The Torqued Wrench: 13 road disc brake questions, answered
Our tech editor Caley Fretz provides more information about disc brakes for road bikes
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Let’s work under the assumption that you are interested in disc brakes. Perhaps it’s the poor weather performance that’s intriguing, or the controlled power. Perhaps you’re a mountain biker, you’re familiar with discs, and a good road version simply can’t hit shelves soon enough. Regardless, I’ll save the “do discs belong on the road” debate for another time.
Today, let’s look at the nuts and bolts of disc use. Here are 13 of the most common questions I’ve been fielding recently on the introduction of drop-bar discs.
1. Should I buy the current options (Shimano R7845 or SRAM HRD) or wait for refinements?
SRAM’s recent recall will certainly turn a few people off, but that seems to be a short-term manufacturing issue rather than an inherent engineering problem.
Shimano’s system hasn’t really been out long enough to find out if that it has any similar issues. Despite the company’s legendary attention to detail, it did have a few snafus with the release of Dura-Ace 9000 (shifters breaking cables, for example, and creaking cassettes), and as a result it made some running changes.
Shimano will likely have more options within the next year. A lighter Dura-Ace option would be our first guess.
SRAM already has two versions, Red and S700. A Force-level 11-speed option would make sense.
As with every new technology, waiting until Disc 2.0 is certainly the safe bet. You’ll have many more frame options, more wheel options, and more brake options within the next 18 months. That said, after extensive time on both the Shimano and SRAM systems, both seem to be ready for the big show.
2. Will they overheat?
Assuming proper maintenance and setup, no. Both SRAM and Shimano claim to have done their homework, including work in their R&D labs and real world testing.
For example, both strapped weight to already large riders and sent them down some of the toughest descents in the world. They slammed on the brakes from high speed, repeatedly, and dragged the brakes all the way down, as many amateurs are likely to do. In both circumstances, heat buildup in the caliper plateaued well below the boiling point of the brake fluids. It gets a bit hotter on the rotor, but there’s quite a bit of material between the rotor and the hydraulic fluid, which acts as a heat sink.
Would we run hydros on a tandem? No. SRAM has a 250-pound weight limit that we would certainly respect as well. Shimano has no weight limit as long as the brakes are used with its IceTech pads and new Freeza rotors.
3. What happens if they do overheat?
It’s possible to glaze the pads, which would decrease brake power but not remove the ability to stop entirely (it’s called friction fade). The real danger is boiling the hydraulic fluid, vaporizing the fluid into a compressible gas. Since hydraulic designs are dependent on incompressible fluid, and gas compresses, vaporizing the hydraulic fluid would result in brake failure. Pull the levers all the way to the bar and nothing will happen.
Thankfully, this process does not generally happen in an instant. The brakes will go a bit mushy before they fail completely — mountain bikers who were riding in the early days of disc brakes are likely familiar with the sensation. If this ever happens, stop and allow everything to cool.
Obviously, any sort of fade is highly dangerous. But discs are not the only brake system that will fail with excessive heat. SRAM was able to blow a tire off its rim after five minutes at 550 watts (on a dynamo tester), but saw zero damage to its disc brakes after 12 minutes at 800 watts. In other words, if you’ve never blown a tire off a rim due to heat, you’ll certainly never boil your road discs.
4. Do they make noise?
Yes, two different types of noise. Discs will occasionally squeal, particularly when wet or if the pads or rotors have been contaminated. In our testing, SRAM’s brakes make more of this sort of noise than Shimano’s. Both are quiet the vast majority of the time.
If your brakes squeal all the time, something is wrong. Clean the rotors with alcohol and put some new pads in.
The second noise will be familiar to anyone who has used discs in the last decade: the ting-ting-ting of an improperly centered rotor. Even with perfectly setup calipers and a centered rotor, the ting will occasionally show up mid-ride. The rotors themselves can warp a bit under excessive heat. More common, the whole hub can simply shift a small amount in the dropouts. Quality quick releases are a must with disc brakes, and will go a long way towards eliminating this noise. (Do you hear me, wheel manufacturers? Your external cam skewers go from crappy and annoying to dangerous when used with discs.)
The real solution is the thru-axle, which eliminates all movement, and thus noise. Hopefully the industry will move in that direction soon. It has already made the swap on mountain bikes.
5. Are they easy to maintain?
There are no cables and housing, of course, so maintenance will be less frequent. But all hydraulic discs will need to be re-bled every so often (most users should get a solid year of use before it’s required). For any competent mountain bike mechanic, this is an easy process. It’s far easier than properly setting up a set of cantilever brakes, for example. The bleed protocols are identical to the brake’s mountain bike brethren.
Pad selection is important, and dependent on conditions. Metallic pads will be louder, but last longer in bad weather. Resin pads are quieter, but will be chewed up quicker. For most road use, resin pads are the way to go. For all-conditions ’cross, go with metallic.
Hydraulic pistons self-adjust as pads wear, requiring less day-to-day maintenance than mechanical options. This also means they are better suited to situations where pads will wear very quickly — something like ’cross nationals last year, for example.
Overall, both the SRAM and Shimano hydraulic systems should be mostly “set and forget,” requiring less maintenance over time than rim brakes.
6. How much weight do they add?
The complete Shimano R785 system (shifters, hoses, oil, brake caliper, rotor, hardware) comes in at about 1,065 grams, approximately 340 grams heavier than an equivalent Ultegra Di2 6870 system.
SRAM’s Red-level HydroR Disc system is lighter, about 900 grams for the shifter, 160mm rotor, caliper, hose, oil, and hardware. Mechanical SRAM Red is about 850 grams for equivalent components — so the disc system is certainly within spitting distance.
