Road Bike Disc Brakes Are Coming, But Will They Work?

Disc brakes are coming to road bikes. You don’t need to look any further than SRAM’s new hydraulic offerings as proof; they wouldn’t make it if major frame manufacturers weren’t ready to spec it. But is this progress being driven by the need for improved performance or consumer demand? And are there true performance gains to be found?

After a massive wreck thanks to disc brake failure, I went looking for answers.

What I found, outlined here and in Part 2 (wheels) and Part 3 (frames/forks, articles coming soon) might surprise you. No one seems to argue that they’re coming and that, eventually, the performance will be there. But the tone coming from the people that make the parts is cautionary. People, including me until very recently, tend to think they can look to mountain bikes’ success with discs and translate that to the road. Unfortunately, that doesn’t seem to be the case. At least not in the ultralight, super sleek package everyone’s expecting.

The biggest challenge in making disc brakes strong enough and able to withstand several miles of continuous braking is balancing weight and aesthetics with performance. Let’s face it, if they’re heavy or ugly, you’re not going to buy it.

It’s no secret that several manufacturers are working on road disc brakes. SRAM’s announced it, Magura and TRP are rumored to have something in the works, and we suspect Shimano’s not going to sit idly by. And surprisingly, Hayes has been there and done that. Because no one’s systems (or at least their specs) are official yet, we asked these brands to answer a few hypothetical questions to see how they might address these issues…

MY DISC BRAKE FAILURE

After a solid three hour gravel road ride led by our friend Joseph from Boone Bike & Touring, we were about to wrap things up with a nice, fast descent down the smoothly paved Junaluska Road. Rolling across the top of the final crest, Evan led the way with me hot in pursuit on my Project Monstercross Moots.

Braking was handled by a first gen TRP Parabox clamping on Ashima’s new and ridiculously light Ai2 rotors. It’s important to note that the original Parabox is meant as a cyclocross brake system. The rear caliper and brake pads are smaller than the front, and the Ashima rotors are about as minimalist as you can get. In other words, they’re perfect for ‘cross where speeds are low and braking is rarely a life or death matter.

The chart above shows elevation drop and speed. As we began our descent, it took only a few seconds to get up to speed. Being my first time on this road, I kept light pressure on the levers, dragging my brakes to keep my speed around 30mph on a very curvy, steep road. Evan was dropping me on a Canti-equipped Raleigh ‘cross bike, and I was losing Joseph, who happened to be riding the Specialized Disc Crux we have on review and maintaining a more intelligent pace.

Coming around a corner, I pulled my brakes a little harder and the levers went to the bar. I held them there. I was still accelerating. In the span of one second, I realized that a) I had no brakes, b) if I kept descending it was only going to get worse, so c) I better crash now and minimize the damage.

Fortunately, I was headed into the side of the mountain rather than off the side. As quickly as I could, I tried to push the bike out from under me and let it ghost ride. I didn’t think about what, exactly, I would do without a bike under me, but I knew I didn’t want to get tangled up in it at 35mph. That’s all I remember until I stopped, lying in a ditch in the fetal position gasping for air.

The brake fade that led to my accident is something I’ve experienced on my mountain bike on several occasions with multiple brands of brakes. The difference is, on a trail it’s easier to run off the side, skid out or drag a foot to reduce speed. There are escape routes. They may not be pleasant, but they’re not a guardrail or oncoming vehicle. Plus, most trails (other than World Cup DH courses, perhaps) are designed to break up massively fast descents with small rollers or short climbs to keep the rider’s speed in check. Lastly, you’re rarely going 30+ mph on the trail.

On the open road, there are fewer options and arguably worse obstacles. Brake fade or failure there can more easily result in serious injury or death, both due to the speeds involved and because roadies generally wear less (if any) protective gear. The descents are longer and faster. It’s worth repeating: People ride longer, faster descents all over the world every day.

Thus far, I’ve been an fervent proponent of disc brakes on road bikes. There’s still a chance my next new road bike will have them. But this experience has shed new light on some serious obstacles that need to be overcome before I’ll fully commit.

WHAT THE BRAKE MANUFACTURERS ARE SAYING

First, a few assumptions we’re working from: Weights will need to be low, and aesthetics will dictate a small, sleek appearance. All of which minimizes surface area to improve cooling. While DH bikes can get away with 180mm to 200mm rotors and multi-piston calipers, road bikes aren’t likely to see anything over 160mm and single piston designs.

No one wants to tip their hat as to actual product offerings, so answers here are based on the hypothetical scenario that each is or will be offering a hydraulic road brake system. No one would confirm or deny anything.

Here’s who’s talking:

• Wayne Stetina, Shimano Road Products Specialist (U.S.)
• Stefan Paul, Magura Bicycle Products Manager (Germany)
• Lance Larrabee, Marketing Director for TRP (U.S.)
• Joel Richardson, Hayes Brakes Product Manager (U.S.)

