Road Disc Brakes: The Pros and Cons

When disc brakes appeared on mountain bikes, there was none of the hue and cry currently heard about the technology’s adaptation to skinny-tire bikes. Indeed, power, modulation, and control were things dirt riders had only dreamed about when skittering down steep inclines of any condition; dry, muddy, or rocky. Thus, as soon as disc brakes were advanced enough to be semi-reliable, mountain bikers took to them like sharks to seals. Even cable-actuated disc brakes were worlds ahead of the best rim brake offerings of the day. Things are a little different this time around, however. While no one disputes that road disc brakes are an improvement over rim brakes in terms of power and control, the main sticking points center around their impact on racing and availability.

First, the facts: disc brakes are better; they are more powerful, feel better, and offer better modulation than rim brakes. But, depending on your situation, the previously mentioned advantages might not be worth the trade-offs.

Shimano's Flat Mount. Will we ever see it?

Shimano’s Flat Mount. Will we ever see it?

As of yet, there is no industry-wide mounting standard between brake and frame manufacturers. Current mountain-style post mounts on the seat stay probably won’t work, as the road stays tend to be much more minimal than on mountain bikes. Since chain stays are usually beefed up on road bikes anyway, it makes sense to mount the calipers there, both structurally and to tuck them out of the wind as much as possible. Shimano’s flush Flat Mount, on the chain stay and fork leg, is a promising start, but given the Japanese component giant’s aversion to sharing, chances of a quick solution are slim.

Weight is often cited as a drawback of road disc brakes, and, as a system, discs are heavier, but not by much, maybe a couple hundred grams total. As most of the weight increase is at the center of the wheel, this is not as much of a disadvantage as might be expected. Since reinforced brake tracks at the rim can be eliminated, rim weight is actually reduced, which will make the bike feel lighter when pedaling.  For comparison’s sake, the $1170 Industry Nine Torch Trail 29” disc mountain wheelset weighs 1580 grams, with tubeless tape and valve stems installed, while the $1350 HED Ardennes Plus FR wheelset weighs 1514 grams without tape and stems. Once brake tracks are done away with, road wheels will be drastically lighter than before. Nixing brake tracks also means no brake pad/rim material compatibility issues. Long descents on carbon rims will cease to be an exercise in faith and karma since heat build-up at the rim, which has been known to cause tubes and tires to explode and wheels to shatter, will no longer be an issue.  Finally, wheels will be stiffer with the inclusion of through-axles, which are practically universal on mountain bikes. Also, road bikes are so ridiculously light these days that pros are forced to add ballast to reach the UCI’s minimum weight requirement of fifteen pounds, negating any weight penalty disc brakes incur.

If disc brakes do have the potential to negatively impact performance it will be because of aerodynamics. Pros and competitive amateurs will notice the up to ten watts of drag rotors create, depending on their size and yaw angle. Most of us will never even realize the rotors are there except when we glance downward, and you know what they say about staring at your front wheel…

Safety issues are an oft-cited deterrent for adapting disc brakes to road bikes, and perhaps the most realistic reason to be wary of them—if you are a pro, or are involved in a lot of pile-ups in your amateur racing. While many mountain bikers have been inadvertently branded by a searing-hot rotor during an oxygen-deprived tube change in the middle of a down hill section, once is usually enough to learn to avoid the flesh-charring metal. In a multi-rider crash, however, bikes and bodies fly everywhere, with no way to avoid the Dynamic Discs of Deadly Death attached to every wheel. This of course, is currently just conjecture, since a mass pile-up of disc-equipped bikes and cyclists has yet to happen, but there probably would be some rotor-shaped tattoos given out.

A more realistic concern would be the mixing of disc and non-disc riders in the peloton, especially in wet conditions, long descents, and a mix of both. Even in dry conditions, disc brakes will allow much later braking going into corners. Not only would this confer a mechanical advantage—which the UCI deplores—but if a rim brake rider followed a racer with discs into the turn they would likely crash as a result of not being able to scrub speed fast enough. This discrepancy comes from the greater modulation (which would mean nothing without their corresponding increase in power) disc brakes provide.


Sram’s updated hydro road disc brake

If you’ve ever wondered why downhill mountain bikers use enormous 8” disc brake rotors, it’s because the larger diameter requires less power to stop its rotation. If you’re thinking that a rim is the biggest rotor possible, you’re correct, and much more force is required to stop the smaller rotors compared to a rim: approximately 1000 pounds of force vs 200.  This “larger window” means there is a finer application of power for the same distance of lever pull, giving you more braking control, which is why feathering the brakes is easier and more effective with discs, and why it’s possible to find and maintain the point just before the tires lock up and skid, which results in loss of bike control. Also contributing to the feeling of increased power is hydraulic fluid and disc pads, both of which are non-compressible.  Increased modulation and power is a good thing, but only if everyone in the race can take advantage of it.

Team mechanics bemoan the added complexity of maintenance and in-race support. Wheel changes will take a bit longer to line up rotors with calipers and secure through-axles. Maintenance will definitely be more complex, with bleeds, burning in pads and rotors, and rotor truing added to post-race duties. Hopefully, with the increased revenue road disc brakes bring, sponsorship dollars will increase along with the mechanic’s paychecks. For most of us though, there will actually be less maintenance with disc brakes. One or two bleeds per year should suffice, and pad changes are so easy they can practically be done in the dark. Hydraulic housing rarely, if ever, needs replacement. For the average to advanced recreational cyclist, disc brakes present tons of upside with hardly any drawbacks.

Here’s a quick rundown of the advantages road disc brakes will provide:

  • Lighter wheels
  • No pad/rim material compatibility issues
  • Stiffer wheels with through axles
  • Better braking performance in ALL conditions
    • More power
    • More modulation
    • More control
    • Consistent performance
    • Less and easier maintenance