Road bike disc brakes: everything you need to know
What they are, how they’re different, and why you should care
Disc brakes are coming to road bikes in a big way - but why? And why should you care?
Road bike disc brakes are the hottest topic when it comes to road bikes and equipment, and to say that the debate is intense would be quite the understatement.
Some can’t wait until they’re standard across the board while others want to kill them before they can lay eggs. Most, however, seem to be on the fence about technology but are at least interested to learn more.
So what exactly are road disc brakes and why all the fuss? Are they really any different from other recent advancements in bike technology? Hide your head in the sand if you wish but unless you’re planning on ditching the sport altogether, you’d best read up because they are going to impact on you one way or another – and probably sooner than you think.
What are road bike disc brakes, anyway?
The most fundamental difference between traditional rim brakes and disc brakes is where the braking forces are applied. As the name suggests – and just as it’s been done for decades – rim brakes clamp directly on to the sides of the wheel itself. In this way, the rim serves as a main structural component of the wheel, the mounting base for the tire, and the braking surface all in one.
In contrast, disc brakes move all braking duties to a separate rotor that is much smaller in diameter and mounted directly to the hub – much like everyday automobiles or motorcycles. The brake caliper is still mounted to the frame and fork but it’s situated much closer to each wheel axle.
Another key difference is how each type of brake is usually operated. With few exceptions, rim brakes are ‘cable actuated’, meaning the levers are connected to the caliper with braided steel cables that slide through some sort of housing – just as has been done for ages. You pull the lever, which then pulls on the cable, which then forces the caliper to clamp down on the rim.
Disc brakes are more often of the fully hydraulic variety, where the cable and housing are replaced by a non-compressible fluid and a hose in a fully sealed system. When you pull the lever on a hydraulic brake, it pushes a plunger in a ‘master cylinder’, which then pushes that fluid through a hose to the caliper at the other end. That hydraulic pressure is what pushes the caliper pistons out and clamps the pads on to the rotor.
Why disc brakes are better than rim brakes
Disc brakes offer several key advantages over rim brakes.
First, they generate much more stopping power, meaning there’s less force required at the lever to generate the same amount of deceleration as on a rim brake. This can be a big help on long and steep descents where ‘arm pump’ can eventually set in, or for heavier riders that have struggled to find enough power from traditional brakes (the same issue applies to heavier bikes, such as tourers and tandems).
An added bonus is that braking power can be quickly and easily boosted (or tempered, depending on your preferences) by changing rotor sizes. A bigger rotor will increase the mechanical leverage while a smaller one will save weight for riders that just don’t need the extra braking force.
Granted, stopping power on any wheeled vehicle is inherently limited by traction. And as many of you will know (possibly through painful experience), it’s already quite easy to lock up a wheel on a bike with rim brakes – which brings us to advantage #2: control.
Disc brakes offer better modulation than rim brakes, meaning it’s easier for the rider to precisely meter out exactly how much clamping power is generated. Peak stopping power occurs just before the point of lock-up and disc-equipped bikes are better equipped to flirt with that edge without crossing over. Disc brake power also tends to be more linear and predictable than on rim brakes, and it’s far more consistent in varying weather conditions, especially when compared with using rim brakes on carbon rims – a combination that yields notoriously poor performance in the wet and yet is also prone to overheating when dry.
All of these advantages are boosted further for hydraulic disc brakes, which have reduced power and frictional losses from lever-to-caliper than cable-actuated ones.
Why rim brakes are better than disc brakes
Especially in these relatively early days of disc brakes on road bikes, rim brakes still hold the upper hand in several key areas.
Their biggest advantage is on the scale. Although the disc and rim brake components themselves aren’t too different weight-wise, the requisite disc-compatible wheels, frames, and forks are generally heavier as well. On average, a good rim brake-equipped bike will be about 500g (roughly a pound) lighter than one with discs – and that’s for a high-end example, all else being held equal.
Sticking to the performance front, disc brakes are also less aerodynamic than rim brakes – although the exact degree of that difference is still up for debate.
The simplicity of cable-actuated rim brakes has plenty of upsides, too. Parts are generally cheap and very widely available, there’s a very high degree of compatibility between multiple brands and vintages, and the systems are easy to repair when needed – even on the side of the road or in the middle of nowhere.
Finally, there’s the issue of aesthetics – an area where rim brake fans are especially defensive. Simply put, bikes with traditional rim brakes are often pegged as being far more elegant looking than ones with disc brakes. That’s obviously a matter of opinion but as we all know, deeply held personal opinions and beliefs are powerful things.
How disc brakes are going to change road bikes
Cast aside the various conspiracy theories surrounding why the industry seems so hot on disc brakes right now. Of course, companies would love to sell more bikes and gear than they do now. History has repeatedly demonstrated, however, that major changes in bike technology have only shifted the types of bikes and gear that people buy, not the grand total.
The fact of the matter is that the bike industry sees the road disc movement as a way to advance bike technology forward in a meaningful way in one big step. Bikes equipped with brakes that work better are safer, full stop (pun intended).
But what are those bikes going to look like? We’re already getting a taste as the market is already getting flooded with options.
As already mentioned, disc-equipped bikes are heavier and less aerodynamic than comparable ones with rim brakes. But for many riders, the other freedoms afforded by switching to disc brakes more than offsets those setbacks.