BIKERUMOR: First things first, assuming no leaks or loose parts, what causes disc brake fade or failure?

Shimano: Constant dragging can be death to brakes. Heat causes failure. The smaller the rotor, the bigger the problem. Larger diameter wheels need larger diameter rotors.

Magura: Fade on disc brakes is caused either by glazing pads (the friction coefficient is decreasing, requiring much more hand force to achieve the same brake force) and/or by overheating/boiling of hydraulic fluid, no mater if DOT or mineral oil, leading to spongy feeling and even to the possibility of a full loss of brake power, because the brake lever is touching the bar without pressure point.

Glazing pads can be avoided by following the correct bed in process and having the right compound, matching to rotor material/ surface and heat demands, which is generally already chosen by the disc brake manufacturer. So keep always with original brake pads and rotors.

Boiling oil, that is a compressible fluid, is even worse, because the brake can fail completely. This can be caused by constant dragging and/or steep gradients, pads with low friction coefficient. DOT fluid has a higher boiling point than mineral oil, but mineral oil will keep the boiling point for ever, whereas DOT attracts water over time, even through seals (and its boiling point) will decrease over time and will be worse than with mineral oil. That’s also the reason why brake fluid on cars has to be changed every 2 years.

Having big heat dissipating surfaces is good. Small compact surfaces generate heat build up.

TRP: Heat is the number one cause, whether it’s cars or motorcycles or bikes. And I think there’s potential to get them hotter on the road.

BIKERUMOR: In a nutshell, how do you prevent brake fade or failure?

Shimano: You either get a bigger rotor with metallic pads which can operate at higher temperatures, or you get something like our IceTech that can keep cooler for longer and dissipate heat better and quicker. Even the aluminum flanges on our CenterLock rotors is going to dissipate heat better than a one-piece steel rotor.

Magura: Choosing the right friction parameters of pad/rotor, use original parts, avoid drag braking (short hard braking is better for recovery in between), use big rotors (hand force and pressure to the pads will decrease, thus thermal load).

TRP: You want to have a rotor that’s designed to remove the gasses formed by the pads heating up. Good airflow through the brake will keep it cool. And the last thing you want is for the caliper to get so hot that it vaporizes the fluid. If the heat transfers too much to the caliper, it can boil the liquid.

BIKERUMOR: More specifically to road bikes, what are the key design differences you had to consider versus your mountain bike brakes?

Shimano: When you take the braking surface off the rim on a mountain bike, you can reduce material there and weight. I don’t believe we can do that on a road bike. If you eliminate the brake surface on a road bike and reduce material, I think you’re going to make it too fragile. It would be unable to maintain its stability.

Weight is more critical. The brake system itself isn’t so much the problem as redesigning the frames for the different torsional loads and redesigning the front wheel to accommodate for the asymmetrical dish. Radial lacing designs won’t work, I think the wheel manufacturers have a bigger challenge than the brakes manufacturers.

It’s not a design difference, but initial designs might be limited because of the upfront investment needed to produce something that may have a limited audience and limited number of frames capable of using them. So the initial cost difference between standard brakes and hydraulic disc brakes might be more than people are expecting. I was told the cost of our original Di2 prototypes were around $30,000 each, so when you really start getting into considering production, you want to minimize the number of prototypes you have to go through. We’re studying this very hard.

Magura: We’re only offering hydraulic RIM brakes for TT and Tri-bikes (at least at the moment, brakes for normal road bikes might follow!?!), no disc brakes. Hydraulic rim brakes are lighter than disc brakes if you look at the complete system. The brakes itself might be equal, but forks and frames for disc brakes have to be beafier and made stiffer to take the loads from disc brakes with their asymmetrical force input. The system of road frame and fork for discs is minimum 500gr heavier than for rim brakes. Weight on disc brakes depends also a lot on the rotor size, the bigger the rotor, the more weight. In order to achieve high heat loads on disc brakes, especially on longer, steeper descents, bigger rotors are neccessary, at least 180mm front, increasing the weight additionally. Rim brakes are more aerodynamic than disc brakes, they can be hidden in the same shape as frame and fork, Disc brake calipers and rotors always protrude the shape of frame and fork.

With rim brakes you already have the biggest possible rotor on a wheel: the rim!

Rim brakes don’t suffer from heat build up/overheating on the hydraulic system, as the distance friction partners (rim/pad) is really far away from the hydraulic piston, so no expansion chamber is even neccessary.

TRP: We’ve done a lot of testing with the Parabox knowing that what started on the ‘cross bikes is moving to the road. With our new system, we’re going with a larger caliper on the rear and we’re recommending everyone use a 160mm rotor on the rear. We’ve cut out some material, but we’re making sure there’s enough material on the rotor to give you both stopping power and enough heat dissipation.

