trail wrote:
jstonebarger wrote:
And I've never understood the modulation thing. Depending on how they're set up, I'm happy with the control I get with cantis, mini Vs, and calipers. I don't notice my disc brakes being all that much easier to finesse. Is modulation something measurable? Of course, I don't need much hand strength to use my disc brakes. One finger instead of two. But that's because they're hydraulic, isn't it?
If one setup does offer more modulation than another, how much is that because it's disc or because it's hydraulic?
It's tough to really get a well-articulated straight answer on the "modulation" story.
There are lots of physics and engineering trade spaces involved. Pad/rotor surface area. Pad-rotor velocity (higher at the rim). Effective distance of the rotor/rim from the axle and the effect on stopping torque (much more torque at rim than near the axle). Pad/rotor materials (far more limited with rim brakes). Force transmission mechanism (e.g. hydraulic vs. cable). I'm probably missing some.
My lay research hasn't resulted in a totally clear answer. It seems that the answer to what "modulation" means at the brake lever is represented by the below graphic (stolen from here).
Like I said, I don't know the real answer as to why disc brakes have more "modulation" but I have two suspicions.
One suspicion is that disc brakes are located in a location where they have less mechanical advantage over the wheel. That seems like a bad thing at first. But it might be good in terms of "feel." As a simplistic example, lets say you need 10 N-m of torque at the rim to lock up the wheel. With little high-friction pads. When translated to force at the lever it's a pretty small range of forces, and it may be hard to provide "feel" to the rider. Anecdotally, brake manufacturers have to build some flex into calipers to avoid having excessively "grabby" feel to the brakes.
Disc brakes need to provide much more torque down near the axle to reach lock up. Let's call it 100 N-m. And they have much more pad area to work with, and they can work with different (lower) pad frictions. That can translate to a wider range of forces that can be "felt" at the brake lever. And it's my understanding that disc brake systems can be built much stiffer, so that modulation occurs at the pad-rotor interface rather than in caliper flex. More feel to the rider.
I think the cable vs. hydraulic differences are just in lower friction for hydraulic (more feel to the rider, just like less caliper flex), and less flex in the material itself (cables stretch a little bit, while fluids are almost perfectly uncompressible).
This is just my Google-fu. An actual brake engineer might come on here and take me to school. And I hope one does. Because I'd like to learn. There's precious little good information on this in the bike media provided to consumers. The maxim that "disc brakes provide more modulation" is just repeated over and over with little explanation, even by "tech-savvy" industry writers.
While I'm not a brake engineer, I have tested literally hundreds of different brakes (mostly mtb, still relatively few road discs out there). The amount of "modulation" on discs runs a wider gamut than on rim brakes. I have a set of sample mtb disc brakes that have about as close to zero modulation as possible. And these are mechanical, not hydraulic, so even with a bit of housing compression they're still more solid feeling than any hydraulic I've tried. They feel rock solid at the lever, and lock up almost instantly. I can easily skid with just my pinky. I've never come across a hydraulic brake that grabbed as hard as these do. But these also have pads about the size of dime, and would fade almost instantly. Hard to keep from locking them up at the start of a descent, and no brakes at all halfway down...
"I'm thinking of a number between 1 and 10, and I don't know why!"