“Disc brakes”, I hear you say, “not this petty argument again.” You would be right and I cannot agree enough how sick I am of people complaining that they don’t need them and that they are fine with rim brakes and vice versa. This isn’t what this article is about – instead, I am going to highlight how I feel they have evolved over the last couple of years and how we are getting even closer to a better set of standards. Rim brakes are still brilliant in their own right as they allow you to have a generally lighter bike and still offer excellent braking and it’s good to see brands such as Trek, who offer consumers the choice rather than forcing you into one type of braking. Looking at a lot of the 2017 model year bikes and the majority of bikes for 2018, it is clear that disc brakes have evolved a lot since their inception into road biking. Let’s take a look at this apparent path of evolution.
When disc brakes were first announced on road bikes, many of them were mechanical disc brake systems (operated by a cable, not hydraulic fluid) which in my opinion, are lacking. I can see the benefits to mechanical for those who tour in that it is easier to get hold of a cable as opposed to specific hydraulic fluid. However, mechanical disc brakes are subject to cable stretch and require a lot more maintenance than hydraulics which self-adjust as the pads wear. You don’t get as much modulation and feel or bite compared to hydraulics. In my opinion, they’re not really good for anything and brand such as TRP introduced the Hy/Rd which is a part hydraulic system but to be honest, I would agree with Matthew Allen’s assessment in a recent BikeRadar article that this is a solution to a problem that shouldn’t exist in the first place. Of course they’re cheaper than going the whole hog to a hydraulic system but as you will see, as is the case in the bike industry, now that disc brakes on road bikes are no longer in their infancy, they have been subject to trickle down to lower levels.
Hydraulics were introduced onto road bikes by SRAM and Shimano which instantly propelled this technology and improved the quality of braking on these various road bikes that came equipped with them. However, hydraulics were still in their infancy. Both Shimano and SRAM’s levers were considerably more bulbous in order to house the reservoir for the hydraulics.
Shimano weren’t confident from the outset of their hydraulics and didn’t incorporate them into their groupset line, instead labelling their levers with part numbers – R785 for the electronic-hydraulic lever, RS685 for the mechanical-hydraulic lever that was aimed at the Ultegra level, RS505 for 105, RS405 for Tiagra. The latter two are perhaps the most ugliest shifters I have ever seen (although this is personal opinion) and also affect fit of the bike as it means that your hands are further out on the hoods. As I work part-time in a bike shop, these two models do complicate the fitting process as you have to account for this. These later models have been introduced throughout the years and it’s refreshing to see Shimano offer hydraulics at the Tiagra level. Whilst these systems all worked well, it was interesting to see that Shimano weren’t confident enough to incorporate these levers into their official groupset line.
SRAM did however incorporate hydraulic braking into their line of groupsets but then famously suffered a widescale recall in late 2013 due to some quality control issues. Although the replacement was still considerably more bulbous, it was a lot smaller than the original iteration.
It is very interesting to note Shimano’s recently announced Dura-Ace and Ultegra groupsets and Shimano have now totally revamped the levers, rotors and callipers and have branded them into these levels. It is astonishing to see, the Dura-Ace version in particular, maintain the same shape as the cable-actuated version and this is proof that you can get these levers to the same size in order to get the same feel. If you’re going to go for Shimano disc brakes, I would say now is the time where you can finally feel confident.
SRAM haven’t yet announced their next generation groupsets but I am confident that they will follow suit with the way that Shimano have done. This just leaves Campagnolo alone in the water, who are only just beginning to bring out their own hydraulic systems.
The actual brake itself though isn’t the only thing to consider when it comes to disc brakes. You also need to consider the size of rotor, how the calliper attaches to the frame and how the wheel attaches to the bike. This is why disc brakes have been quite a complicated affair over the years and why they haven’t yet been introduced into the pro-peloton. If you needed a spare wheel from neutral service, how would you know if the wheel you were getting would fit the same standards as your bike?
When it comes to rotor size, the bigger the rotor, the more powerful the brake. With road bikes, you don’t need anywhere near as big a rotor as you would on a mountain bike and at first, the standard seemed to be 140mm. However, other brands spec 160mm and some brands spec 160mm front, 140mm rear such as my Norco cyclocross bike. This standard still isn’t set in stone but with brands releasing adaptors to convert between the two, this certainly helps and I suspect 160mm will be the preferred size as time goes on.
In terms of mounting, the two types are flat-mount and post-mount. Post-mount pretty much follows the IS standard it replaced from mountain bikes whereas flat-mount discs sit flush against the frame and make hose-routing (particularly when the hose is routed internally through the frame) better. Again, you are able to convert between the two depending on what standard your frame is but in my opinion, this makes the bike look quite ugly. Each to their own though.
