Disc brakes have been used on the road for years but they’re only now hitting the mainstream as big brands offer more disc-equipped bikes covering all sectors of the market from gravel bikes to endurance bikes to full-on road race rigs. With the UCI now allowing them within the professional peloton, disc brakes are going to become an ever more familiar part of the road riding landscape.
What are the advantages of disc brakes?
Let’s start right at the beginning. We’re all familiar with traditional bicycle rim brakes where the brake pads operate on the wheel’s rim, right? With a disc brake the pads instead act on a metal rotor that’s attached to the wheel’s hub. Simple enough.
What’s the advantage of that? People sometimes say that they can easily lock up the wheels of a road bike, stopping them totally with very little effort, so there’s no point in having any more power.
One key point is that disc brakes can offer more control than rim brakes before they lock up. You have more of a braking range with which to work, so you’re less likely to skid.
SRAM’s Product Manager Paul Kantor told us, “I can certainly lock up my wheel with a carbon rim and a mechanical rim brake, but if I do that I’m still just sliding down the road and that sucks. Locking up the wheel isn’t hard but if that were our goal we’d just sell you a stick that you could ram in there and you’d be done!
“Everything we’ve focused on is prior to locking up the wheel. You can take off more speed with greater control prior to locking up the wheel. Some people refer to that as ‘modulation’. That’s a term we don’t really care for, but it’s fine. We’re really talking about decelerating substantially without locking up the wheel.”
With a rim brake system, you want the wheel rim to be as light as possible, you need it to be strong, and it also has to provide the braking surface. Anyone who has ever ridden carbon-fibre rim brake wheels knows that while they might be lightweight and fast, the braking performance isn’t brilliant, especially in wet conditions.
A disc brake system allows manufacturers more scope to innovate with the braking surface because it doesn’t need to operate as the wheel rim too. Shimano, for example, equips its hydraulic disc brakes with what it calls Ice Technology that features a clad rotor blade with a three-layer sandwich structure of stainless steel and aluminum. It says that this provides a better radiation performance that reduces the temperature while braking.
Plus, with a disc brake the braking surface, the rotor, is much further from the road than it is with a rim brake so it’s less likely to get wet from surface water. There are holes in a rotor that allow water to get out from underneath the brake pads too. The result is that disc brakes are far less likely than rim brakes to be affected by water or gunk thrown up from the road.
With a rim brake, the tyre has to pass through the brake calliper, but that's not the case with disc brakes, so they free up space for wider tyres. That's part of the reason why discs have become popular for endurance and adventure bikes.
Linked to that, a side benefit of disc brakes is that your wheel can go out of true without rubbing on the brake pads.
Mechanical or hydraulic disc brakes?
You can divide disc brakes up into two types: mechanical and hydraulic.
Mechanical disc brakes are operated by a cable, like the vast majority of rim brakes, while hydraulic systems use fluid to transfer the force from the lever to the calliper.
Pulling the brake lever in a hydraulic system moves a piston inside the master cylinder which forces brake fluid towards the brake calliper. This acts on the pistons in the brake calliper to push the brake pads against the disc rotor.
Mechanical disc brakes tend to be cheaper. TRP’s Spyre mechanical disc brakes, for example, are priced £69.99. You can use them with standard (non-hydraulic) dual-control levers. The Spyre is a dual piston design meaning that two pistons move equally against the rotor, as opposed to the single piston design of the Avid BB7 and Shimano CX75 mechanical disc brakes.
Hydraulic brakes are higher end and they perform better than either rim brakes or mechanical discs in just about every respect, but they’re more expensive. A SRAM Rival 22 hydraulic disc brakeset (you get both the shifter and the brake calliper), for example, is £302.
Hydraulic systems are more efficient than mechanical disc brakes so you need to apply less pressure at the lever for an equal level of braking power. This means you can get better modulation.
A hydraulic system is sealed so no contaminants can get in to affect braking performance, and complicated internal cable routing isn’t a problem, whereas it can add friction to a cable setup.
TRP’s Hy/Rd brakes are unusual in that they’re cable operated with hydraulic power in the callipers. We found them powerful, easy to live with, and the best solution so far for disc brakes with conventional brake/gear levers.
What about heat?
Whatever type of system you use, braking produces heat. There used to be concerns about the safety of carbon rim brake wheels during prolonged, heavy braking – we’re talking about Alpine-style descents here – but manufacturers seem to have got that technology sorted.
