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.

by Dave Atkinson   January 15, 2013  

Colnago C59 Disc - front disc

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.

18 user comments

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Chainline/heel clearance is another factor to consider since the industry seems to be heading towards settling on the 135mm standard as opposed to traditional road 130mm. Factor in a 68mm BB shell, the narrow q-factor of a road chainset and short chainstays and you've got a combo that wasn't srictly designed to be run together.

posted by Alb [72 posts]
16th January 2013 - 0:40

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What do you think about concealed cables/hydro pipes, Dave?

One of the off-putting things about discs is the cable running down the back of the fork. The rear isn't so bad as it can be incorporated into the frame but the fork cable/pipe problem needs to be resolved.

Twitter: @velosam

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posted by SamShaw [205 posts]
16th January 2013 - 10:48

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The force applied at any point however *is* a function of the contact patch. (But I agree a road tyre will have more than enough traction available).

posted by Paul J [431 posts]
16th January 2013 - 11:30

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Alb wrote:
Chainline/heel clearance is another factor to consider since the industry seems to be heading towards settling on the 135mm standard as opposed to traditional road 130mm. Factor in a 68mm BB shell, the narrow q-factor of a road chainset and short chainstays and you've got a combo that wasn't srictly designed to be run together.

yeah it's interesting that that's the way everyone's seems to have chosen. tyres are narrow so sensible chainstay design should mean that heel strike shouldn't be any more of an issue than it is now. chainline's more of an issue

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posted by Dave Atkinson [7044 posts]
16th January 2013 - 11:56

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SamShaw wrote:
What do you think about concealed cables/hydro pipes, Dave?

One of the off-putting things about discs is the cable running down the back of the fork. The rear isn't so bad as it can be incorporated into the frame but the fork cable/pipe problem needs to be resolved.

i don't think concealed cables are a great idea; the more tortuous the routing the worse the performance.

with pipes though it doesn't matter: no amount of toruous bends will have an effect on braking performance. that's one of the major beneifts, especially for TT ans aero road bikes. now that the standard is to fit the rear calliper between chain and seatstay, the obvious solution for the rear is to run internally through the down tube and along the chainstay. the hose can go inside the fork too, and i think that's where it'll end up in some cases, although there's the engineering problems associated with putting a hole in a high-stress area to be resolved. the other and perhaps more elegant solution is to design the fork leg with a channel at the rear and use a thin cover to conceal the hose and make the shape of the fork leg.

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posted by Dave Atkinson [7044 posts]
16th January 2013 - 12:04

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SamShaw wrote:
What do you think about concealed cables/hydro pipes, Dave?

One of the off-putting things about discs is the cable running down the back of the fork. The rear isn't so bad as it can be incorporated into the frame but the fork cable/pipe problem needs to be resolved.

Plastic Clip on shroud over the cable ? The only hose you'd
see then would resemble a caliper brake version as it
goes from the crown to the lever.

Can't see heel strike being a real problem, after all
in the greater scheme of things we're talking 2.5mm each
side extra width. Yes some will have problems with this
but most of us wouldn't notice it.

still on the 3rd switch-back of Bwlch !

posted by therevokid [592 posts]
16th January 2013 - 12:05

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I have skidded on my road bike (with race tyres and the like) from braking hard. I think, for me, this is the fundamental issue.

It maybe my bike handling skills - to be honest I don't remember where my weight was distrubuted, in each case it was an emergency brake and so I didn't have time to think about positioning. But the point remains, I skidded and so increased braking power on the rim will not help me for all the engineering.

posted by Colin Peyresourde [978 posts]
16th January 2013 - 13:58

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"Road bike tyres therefore have more traction than MTB tyres can ever have offroad" - that depends totally on the surface. Ever ridden Moab? Modulation is *critical* because your tyre sticks like the proverbial.

Agree re: Thru-axle. Ive not owned one in 20yrs of MTB/DH. Maybe that marks me a DH puss.

Ease of maintenance (disc better than rim): modern dual-pivot cartridge brakes are sooooo easy to maintain, compared to say V-brakes or canti's. Is this really an issue?

Weight & aero - I see this as a major departure from all the gains of the last decade.

Heat: Heat just isn't an issue for 99% of MTB as they are almost always short grabs of stop, so it didn't need 'solving' beyond a good open reservoir design. Granted not all folks ride 60-minute descents regularly, so this probably isn't much of an issue for the UK Audax crowd.

I was told there would be Cake. Luckily there's http://TestValleyCC.org.uk

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posted by KiwiMike [376 posts]
16th January 2013 - 14:05

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KiwiMike wrote:
Ever ridden Moab? Modulation is *critical* because your tyre sticks like the proverbial

no, much as i'd like to Smile

the amount of traction available to mtb tyres on slickrock is probably similar to road tyres on tarmac. ie, plenty

i didn't say disc brakes were necessarily easier to maintain than dual pivot brakes. my point was that it's not really an issue, when it's sometimes touted as such

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posted by Dave Atkinson [7044 posts]
16th January 2013 - 15:09

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Incidentally I've been having a chat with John and Sean on twitter today and the question of hydraulic rim brakes came up, which I didn't mention above really. I think they've got a much better chance of being adopted quickly for the pro peloton than discs. They offer some of the advantages of hydraulic discs - increased power, no problem with wiggly cable routing - without many drawbacks. obviously your rims will wear down and you can't run an out-of-true wheel, but these are not really concerns for a pro. they're retrofittable to existing frames too.

