Road discs - Industry led bling or the next use of a proven technology?

The industry seems to want them, some riders don't. Personally, I do. There are problems, but they're not insurmountable.

Road discs then. Everyone's talking about them, and quite a lot of people are trying to make them. But they're not ready, yet. Why is that? Personally, I think it's because making discs work in a road bike situation has been a bit harder than people expected. Even though the technology is well proven in a MTB context, there are different considerations and the usage is significantly altered on the road.

Even so, the industry is pressing on. We had a ring round some of our contacts to gauge the mood, and although some were decidedly more cautious than others, most tended to agree that it's coming. The main mantras of the disc advocates are pretty simple:

  1. They work better, in all weathers
  2. Your rims don't wear out

Not everyone thinks road discs are a great idea though. Today I listened to the Velocast Tech Podcast with John Galloway and guest Sean Lally from Cycle Systems Academy. Neither of them seem to be very keen at all, and there was much discussion as to the merits or otherwise of switching braking over. Some of it was more conjectural, and included famous episodes in the early life of road discs such as Tyler from Bikerumor's brake fade crash. Hey, it's hell being an early adopter. Other parts of the podcast dealt with the technological reasons why it wouldn't necessarily be a good idea. And for the most part I don't really agree, although the podcast is definitely worth a listen for their views. Here's my take on some of the points raised. Do discuss. I know you will.

Lack of traction: One of Sean's points, and the most contentious for me because as far as I'm concerned it's simply not true, is that road tyres don't have enough traction to apply the higher braking forces capable with a disc brake, leading to the danger of skidding out. I dropped out of my Mechanical Engineering degree in disgrace after a year, so I'm not your first port of call for technical stuff, but even I can understand the friction equation. The force required to move two surfaces against one another is a product of the coefficient of friction between them and the force holding them together, in this case applied by the mass of the rider (and bike). It has nothing to do with contact patch size and very little to do with tread pattern. Road bike tyres therefore have more traction than MTB tyres can ever have offroad – the coefficient of friction being much higher between tarmac and rubber than between dirt and rubber – and about the same as they'd get on the road, because the rubber and the weight of the rider are basically the same. Lack of traction on the road is just not an issue. Increased traction leads to other problems though.

A thru axle is a step too far: thru-axle technology has been touted by some as necessary for conteracting twisting forces from braking. Certainly, I'd struggle to see how that'd catch on in the peloton: switching a thru-axle wheels isn't exactly onerous but it definitely takes a lot longer than a QR. But too far? is an asymmetric head tube a step too far? A press fit BB? Di2? Who decides? It's kind of moot, as I don't believe a thru axle is necessary for a properly functioning disk brake system. If it were, my MTB wouldn't work. But drawing a line in the sand and saying development of the road bike mustn't cross it is what the UCI do.

Disc brake wheels/levers/brakes/whatever are ugly: It's a commonly held view, but it's not a reason to halt development. To me it doesn't really mean much, if they're a proven performance advantage. I think that plenty of carbon frames are really ugly. Ugly is normally a product of how far the look of something deviates from the accepted norm. Currently the norm is radial front wheel lacing and callipers. Discs look a bit odd.

Frame and fork design: I talked to one manufacturer at Eurobike about their disc road bike and the only bit of the frame they'd redesigned was the chainstay that held the calliper. Everything else was FEA and real-world tested and was fine with the new loads. Frame design has been tending towards lighter and lighter structures for ever but we know that the production frames we have now are heavier than what's required to pass the CEN tests. When Cannondale launched their 695g SuperSix Evo frame they told us that the frame was capable of passing the tests at 620g; That's 75g of carbon you can repurpose to specifically counteract the braking forces if you want.

Comfort's another factor. The places where you tune the frame itself for comfort (seatpost and seatstays, primarily) aren't heavily affected by the braking forces; mostly it's the chainstays and bottom bracket, which on a Carbon race frame are already pretty monolithic and built for stiffness for maximum efficiency of power transfer. They don't twist when a sprinter is putting 1500W of power through them in a dash for the line, and they'll cope with big forces at the other end, too. Plus, moving to discs negates the need for a brake bridge or a stiff enough seatstay to deal with forces from the calliper, meaning more work can be done there to improve comfort.

Forks are more of an issue because the bit you need to build to be stiff enough to not bend is also the bit that you'd want to tune for the ride. But in an industry where 'laterally stiff yet vertically compliant' has become a running joke, it's surely not beyond the wit of the engineers to build in the right stiffness and compliance in the different planes. We've tried some of the early road disc forks though, and they've been a pretty mixed bag so far.

Aerodynamics/weight: Nobody's really tried to optimise a disc brake wheel in an aero setting. So we're not really in a position to judge whether the disc/rotor disruption can be offset by different rim design. My guess: not quite, though we've talked to manufacturers who have said that the aero disadvantage of a disc and calliper is pretty minimal. After all, the disc presents almost no surface area at the front and the front calliper is hidden behind the fork leg and could presumably be integrated into it. On top of that, the fact that you can route disc hoses however you like with no loss of performance means fully internal routing is possible.

The weight of a wheel is an issue but rotating weight is more cruicial, and discs add weight at the centre with the promise that it might be possible to shave some weight off the rim if a braking track isn't required. 100g more rotating weight where the disc is only needs about a 25g weight reduction at the rim for the same angular momentum. That's probably attainable. Radial lacing is out though, as the wheel won't be strong enough. So longer spokes mean more weight again.

Ease of maintenance: we're talking about the same basic systems as you have on a MTB. Okay, they're not as easy to service as a cable but I don't hear many people moaning about the fact that MTB discs aren't user serviceable. They don't moan because, for the most part, the systems don't break. Hydraulic discs are easy to set up and they self centre. For day-to-day use they're much less hassle than rim brakes.

Heat: Tour de France riders don't drag their brakes all the way down the Galibier – just sayin' – but plenty of amateur cyclists will do precisely that and heat build up is, for me, definitely the main issue with road discs. The forces are bigger than with MTB discs, the speeds higher and the descents are longer. All of those things mean more heat. But if it can be solved for MTBs and F1 cars and touring motorbikes and every other secnario we've yet tried, surely it can be solved for road bikes? Technologies like Shimano's wafer-construction ICE rotors and heatsunk pads are still in their infancy, and we're not at the point where we've stopped innovating. There's more work to be done; personally I think the problems with heat dissipation are the reason we haven't seen production systems yet. But their time will come.

Dave is a founding father of road.cc, having previously worked on Cycling Plus and What Mountain Bike magazines back in the day. He also writes about e-bikes for our sister publication ebiketips. He's won three mountain bike bog snorkelling World Championships, and races at the back of the third cats.

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