When it comes to disc brakes, fade (the loss of braking power) can occur as a result of the buildup of heat in the system.
Shimano identified overheating as being of particular concern for discs on the road, the longer, faster descents (and smaller rotors) being likely to result in rotors and pads heating up more than they do off-road.
As mentioned above, to counter this Shimano’s IceTech rotors use a three-layer sandwich structure of stainless steel and aluminium, the aluminium being included because it transfers heat better than steel. These have wavy aluminium sections inboard of the brake track designed to maximise surface area for improved cooling. The pads have cooling fins that are made from aluminium for the same reason.
Speaking about the development hydraulic brake systems, SRAM’s Paul Kantor said, “Where you might have trouble is with some big guy riding down the Stelvio for 45mins dragging the brake, and we were worried that we’d have boiling issues there. But what happens is that the brake reaches a steady state where the heat isn’t increasing.
“What we did see is that when we decelerated from 50-60kph down to 15kph in 1-2sec increments on a switchback descent there was friction fade where you’d lose some of your coefficient of friction in the pad. That is much better than having the system boil.
“We could induce this on a 140mm rotor so we altered the backing surface on the pad to dissipate that heat way better. So now we have a very small window where you could induce some friction fade if you were really trying to do it but we have had next to no issues with boiling the system.”
Discs are available in different sizes. All other things being equal, a large disc will slow you down faster than a small disc.
Shimano's road disc brake system has been designed for use with 140mm or 160mm rotors, the idea being that users can choose the size to suit their weight and intended use.
SRAM’s Paul Kantor said, “We recommend 160mm rotors front and rear for road use and 140mm is fine for cyclocross. We have tested 140s extensively but we like the margin of safety that 160s offer for the road.”
Focus has told us that in testing it found 160mm rotors preferable to the more common 140mm, handling the buildup of heat more effectively. This goes against the trend for smaller rotors, which is largely the result of Shimano recommending 140mm rotors for all but the largest cyclists.
The choice is yours but if you’re a larger rider you might want to start with 160s and see how you go.
There are different standards for fixing disc brakes to road bikes but Shimano’s road-specific Flat Mount system, announced in 2015, is becoming dominant.
"This new design allows consumers to move away from the mountain bike history and look, which has been used until now, using a method better suited to high performance road bike riding,” says Shimano.
Shimano’s Flat Mount is an open standard meaning that other manufacturers are free to use it. SRAM has recently adopted it too.
With Flat Mount the brake callipers attach directly to the frame or fork, offering a cleaner and more minimalist appearance than with a post mount system. It also provides a more compact packaging of the brake calliper, which is a particular benefit at the rear triangle.
The bolts thread into the bottom of the calliper rather than in from the top as is the case with post mount brakes. At the chainstay that means the bolts no longer thread into inserts in the frame, but pass through the chainstay from the bottom so there’s less chance of damaging an expensive carbon frame. Because the bolts thread in from the bottom of the calliper, the front brake must be used with a slim adaptor.
Quick release or thru-axle?
A disc brake puts forces on a wheel that are different from those of a rim brake, so keeping that wheel in its correct position and avoiding flex in the axle and dropout become challenging.
One way to keep the wheels in place is to use thru axles where the ends of the dropouts are closed and a removable pin goes completely through the axle to hold it in place. This adds security but it also adds a little weight and makes swapping wheels a touch more difficult.
Focus has come up with what it calls Rapid Axle Technology – RAT, for short – design to simplify the process. The RAT thru-axle is a design that requires just 90° rotation of the axle in the dropout to close the lever.
Some disc brake road bikes use standard quick releases, like the Pinarello Dogma F8 Disc, while others use thru axles, like the Focus Izalco Disc. Others go with one quick release and one thru-axle. We’re still in a period of change and it’s not clear how the market is going to settle, or whether it will settle at all; it could be that different manufacturers continue to use different systems.
Many road bikes these days are designed with a focus on aerodynamics, partly because the UCI has a 6.8kg minimum bike weight limit for racing. There’s little point in a manufacturer focusing on bringing down weight but it can reduce drag in order to gain an advantage. How do disc brakes fit into this picture?
Well, as our man Dave discovered when he visited a wind tunnel with Swiss Side, disc brakes, in their current incarnations, aren't particularly aerodynamically efficient.