Without having to worry about accommodating a caliper, disc-equipped road bikes can now more easily fit higher-volume tires and wider rims for improved traction on a more diverse mix of terrain. Since the seatstays no longer have to be reinforced to accommodate a rim brake, they can be made more flexible.
Wheels will likely change with the move to discs, too. Without the need for a conventional brake track, clincher rims can instead be designed more for aerodynamic performance – and we’ll likely see more carbon clinchers since overheating as a result of prolonged braking is no longer an issue.
Expect the move to disc brakes to also come with a concurrent transition to thru-axles. Although it’s still faster to install and remove a wheel with quick-release skewers, there’s still too much variability in how the wheels fit into the frame and fork. This can lead to issues ranging from annoying (pads rubbing on rotors) to terrifying (unanticipated wheel ejections under hard braking).
Thru-axles instead use closed dropouts that more consistently place the wheel in the same spot relative to the brake caliper, and are safer to use in general with less chance of user error. Expect these to become the norm.
Moving forward, we expect that companies will continue to figure out how to more cleanly integrate disc brakes into bike designs, which should not only narrow that aerodynamic performance gap but will likely decrease the added weight, too. Keep in mind that these are still early days in the road disc brake evolution so things are bound to get better.
Compatibility and standards issues
There’s no gentle way to say this: disc brakes are a nightmare in terms of compatibility with existing rim brake equipment.
Disc brakes not only require dedicated fittings on the frame and fork for the caliper but, ideally, localized reinforcements to handle the added stresses applied. Meanwhile, the wheels require hubs with either a six-bolt or Shimano Centerlock splined interface to attach a rotor. Neither of these can simply be added on after the fact.
Likewise, companies that are on-board with discs see them as the future – so bikes and frames that are equipped with them aren’t likely to allow buyers to switch back to rim brakes. Sorry, it’s a one-way door.
Worse still, even the methods by which disc-specific components are attached to a bike are still in flux. While there are indications that the industry is settling on 12mm-diameter thru-axles and the new ‘flat mount’ caliper mounting standard, these are yet to be well established or official. Such stability would be a boon for racers and general enthusiasts alike for a lot of reasons, but in the meantime things are uncertain.
There’s virtually no mixing and matching allowed between disc brake brands, either, at least as far as hydraulic options are concerned. While there’s a fair bit of flexibility for combining different makes and models of rim brakes, especially when you factor in smaller aftermarket brands, disc brakes are much more limiting.
SRAM hydraulic disc brake calipers can only be paired with SRAM levers, for example, and the same goes for Shimano and Campagnolo. Cable-actuated disc brakes from TRP, SRAM/Avid, Hayes and others offer more flexibility but even then, differing cable pull ratios between the various makes, models, and even years of levers have to be considered for optimal functionality.
Implications for long-term maintenance and serviceability
Hydraulic disc brakes have long been widely accepted in the mountain bike market and if you take that history as a reliable indicator, disc-equipped roadies are in for a bit of a mixed bag in terms of long-term maintenance and serviceability.
On the one hand, hydraulic disc brakes are fully sealed from the elements and require little-to-no everyday maintenance – most of the time (cable-actuated disc brake maintenance will be more inline with conventional rim brakes). There’s no housing for grit to get into and no cables to fray. As an added bonus, they even self-adjust for pad wear so the lever pull stays consistent over time. Aside from occasionally bleeding the system with fresh fluid – most companies recommend doing this about once a year – there’s not much to it.
Bleeding hydraulic systems does require special tools with a full home kit running about $55 / £45 (plus a few extra for fluid annually). Alternatively, having a shop do it will cost about $60/£40 each year, give or take.
Disc brake pads also tend to be slightly more expensive (about $80 / £50 vs $60 / £40 per full set of good ones) but the real-world differences are quite minor when you factor longevity into the equation. Keep in mind, however, that regularly riding in wet, gritty conditions can skew those figures dramatically.
Conventional cable and housing is far from cheap, though, especially if you prefer to use higher-grade stuff (as you should), so while rim brakes hold an advantage here, the differences aren’t as dramatic as it might seem.
If something actually breaks, however, rim brakes hold a big edge since it’s much easier to diagnose – and repair – a problem. For most users, hydraulic disc brakes will be akin to electronic equipment: while you can often figure out an issue on your own, most cyclists won’t have the equipment or knowledge to do so.
Master cylinders can (and do) fail on occasion, for example, and caliper pistons have been known to crack. Parts can be obtained – albeit usually with some headache involved – but it’s invariably a messy and tedious job.
If you’re the type to run things into the ground, though, keep in mind that there are definite upsides to not subjecting your rims to regular wear. Whereas it’s very expensive to replace a rim that’s been worn down from long-term braking, disc brakes only require a new rotor. Speaking of which, those rotors are also less likely to go out of true than a rim.
Where we go from here
You can fight and resist disc brakes on road bikes all you want but the tide has already started to shift and there’s little you can do to hold back the water.
Will disc brakes replace rim brakes completely? Perhaps not. At the very least, rim brakes will likely live on with smaller brands and niche applications.
But over the long term, our money is on discs emerging as the dominant technology. Discs were barely a blip on the cyclocross radar just a few years ago, for example, but several major companies have already eliminated rim brake options completely from their current lineups – and the UCI has now paved the way for their use in the Tour de France.
The case for adopting them immediately might not be ironclad at present – after all, even good rim brakes are overkill for some already – but regardless, it’s increasingly clear that disc brakes are going to have an impact on roadies one way or another in the coming years.