We had to develop it to handle the additional heat that will accumulate on long descents. With road bikes you’re on the brakes for longer period of time and there’s a lot of stress. The amount of grip on road bike tires is more than what most people think, so you do need big powerful brakes. I’ve experimented with a 180mm rotor on the front and it’s nice. Compared to mountain bikes, the piston and rotor sizes are quite similar from what we’ve found.

BIKERUMOR: Will pad compound or rotor surface materials need to change? Which of the two is more important on the road?

Shimano: No. Generally because the bikes are heavier and the grades are steeper, you would expect there would be less heat build up issues on a road bike. I think technique will help too, riders may need to brake hard for a short distance, for instance coming into and going around the corners, then let off so they can cool. I think in your situation, you were on the brakes the whole time and they simply got too hot. Metallic pads are going to be noisier and take a little longer to warm up, but they’ll work better once things get hot. With IceTech, though, we were able to put resin pads under riders that were previously needing metallic pads and give them better, quieter performance just because they system cooled so much better.

Magura: Pad compound and rotor surface on MTB disc brakes is already on the top. If we had better friction partners, we would use it on both MTB and road.

TRP: We’re not exploring any changes in pad compounds or rotor materials, it’s more about rotor shapes to disperse the heat. The pads in the Parabox are semi-metallic and that’s what we’ll likely stick with.

BIKERUMOR: What about rotor or caliper shape, or pad size – can those dramatically improve performance?

Shimano: They all impact it, and everyone has their own strategy to get the lightest most powerful brake that won’t overheat and still do the job. If you look at race motorcycles or high performance cars, you get multiple pistons or one large one. When you add pistons, you get more twisting of the caliper, which means you need a bigger stronger caliper and rotor. A longer, thinner brake pad will give it more surface contact and friction. It’ll also give you more area to dissipate the heat, and the caliper can be made stiffer. For your size, you might want 180mm rotors and IceTech rotors. Under the same conditions, our IceTech system can run up to 50º cooler.

Magura: Shape and size is already on the optimum for the actual moment on MTB disc brakes. Using the same parameters on road brakes will give better performance on road disc brakes, as the average speeds are higher than on MTB, giving a better cooling effect/better heat transfer into the surounding air.

Increasing pad size would increase weight again (more material of the pad itself and on the caliper to take the volume of the increased pad).

TRP: There will be some new road specific designs, all in the interest of better performance but also in terms of appearance. It’s got to look a little more subtle, and ours will be sleeker than our mountain bike calipers. We’re experimenting with different piston materials and shaping the caliper body to get more airflow through there and radiate the heat better.

BIKERUMOR: Mechanical versus hydraulic discs for road, any performance or safety difference with regards to brake fade?

Shimano: There’s absolutely no difference in brake fade. The benefit to hydraulics is better modulation, there’s a more linear progression of braking power. Plus, the hose won’t get contaminated like cables and housing.

Magura: Mechanical disc brakes will not suffer from boiling oil, but from melting of other components, spongy feeling and bad modulation from mechanical cable and housing. If mechanical disc brakes would be top, then MTBs and cars still would use them!

TRP: You don’t have the fluid issue, but I don’t know that one might be better than the other. You can develop more power with a hydraulic system, and the calipers will compensate for wear, but you have the potential to boil the fluid.

BIKERUMOR: Set up is going to be key. What can you do to ensure OEM spec’d bikes are set up properly, particularly with hydraulic disc brakes whose lines run inside the frame, requiring them to be disassembled and bled prior to the sale?

Shimano: You keep seeing evolution of things where we have the one-way bleed for our mountain bike brakes that makes it virtually impossible for air bubbles to get trapped. Once it goes to the factory, it’s out of our hands. It’s got to be the dealer that’s properly trained to assemble and tune the bike before it gets into the consumers’ hands. All of our systems are sent to the OEM manufacturer pre-bled and are installed. Once they start running hoses inside the frames, it’s going to be a colossal pain in the ass. I think the potential for routing hydraulic brakes internally is not so great because of the one by one labor that would be involved. I don’t know if I’d want that. I guess you could develop joints, like a banjo joint, that could be mounted on the entry and exit points on the frame.

Magura: All Magura brakes, also our RT rim brakes are already designed with service in mind. We use the same tools as on any other of our hydraulic Magura brakes, we use the same Royal Blood mineral oil, we teach our OEM customers how to route lines, how to service, how to assemble. The same we are doing already on the MTB range brakes.

TRP: Every single brake that comes from our factory is cycled 300 times, pressure tested, held for 36 hours, then pressure tested again. Of course, when you start running this inside a frame, that all changes. We’re working on a system that allows you to maintain the pressure in the system while disconnecting the hose. This will allow you run the hose inside a frame without losing pressure (Editor’s note: this is very exciting!).