Jean-Paul Ballard of Swiss Side told us, “We've measured a 16% increase in wheel drag between a disc-braked wheelset and a standard wheelset.
“We performed a direct back-to-back test of the Zipp 303FC in standard version and disc brake version, for our own competitor comparison purposes. That 16% is a constant offset in the performance curve across the entire cross wind angle range."
The extra drag comes from three sources. The rotor itself adds drag, and because disc wheels need more spokes to cope with braking forces, there's more drag there too. On top of that, a disc hub is generally bigger than a standard hub and that increases drag as well.
Disc brakes could get more aerodynamically efficient over time, but probably not by a huge amount, and that’s one reason why they’re unlikely to take over completely from rim brakes, particularly when it comes to racing. Speaking of which…
Like it or not, what the professionals ride has a massive influence on the road bike market. After all, that’s the main reason that sponsorship exists. When pro riders use a particular product others follow, and that’s why it’s so significant that the UCI is now permitting disc brake equipped road bikes in the peloton.
Pro teams were initially allowed to try out disc brakes in races towards the end of the 2015 season and the UCI has recently confirmed that disc brake trials will continue through 2016 too. If everything goes to plan, disc brakes will be permitted on a permanent basis from 2017. There would have to be a massive change in momentum for this not to happen.
The extent to which teams and riders will use disc brakes on a regular basis is unclear. Will they become the default option or will they be used only for particular races/stages? Nobody knows for sure, but we can be pretty certain that disc brakes will become a familiar feature of top level road racing very soon.
Whether or not road racers are won over by disc brakes, brands will almost certainly encourage teams to use them as a way of legitimising and validating the technology in the eyes of the bike buying public, and ultimately selling more disc brake road bikes.
Whether you’re an early adopter looking to upgrade or thinking your next bike will have discs, here’s our selection of the disc brakes we currently favour.
Buy a mid-range disc brake equipped road bike or cyclocross bike at the moment and there’s an extremely high chance you'll end up with a pair of TRP Spyres bolted to it. In the tidal wave of new disc bike drop bar bikes appearing on the market, the Spyre has become the benchmark for ease of setup, use and reliability.
These are excellent, quite possibly the best mechanical disc brake solution out there - more expensive than its predecessor but less expensive than a hydraulics.
TRP Hy/Rd disc brakes combine cable actuation with hydraulic power right in the calliper. They're powerful, easy to live with and the best solution so far for disc brakes with conventional brake/gear levers. After a month or so testing these brakes in all conditions, we found them to be more powerful and controllable than rim brakes and easier to set up and maintain than mechanical discs, and they win over stem-mounted converters in their simplicity with no noticeable loss in performance.
Shimano’s hydraulic disc technology has trickled down to mid-range 105 level. You get the front and rear shifters, disc callipers, pads, cables and hoses included in the £399 package. The callipers are more compact than the post mount ones and they come with Ice-Tech resin pads with heat-sink fins to help with cooling. The front brake has a reversible plate that allows you to run either a 160mm or 140mm rotor at the front. the rear flat mount calliper bolts directly through the frame if you're running a 140mm rotor; if you want a 160mm at the back you need an extra plate between the calliper and the chainstay.
This is a very good first incarnation of road hydraulic braking from Shimano. In use, the brakes are really excellent, with significant improvement in modulation compared to mechanical rim brakes. We racked up hundreds of horrible, wet, dirty commuting miles with these and they brakes operated reliably throughout with no reduction in power or control. If you can afford them, these brakes are very much recommended. They're not without their glitches, but they outperform rim brakes in pretty much every situation.
SRAM's Rival 22 hydraulic groupset is the lowest tier of its road disc line-up, but for many it provides the ideal combination of performance and price to fit to an all-purpose bike. The hydraulic levers look bulky, but ergonomically they're easy to use and comfortable (with a caveat if you have really small hands). The braking offers great modulation and plenty of power for very little effort. We did get some fade when we dragged one of the brakes on a two-mile descent. Trying to cook both brakes at the same time was impossible: if you're generating enough heat to affect the system then you'll be at a standstill in no time.
Mat has worked for loads of bike magazines over the past 20 years, and been editor of 220 Triathlon and Cycling Plus. He's been road.cc technical editor for seven years, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. We send him off around the world to get all the news from launches and shows too. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a past winner of the Cycling Media Award for Specialist Online Writer.