Can’t Stop Disc Brakes

Is it performance or fatalism?

Road bikes are going to get disc brakes. This much has been clear for the past few years.
But there’s much to suggest the reason is fatalism, not better braking. And just about every ode to the greatness of disc brakes reinforces this feeling. There’s little that demonstrates that disc brakes are being embraced because they’re better. There’s much that demonstrates that discs are being embraced because they’re inevitable.
The biggest issue is the data. Which is glaring in its omission. The bike industry spends lots of time and effort reporting on test data. If someone has an expensive bike to sell, they’re most likely going to tell you how stiff the bike is, how compliant it is, how aero it is, either compared to the competition or the previous iteration of the same model. Wheel manufacturers, many of whom are also in the brake business, take all their wheels to the wind tunnel and boast about numbers. Percentages are bandied about: 15% stiffer, 12% more compliant, 8% less drag, and so on. There are safety standards for helmets. There are safety standards for bikes. Just about everything else these days is taken to a wind tunnel to prove how slippery it is against the wind. Even disc brake bikes. But somehow, testing disc brakes for the mundane job of stopping doesn’t get the same attention.
Surely, component manufacturers have test rigs and are testing out brakes in the privacy of their own factories and warehouses. But the results of those tests are somehow not worthy of putting out in ads or press releases. It’s hard not to wonder if disc brakes don’t seem all that impressive in stopping tests.
Some disc brake lovers seem to acknowledge this. First, they discuss greater stopping power. Then, they point out that really, the beauty of disc brakes isn’t so much the stopping power, but the modulation. You can control speed better by braking later, scrubbing off speed quicker, and getting back to pedaling sooner. Is this true or is it false?  And considering how most of the stopping of a bike occurs at the front wheel, does rear brake power or modulation matter at all? 
There are plenty of competent, even outstanding people out there road-testing bikes for magazines, but they can be mistaken. Two examples of how people can make errors. First, tire pressure. When people have tested tire pressure without knowing what the pressure is, they have been known to report that over-inflated tires feel faster. Testing indicates they’re slower. When full-suspension mountain bikes were making inroads, some pro’s did a lap of a cross-country course on a hardtail and then repeated the run on full-suspension. They reported the run on the full-suspension felt slower, but timing indicated they were faster.
Surely, there’s a way to test how fast a wheel or a bike can be brought from 30mph to 20mph via braking without skidding.
The “without skidding” part is important. Which is why modulation might really be the issue. If the stopping power is so great that the tire loses grip on the road surface, the “better” brakes could lead to a crash.
Wouldn’t it be great to know what the differences are?  If there are differences at all? How much?  Give us some numbers. Start with a hand force of X translates into the calipers applying a force of Y, and then stopping the bike in Z feet. A test rig should be able to allow us to see what happens when forces are made commensurate, like applying the same force to a caliper, be it for disc or rim brakes, and then measuring the force at the lever and how that influences stopping.
It would be nice to know if brakes could be too powerful. By this, can finger-squeezing sensitivity can be such that a one might not have the ability to modulate easily enough. Maybe there’s an ideal amount of resistance one should feel at the lever.
Since the UCI has gotten in the business of certifying bikes, testing brakes would be a natural, and welcomed, growth. Start with current rim brakes and aluminum rims. Move on to carbon rims and carbon-specific pads. And the latest generation of brake tracks, both in carbon, and aluminum. Perhaps ban, for racing, brake/pad/rim combos that don’t stop as well as others. And then nudge the industry, with progressively higher standards, to improve so that everyone has a system that works well.
And there have been efforts over the years to improve braking with rim brakes. We’ve seen it at the rim, the pad, and the caliper. Have these done anything?  There’s speculation that the move to discs is, in some ways, a response to poor braking on carbon rims. There are indeed plenty of carbon rims out there that don’t inspire confidence, but for both carbon and aluminum, there are brake tracks that have tried to address braking concerns. Same with pads and calipers. SRAM even has a hydraulic rim brake—it would be great to know how it compares to both cable-actuated rim brakes and hydraulic disc brakes.
There isn’t any magic in disc brakes. It’s about friction. It would stand to reason that if one system stops faster, it generates more friction faster and possibly more easily. If discs stop better in the wet, this is probably why. But it also suggests a limitation–too much heat is created by the better-stopping system.
