Each time I write about the proliferation and advantages of disc brakes on road bikes, a litany of naysayers parry. Common criticisms: disc brakes are unreliable and finicky, heavier than rim varieties, hinder aerodynamics, fade because of overheating, prevent fast wheel changes, and are dangerous when ridden together with rim brake–equipped bikes because of the differences in cornering speeds and stopping times. Most of these arguments are refutable.
Safety has been at the heart of the most substantive debate over the technology, driven mostly by racers who worry that sharp, hot rotors could cut or burn riders in crashes. And the UCI didn’t help the issue last year with its dithering. At the beginning of the 2016 season, the organization approved disc brakes for widespread use in the pro ranks. But after a rider was cut during a pileup at Paris-Roubaix and blamed the wound on a metal rotor, the organization yanked the program, despite doubts that the rotor caused the injury. Discs are returning to the Pro Tour for 2017, this time with rounded edges and possibly protective casings to address racers’ concerns. Their return is a signal that, like it or not, this is the future.
The other common argument I hear against discs is that they are a solution to a problem that doesn’t exist—rim brakes work fine. (Having descended countless wet mountain roads on carbon wheels with rim brakes, where stopping power all but disappears, I beg to differ.) But rather than simply continue to rail on the subject, this year I decided to put the debate to our test team in Sedona, Arizona, by offering up two comparably spec’d bikes—one disc, the other rim—for comparative feedback.
On one side, we had the Trek Domane SLR 8. Since the new Dura Ace 9100 wasn’t available at test time, this is not a stock build, but a one-off we received earlier this year. It’s equipped with Dura Ace 9000 parts (including rim brakes), upgraded Bontrager carbon Aeolus 3 clinchers, and stock 28-millimeter Bontrager tires. The bike weighed a feathery 15.3 pounds.
On the other side was a stock Trek Domane SLR 7 Disc, which comes with Shimano Ultegra Di2 components, Shimano RS805 hydraulic flat-mount brakes (which were the company’s top model prior to the release of the new Dura Ace 9120 and 9170 models), alloy wheels, and 32-millimeter Bontrager tires. It weigher more than two pounds heavier at 17.6 pounds.
While not a perfect head-to-head comparison, I opted for these specs because they are a realistic representation of what consumers can expect to buy. The frames are the same, while the wheels, components, and tires on each bike are optimized for their respective brake setups. If anything, the rim-variety SLR 8 had the clear advantage from the outset because of its feathery carbon wheels and overall light weight.
About that weight differential: it’s not all attributable to the discs. In a straight comparison of the new Dura Ace 9100 groupsets, the disc-brake setup is 348 grams (about .75 pounds) heavier than the rim variety. So, yes, you still pay a weight penalty for discs, but it’s not huge. And manufacturers are building race-ready disc bikes that meet the UCI’s minimum weight requirements, so it’s not really an issue for racers or for the rest of us. On our two testers, the components, wheels, and tires account for the rest of that difference.
Despite the SLR 8’s clear weight advantage, testers unanimously preferred the SLR 7. “They are both comfortable, quick, and super fun to ride,” one tester said. “But I feel more confident on the disc version.” Testers found the modulation on the discs more nuanced and subtle, with just a slight touch of the brake providing microadjustments, while the rim breaks took much more effort and were less predictable. Most people commented on how much less hand pressure it took to stop with the discs than the rim variety. Several testers even swore that disc brakes made them faster, saying they could hold speed longer into a bend on the SLR 7 because the brakes’ responsiveness and finesse allowed them to slow later, which meant coming out of the turn quicker.
Even though all testing was in a big group, we had no collisions or accidents due to the mix of rim and disc brakes. You simply learn how each performs and adjust your riding accordingly.
As for durability, the discs have held up fine for four months of testing without a bleed or an adjustment. We also had zero days of inclement weather, which I imagine would have amplified the preferences for the SLR 7 given that the carbon rims on the SLR 8 feel far less confident in the rain. (A fact I discovered subsequent to the test.)
While this isn’t a scientific inquiry, it did convince me again of disc brakes’ advantages. They won over our group of testers, too. Even the hardcore road racers in the group preferred discs. On the day we took on Mingus Mountain, a 4,000-foot hill climb west of Sedona, there was a morning scramble for bikes with disc brakes. The only models that weren’t chosen were rim varieties. “A long, winding descent like that? You definitely want disc brakes,” said one avid road racer and tester. And I heard no reports of brake fade on that long descent.
That’s not to say that the SLR 8 is a bad bike. I rode it for the better part of six months as my primary road machine, and it is absolutely fantastic. I’m sure I would have made easy work of the Mingus descent on it. As critics say, rim brakes work fine, especially the high-end versions on this model. Discs simply work better.
I remember when cars didn’t have power steering, and everyone got around just fine. But now that power steering is here, nobody would consider going back. Given the superior performance of discs, in five or ten years, I imagine we’ll all look back at bikes with rim brakes and scratch our heads over why there was ever a debate.
Actually, I’m already scratching.
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