Frame manufacturers say that they need to add 100-150 grams to their framesets, strengthening both the fork and stays.
Wheels need to be beefed up, too. No more radial spoke lacing, and heavier hubs are unavoidable. However, top-tier mountain bike wheels now float around the 1,200-gram mark, and those rims are surely overbuilt for road use. The industry will be able to produce light road disc wheels, with exceptionally light, brake track-free rims.
So what’s the total damage? Discs will add somewhere between 250 and 750 grams, depending on the component choices made.
7. What can I do to make them lighter?
The R785 levers comes stock with what is essentially an XT mountain caliper. The levers are compatible with Shimano’s XTR caliper as well, including the new magnesium version, which would shed about 150 grams, about 75 grams per wheel.
Be warned, though: Shimano’s Dave Lawrence says that the XTR caliper is slightly less powerful than the XT version.
Shimano’s system already comes stock with 140mm rotors, which use the company’s Freeza (sounds like a super villain, no?) cooling fins. We would not recommend swapping to a lighter 140mm rotor, as those fins offer exceptional cooling. A savings of a few grams is not worth poor or even dangerous performance.
SRAM’s HydroR system offers less room for improvement. A swap to 140mm rotors will drop about 40 grams, but SRAM recommends using 160mm rotors on the road. We actually like the feel with 140s, but for safety reasons it’s best to stick with SRAM’s 160mm recommendation.
8. Which rotors should I use?
Weight, looks, and safety are the three issues here. For low weight and better looks (at least for anyone used to a traditional road aesthetic), smaller rotors are better. But for safety and power, at least on SRAM’s system, larger ones are better.
For now, we simply have to advocate using the stock, manufacturer-recommended rotors. Until we get a better sense of what it will take to make these brakes fail, we can’t, in good conscience, tell anyone to seek something lighter or smaller.
9. Do discs harm aerodynamics?
Most likely, yes. Bike brand Culprit did some wind tunnel testing comparing its Legend frame with and without discs, and found that the discs increased drag. The difference was dependent on yaw angle, and ranged from 1-10 watts, with the largest difference at high wind angles.
This is, of course, a massive argument against the adoption of discs for racing. Ten watts is similar to the gains found from a set of aerodynamic wheels — it’s certainly not insignificant.
Shimano’s Lawrence believes that if road racing goes disc, it will go in 100 percent. All or nothing. In that case, the detriment is erased, as it affects everyone. He also noted that his discussions with bike brands on the subject of aerodynamics have been inconclusive — some brands are seeing a negative effect, while others are seeing essentially zero change.
For the non-racer, the difference is truly negligible either way.
10. What sort of wheels do I need?
We’ve covered wheel conversions a few times recently. Right now, there simply aren’t many new options. Shimano has a new 11-speed compatible road disc hub and a few other brands have a road disc wheel available, though most are aimed more towards cyclocross.
Zipp’s 303 disc wheelset is a great option, though expensive, and it comes in tubular and clincher versions. Hed has a disc-compatible Ardennes set, FSA makes a very cheap option (the RD-460), Bontrager has its SSR disc. Many are simply normal road rims with a disc hubs — meaning you get a rim brake surface that you don’t need.
Hopefully, wheel brands are developing disc-specific road rims as we speak. They should be able to tune the shapes for improved aerodynamics and perhaps lighten up the rims as well, as they won’t need to handle brake forces.
11. What about hydraulic rim brakes?
For time trial bikes or aero road bikes with tricky cable routing, hydraulic rim brakes are fantastic. They allow excellent brake feel when a cable/housing system would have excessive friction.
For everyday road riding or a normal road bike, they are less fantastic. The benefit is minimal: a bit more power, consistent lever feel that won’t degrade over time. Contrast that to the downsides, which are inherent in any rim brake: Rims are imperfect, even machined aluminum ones. That means braking will be inconsistent and unpredictable, relative to a disc setup. Carbon rims, of course, never offer particularly good braking. In the rain, every rim gets covered in water, and the braking gets even worse.
In short: little gain, all the same drawbacks.
12. What about swapping wheels?
This is unquestionably a big problem, at least for the road racing crowd. For the average user, using a single set of wheels, a wheel swap takes just a few seconds longer than it always has.
The big issue is with multiple wheels. The tolerances within disc calipers are exceptionally tight, enough so that if a rotor is even a sliver of a millimeter out of spec, it will rub and make noise. This makes using neutral wheel service all but impossible.
Frankly, there is no good solution to this right now. But both SRAM and Shimano recognize the problem and they are endeavoring to fix it. Hopefully they have a solution before the UCI gives the green light to discs in road racing (if that ever happens).
13. What about mechanical options? Or the hybrid mechanical/hydraulic option from TRP?
The power, modulation, and control offered by a hydraulic disc far outstrip those of a mechanical one. They should not truly be considered together. That said, mechanical discs, when properly setup, will still perform better than most rim brakes.
There are enough downsides to going disc, from weight to wheel changes to potential aerodynamic consequences, that it really takes a full hydraulic system to tip the scale heavily in disc’s favor.
TRP’s HyRd option is interesting, in that it adds the self-adjusting pads of a hydraulic system (which are not present on any mechanical system) with the ease of setup associated with a cable actuated brake. But as a whole system, it’s heavier than a complete hydraulic setup, and the lever feel will never be as good when there’s a cable involved. For roadies unfamiliar with hydraulics, or for anyone who hates the look of the new hydraulic levers, it’s a great way to step into the world of hydraulic discs.