BIKERUMOR: And for aftermarket sales?

Shimano: You want to take it to a shop that has a lot of experience with mountain bike brakes. Shimano does a lot of hands on training by our traveling tech reps and has online training for dealers. All of this helps ensure they’re set up correctly, and it’s pretty easy to tell if they’re working properly.

Magura: We always recommend: RTFM! (read the f…. manual!). Our manuals always come with detailed installation guidelines. If you follow them, you´ll be fine, even without bleeding. And if you need to service/bleed the brakes, we offer service kits with Royal Blood mineral oil.

TRP: Like our mountain bike brakes, you’ll need a bleed kit if you need to trim the hose. But our new system we’re working on will simplify that process quite a bit.

BIKERUMOR: What about the bike frames themselves?

Shimano: Frames will have to be beefed up to handle the different torsional loads, particularly on the forks. If you just take a fork that’s developed for rim brakes and add disc mounts, that fork is not prepared to take the braking force on one side down at the end. And the rear spacing will have to change to 135mm. It’s a big deal. The other thing that people don’t think about is that you have to offset the front wheel. To handle the narrower spoke flange width, you’re going to either have to add four spokes to the front wheel or about 40g to the rim to make it strong enough. And you won’t be able to use certain types of spokes, meaning it’s not going to be as aerodynamic. As for material, a carbon frame isn’t going to dissipate heat from the caliper any.

Magura: (Answered above)

TRP: Like everything else, manufacturers have taken material away where it wasn’t needed and added it where it was. With carbon fiber, it’s pretty easy to figure out where it needs to be stronger and make it so. You’ll see wheel and hub manufacturers do some interesting things like bigger and thru-axles. Specialized is already aiming their dropouts slightly backwards to keep the brakes’ rotational forces from throwing the wheel out.

And then I talked to Hayes Brakes product manager Joel Richardson. Most people don’t associate Hayes with road bikes, but it turns out they may just have more experience with road bike disc brakes than anyone at the moment:

“About six years ago, we identified the road market as a very good application for disc brakes at a high level,” Richardson says. “We spent a good amount of R&D time and money to develop hydraulic and mechanical disc brakes for a major OEM manufacturer, one of the biggest. After a couple years of development, we hit major roadblocks. The biggest of which is packaging. The current mountain bike brake designs and mounting standards -six bolt rotors, flange offsets, 74mm direct post mount, etc.- just didn’t work for road. All of these were done for a reason with mountain bikes and they work there. The road we went down was a new standard for mounting the brakes. We were working with a major player in the road bike market, but even with their substantial clout, we found that the required packaging of the product didn’t mesh with what customers are likely to want (read: buy).

“What we found with road bikes was that you can generate incredible heat and forces. There are long descents where you’re dragging the brakes for a long time. You have tiny little calipers with very little thermal mass. And they have tiny little pistons that require very little fluid volume. Then you have tiny rotors with virtually no mass that can’t dissipate heat. When you whittle everything down to a super lightweight package, the only place for all that heat to go is the hydraulic fluid, and you can boil it in no time at all. When the fluid boils, it happens instantaneously and it happens right behind the brake pads. As soon as that happens, it introduces air into the system.

“Another thing people don’t think about is, even though they’re skinny, a grippy road bike tire on hot pavement can create a tremendous amount of torque at the rotor and caliper. It’s much easier to lock up a mountain bike tire on a loose surface than it is to lock up a road tire in good conditions, and that puts more friction between the rotor and caliper when braking, and that means more heat.

“The idea has merit and can definitely be executed. We believe in it and know it’ll happen. We’ve built dyno and real world tests around it and spent considerable resources to explore it. At this time, we’re primarily focused on mountain bikes. We have a mechanical road caliper, the CX5, but, well, we don’t have shifters for the road. So here we are.”

I want to point out that I installed the parts myself, cut the rear hose and re-bled the rear brake and chose to not use TRP’s stock rotors. I do not blame TRP or any other manufacturer for what happened. In hindsight, it was poor parts selection for the actual use. And perhaps I could have used better braking technique – brake hard, release, brake hard, release.

That said, I think it’s not unreasonable to assume most roadies will choose similarly lightweight parts without knowing the performance capabilities and limitations, and many will drag their brakes to limit speed. In the end, all I know is heat was the culprit, but what the exact technical consequences of that heat were on my equipment is unknown other than it failed. And for most riders, that’s all there is to care about.

Stay tuned for discussions with wheel and frame/fork manufacturers over the coming weeks.

Big thanks to Johnny R. for stopping and calling 911, and to Boone’s fire rescue, EMT’s and doctors that took great care of me!

Note: SRAM was invited to participate but has not returned answers as of publishing, which is a shame because they’re the only ones with a publicly announced system. Perhaps they’re keeping quiet until the official launch. If we hear back, we’ll update the post.