Magura make a road version of their system already but it's a converter and as such is only a stopgap. SRAM are going to do hydraulic rim brakes for Red, and they'll no doubt break cover with the hydro levers

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posted by Dave Atkinson [7044 posts]
16th January 2013 - 20:30

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Plenty of touring and 'cross bikes out there with 135 oln rears and 68mm shells, chainline shouldn't be an issue

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posted by ctznsmith [102 posts]
16th January 2013 - 20:31

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Quote:
I have skidded on my road bike (with race tyres and the like) from braking hard. I think, for me, this is the fundamental issue.

The main benefit of disc brakes is not necessarily increased power, but improved modulation.

Another benefit (in the realms of road racing down big alpine passes) is the rims won't overheat, melt glue and roll tubs.

posted by Sam Alison [21 posts]
16th January 2013 - 22:46

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It's true that disc brakes mean that rims could be made to do their job of supporting the tyre and spokes without having to act as a degradable braking surface but I don't see many MTB disc-specific rims that are anything other than a standard rim with a painted over braking surface! I might be wrong but aren't manufacturers slow in getting behind the potential disc movement too?

Rich

posted by griggers [14 posts]
17th January 2013 - 14:13

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I know this is old news, but I've never seen a satisfactory resolution of the "disc brake forces acting on wheel ejection" issues as discussed in

http://www.ne.jp/asahi/julesandjames/home/disk_and_quick_release/

It doesn't get a mention in the above considerations, so I have to assume either the problem never existed in the first place or that it was somehow solved. Comments? (Note one suggestion therein is mounting the calliper on the right side so forces are pushing the wheel up instead of down, which seems a pretty simple change to make if the problem actually existed).

I agree that the "lack of traction" is not an issue and certainly not on (say) a loaded touring bike, where I think a front disc has merit. Brake fade is certainly solvable and if it happens is an obvious design flaw. Let's face it, a lot of bicycles are just a collection of parts thrown together at a price point with the hope someone buys them and then never rides them.

Ride your own ride

posted by CanAmSteve [88 posts]
17th January 2013 - 16:22

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But what of improved braking for carbon rims which, I hear, have poor friction characteristics for rim braking. This could lead to reduced rim wear and more predictable braking. Cyclo cross and cycling in winter conditions should now have less associated wear on rims and brake material. I'm sure someone will come up with a clever 'bolt-on' device which will warn when brakes are cooking on long descents. A simple Red Amber Green temperature sensor on handlebars... There are always solutions to solve the previous solution Wink

If I was only half as good as I am in my own mind.

posted by JulesW [24 posts]
22nd January 2013 - 12:40

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Dave Atkinson wrote:
SamShaw wrote:
What do you think about concealed cables/hydro pipes, Dave?

One of the off-putting things about discs is the cable running down the back of the fork. The rear isn't so bad as it can be incorporated into the frame but the fork cable/pipe problem needs to be resolved.

i don't think concealed cables are a great idea; the more tortuous the routing the worse the performance.

with pipes though it doesn't matter: no amount of toruous bends will have an effect on braking performance. that's one of the major beneifts, especially for TT ans aero road bikes. now that the standard is to fit the rear calliper between chain and seatstay, the obvious solution for the rear is to run internally through the down tube and along the chainstay. the hose can go inside the fork too, and i think that's where it'll end up in some cases, although there's the engineering problems associated with putting a hole in a high-stress area to be resolved. the other and perhaps more elegant solution is to design the fork leg with a channel at the rear and use a thin cover to conceal the hose and make the shape of the fork leg.

Ah, the old toilet paper conundrum!

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posted by Tovarishch [37 posts]
25th January 2013 - 12:00

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griggers wrote:
I don't see many MTB disc-specific rims that are anything other than a standard rim with a painted over braking surface!

A Stan's Crest 29er rim weighs 380g, about 70-80g lighter than a typical road rim - the profile is far squatter and significantly wider than a road rim - very strong too.

Make mine an Italian with Campagnolo on the side

posted by monty dog [346 posts]
16th March 2013 - 17:46

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I'm pretty sure that my first 531 frame bought in 88 for use with I think the norm 7 speed cassette was 125mm cos when I had it repainted and I moved to 8 speed I also had the rear triangle cold set to "traditional" 130mm spacing.

As for grip and power; in my humble opinion the advantage of disks is that 1. even in rain stopping power/ feel is virtually unchanged from dry, 2. Rather thanwadding extra stopping force at the tyre you end up applying less force t the lever for equal stopping effect. Thus less arm/ hand pump on long descents and safer braking from many hand positions.

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posted by bikeandy61 [361 posts]
11th May 2013 - 20:48

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