Keeping in tune with how disc brakes have evolved over the years, it looks very much like flat-mount will be the main standard and is much more predominant on 2017/18 bikes than it has been before. Shimano, who invented the standard, only sell callipers for their new Dura-Ace and Ultegra groupset in a flat-mount configuration.
The final thing to consider is how the wheel attaches to your bike. At first, quick-releases (QR) were used which are good enough but there is always an element of adjustment with them which means you need to position the wheel perfectly so your disc rotor doesn’t rub. There is also the very small risk that the forces exerted by the disc can cause a QR to loosen.
Thru-axles (TA), carried over from the mountain bike world, are now popular on road bikes with disc brakes as it makes the frame much stiffer as TA’s only go into the frame in one direction and it also reduces the risk of disc rub. It is also more difficult overtighten them as a lot of them come with a preloaded torque. Systems such as Focus’ RAT system make this process even easier – the RAT system requires a quarter-turn to undo which makes wheel changes even quicker.
In terms of standards, at first there were many. On the front, 100×12 and 100×15 for example (and on the back, 142×12 and Specialized’s infamous 135×12 (more on that in a minute). These dimensions refer to the size of the thru-axle and the frames that manufacturers released were also specific to these sizes. You can’t change between them because the thru-axles need to be specific to the frame. Looking to the present, it now looks as if manufacturers have settled on 12mm front and 12mm back. However, wheel manufacturers such as Zipp give you different end-caps with their wheels so you can swap between them all and also QR.
You also need to consider how the thru-axle is removed from the bike. Some have a lever-design with a ratchet, some require an allen key to remove them, some such as DT Swiss’ lever can do both. Carrying around an allen key to take your wheel off is a bit of a faff but it does make the bike look cleaner and of course, the industry is always chasing for integration and aerodynamics. When you look at how this aspect could be a dilemma in the pro peloton, it could mean that certain riders can get their wheels changed quicker than others which is a little unfair. But for the likes of you and me who presumably don’t race professionally, surely the added security and stiffness of a thru-axle is a better benefit than a faster wheel change.
So taking all these standards into account, it looks like the industry will go for 140/160 disc rotors, flat-mount disc brakes with 12mm thru-axles front and rear. This would make sense and would mean that because these are all the same, if neutral service were to carry wheels of this spec or if you were to buy or swap your disc-brake wheels from bike to bike, they would interchange together.
Now that we’ve looked at the actual disc brakes, the models and the various standards that go with them, we finally need to have a look at the frames that manufacturers have made to accommodate disc-brakes.
Frames in most cases, need to be specific to the braking. A disc frame needs to be able to react well to the different forces a disc brake exerts compared to a rim brake. Generally, they are a bit beefier and a little heavier to accommodate this. When you look at geometry charts, it’s often quite telling to see how in most cases, manufacturers need to change the geometry to suit.
Have a look at chainstay length for instance. Generally, the shorter the chainstay, the more nimble the bike will handle on the back end and it’ll mean a shorter wheelbase too (distance between the axles, a longer wheelbase means a more stable bike, a shorter wheelbase means more nippy). However, this affects the chainline and the groupset brands specify a chainstay length of 420mm needed to ensure good performance of the gears. You don’t really need to worry about this on endurance bikes as they tend to have a longer wheelbase anyway as they are designed for comfort. However, when brands such as Specialized made disc-brake versions of their Tarmac and CruX for instance, typically bikes designed for racing, you don’t want to change the properties of the frame as if you have longer chainstays than their rim-brake counterparts, this then affects how the bike handles as it’ll have a longer chainstay which will mean more stable.
Specialized infamously tried to combat this by introducing SCS (Short Chainstay System) and to do that, it had to change the rear thru-axle size to accommodate this to 135x12mm. This received uproar as it meant that many wheel brands didn’t support this size and you were only tied to Roval (Specialized’s in-house brand) and a few others for wheels. Cannondale have managed to do this in a slightly more friendly way and additionally create further mud clearance on their cyclocross SuperX bikes by redishing the rear wheel 6mm to the right.
As you can probably tell, there is a lot to consider when it comes to the method of braking you go for when it comes to picking a road bike. But I now feel that with a solid set of standards seemingly set, now is a much better time to make the jump if you’re unsure as it looks like you will be supported in the years to come. My next road bike will certainly be disc as I feel that the manufactures have reached a high-enough level. The manufacturers will only further improve and now that they all seem to have more confidence in their own product and have managed to make the levers an almost indentical to their cable-actuated counterparts, it shows that this technology is being taken seriously.