim, (the disk would be about 26” in diameter), wouldn’t less clamping pressure be required, leaving everything else the same?
Hint, look at a rim and rim brake caliper. Look at a hub mounted disc and the caliper. Notice something? They are exactly the same. Same principle and very similar mechanism. They only differ in pad material. That is why hub mounted discs are somewhat better (but only somewhat). A rim brake could be made that is just as effective as a hub mounted disc if we were willing to use steel rim and sintered metal pads.
Originally Posted by
Koyote
My mechanical disc brakes stop about as effectively as a good set of rim brakes. And they are all perfectly adequate.
My hydro disc brakes generate muuuuch more stopping power than that, which allows me to more easily modulate my speed on some of the very steep descents around here -- I can do it without having to death grip the brake levers.
I haven’t ever had “a death grip” on any brake lever. I’ve mountain bike and ridden heavily loaded touring bikes...both road and off-road...with nearly every brake system around. Even with old center pull Weinmann’s, never had a death grip. The
only brake that I’ve ever had to be cautious with is hydraulic. I never found them to have a middle ground...only on and off.
Bike wheels often develop slight wobbles. If rim brakes were really close to the rim, they would rub. So rim brakes are set up with a fairly large gap between the rim and the brakes. When you apply the brakes, there is a fairly large distance for the brake to travel before it contacts the rim. Because rim brakes are designed for this large movement, less force is applied to the brakes..
If the wheels wobble, fix the wobble. Hub mounted rotors can just as easily develop a wobble and rub. Removing the wobble is much harder than taking it out of a rim. Spokes are easier to minor adjustments because it’s just a simple tension adjustment. Rotors require bending that is far less precise.
You also have the distance argument wrong. Rim brakes don’t need to have a huge gap between the rim and the pad. They are often
set up with a huge gap between the rim and the pad. Most bikes are adjusted so that the pad doesn’t hit the rim until about the lever has moved about half way to the bar. I don’t care what Sheldon Brown says, that makes for very mushy brakes and what many people experience when they use poorly set up rim brakes.
With disc brakes, the pads can be less than half a millimetre from the disk. Because there is much less movement in disc brakes, the same amount of force on the brakes lever results in much more pressure on the disc pads.
And here is the reason that many people dislike rim brakes. If the rim brake is set up so that the brake actuates much earlier in the pull, the brakes feel more powerful and the braking feels better. You can detune disc brakes to the same half lever pull and the best hub mounted disc will be the worst brake you have ever used.
Originally Posted by
Chasboy
yes, I’m just trying to get it clear in my mind that for the disk more pressure is required, contact patch is increased and the technology to exert the pressure needed is available in the caliper design, which apparently it is. Rim brakes don’t exert that much pressure and the rim contact patch as well as it’s stability is not as good as the disk.
A well set up rim brake will exert enough braking force to send a rider over the bars. That’s the limit of braking for bicycle under
any brake.