Friction plays into modulation, but there must be limits. Namely the tire’s grip on the road. How much better can discs be when stopping in the wet?  While it’s hard to get excited about hard braking in the wet, should the bigger fear be how poorly the tire grips the road?  Tire adhesion is dramatically reduced in the wet, and just about everyone who has ridden in the rain has experienced their rear tire losing traction and starting to fishtail.
There are laws of physics at play. The solution to better braking, no matter where the friction is generated, is solving an engineering problem. Either it’s stopping a 622mm rotor (the approximate diameter of a 700c wheel), or a 140-160mm rotor. We have no idea if it’s easier or cheaper to build on current technology or to discard everything done so far and start anew.
Even assuming that disc brakes stop better, that hardly settles the issue. There are several other concerns. Little has been written about servicing, or service life. I assume that most people will be able learn how to service them at home. At the same time, considering all the warnings about contamination of pads, is this a bigger issue for discs than rim brakes?  How long will a set of disc brake pads last compared to rim brake pads? How easy is it to service on the road?  Resin (aka organic) disc brake pads have been known to wear out during a single muddy cyclocross race (disc brake proponents might suggest switching to semi-metallic, only these typically have more power and less modulation). Is this something to worry about when riding 100 miles in the rain?  What about a muddy Paris-Roubaix?  How much attention does a normal user have to give to the brakes?  Is it more or less than for rim brakes? 
What about overheating hydraulic systems?  With rim braking systems, tubulars have been known to overheat and blow out when the brake is dragged for long enough. It happened in the days of aluminum rims even. And some carbon clincher rims have softened enough to cause blowouts. This overheating—in some cases the temperatures generated before these rim-brake issues flare up is higher than the boiling point of some hydraulic fluids–suggests that a much smaller rotor has greater potential to overheat. And Shimano, SRAM, and others have incorporated design elements that are supposed to minimize heat buildup. There have been few publicly-available tests indicating how well these elements work.
Hydraulic systems might need more care beyond just the pads and designing ways to minimize heat buildup. While the hoses are closed, moisture can still get inside. Performance not only degrades, but the boiling point of the fluids is reduced. This can happen whether or not you ride your bike.
And there’s the attention that a rider must heed to making sure her braking system is safe. There have been two recent quick-release skewer recalls on mountain bikes equipped with disc brakes, and the recalls date back to bikes sold in the 1990s, and bikes priced across the spectrum. While the bike industry bears a only bit of responsibility, and there are questions to be asked about both shop and rider education, more striking is that the margin for error in securing a wheel to a disc brake road bike might be dramatically reduced.
In terms of racing and rider safety, there have been interesting arguments on both sides. Here, it would have been great for the UCI to get involved. For example, mixing different brakes in the peloton will not necessarily be dangerous—after all, there are bikes with different braking systems already in the peloton, and they don’t all stop equally well–though it will necessarily complicate servicing bikes on the road. The dangers of exposed rotors are a legitimate concern: if people can be gouged by chainrings that aren’t spinning and are in the middle of the bike, it seems reasonable that rotors that are closer to the edges of the bike and will be spinning and/or hot and have a greater likelihood of contacting riders in crashes.  If racers lose because they can’t fast enough bike change, that’s something the teams and their sponsors will have to live with.
The pro riders will do their job of being test mules for the rest of us. Hopefully rotors won’t flay flesh. The racing experiences will probably help work out standards for things like rotor diameter, mounting points, and axles. In many cases, the “winning” designs will probably be the ones easier to work on and less prone to misalignment rather than the ones that stop best.
Some think this conversion to disc brakes will be good for the bike industry. It will only be good if it ultimately results in more people riding more often.
If we end up with bikes that stop no better, need to be serviced more frequently, go into the shop more often, and cost more to buy and operate, we will have lost. If we’re doing this to be “modern” or to keep up to date seems misplaced at best: they’ve been putting motors on bicycles for years, and by choosing to ride a motor-free version, we’re deliberately limiting the technology at our disposal.
When looking at disc brakes on road and cyclocross bikes, it would be great to know that not only will performance be markedly better, but that the amount of time and effort it takes to keep the brakes going either doesn’t change or gets reduced.
But disc brakes can’t be stopped. Maybe they shouldn’t be, but it would be great to know there’s more than fatalism behind the change.

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