Menu

Technical FAQ: Defending road disc brakes

  • By Lennard Zinn
  • Published Sep. 4, 2012
To road disc or not to road disc? That is a question. Photo: Nick Legan | VeloNews.com

Editor’s Note: Lennard Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.

I received a lot of feedback on last week’s column on road disc brakes. Here’s a sampling.

Dear Lennard,
You are remarkably negative about mechanical disc brakes and disc brakes on road bikes in general. Typically I find your look on things to be well thought out and balanced, but this latest piece comes over as only negative.

It’s true and also mentioned in Avid’s instruction manual that you’ll need compressionless housing for any mechanical disc brake in order to make it work properly. But that kind of housing is readily available. So what’s the problem? You don’t blame Shimano, SRAM or Campagnolo for the need for compressionless housing when using their shifting systems. I have made good experiences with Jagwire’s Kevlar-reinforced Ripcord housing with 5mm diameter, but I know at least two other manufacturers who also offer reinforced linear strand housing designed for being used as brake cable housing.

When using this housing — even full length with a rear brake as I do on my rain road bike and on my wife’s fast commuter Pedelec — and combining an Avid BB7 road (with 160mm BB5 rotors front and rear) with a Shimano ST-6700 or SRAM Force brake/shift lever, the brake works like you want it to do: much power (absolutely on par with a high performance road caliper), very easy to modulate (even better than any road caliper I know), and nearly unaffected by weather conditions. The only negative is some brake squeal that might come up from time-to-time when using metallic pads.

I also do not agree with your assumption that the brake load is higher on a road bike because of its higher speeds. Both types of bikes, modern road bikes with rim calipers and mountain bikes with hydraulic (or mechanical) disc brakes, can easily create more brake force than you are able to get down on the ground. If you apply the brake hard enough, the front tire will either lose traction or you will endo. But when considering the problems of heat dissipation under sustained braking, it’s obvious that the demands on a mountain bike are much higher. You know as much as I do that when riding your mountain bike there are many descents where you have to apply the brakes (nearly) all the time. You can open the brakes for only very short periods of time before you have to pull the lever again. Do you agree that this situation is rather rare on a road bike, at least for cyclists with average descending skills? Working as a cycling trip guide I regularly ride several weeks per year in the Alps or Pyrénées and I know less than a handful of descents all over France or my German Black Forest home region where I have to brake for the majority of the time it takes me to get down.

It’s obvious that you can thermally overload and finally destroy any brake system on any vehicle if you want to or if you don’t know how to descend. People from the Netherlands are famous for burning down their cars’ brakes in the Alps during their summer holiday trips even when driving modern high performance and high quality cars. Because they are scared and don’t know how to use the braking forces of their engines descending in their highest gear. And on group rides or events I have personally witnessed numerous flats because of overheated inner tubes on road bikes as well as on mountain bikes back in the days when most of them still had rim brakes and no tubeless tires.

So it’s obvious that as a rider you have to find a brake system with which the ability to dissipate heat is adapted to your weight, descending skills and terrain you’re riding in. I think for the majority of cyclists, mechanical disc brakes offer the best (say safest) behavior when being overloaded. And there will always be riders who will overload their brakes. Mechanical disc brakes don’t have a fluid which at some point “suddenly” boils and lets you pull the lever to the grip with no more braking action. And they will also not overheat inner tubes, which suddenly burst. If you ignore all signs of thermal overload on your mechanical disc brake (fading brake force, smell, change of noise, glowing rotor) then you will finally destroy the rotor, the pads and eventually the caliper but you will still be able to stop your bike. Then you can either learn how to descend, stop in time next time and give your brakes a break, or opt for bigger rotors. But you will not crash when your brake is suddenly giving up or one of your tires is blown off the rim because of a bursting inner tube.

That’s why, for the majority of enthusiasts, I favor mechanical disc brakes on road bikes. They are easy to maintain and adjust and don’t require special skills and tools as hydraulic brakes do. It’s true and undisputed that they will increase the bike’s weight a little bit and require the fork to be stiffer than some of us want it to be, but they reliably do away with some of the biggest safety problems bikes with rim brakes have: dangerously poor braking performance on wet roads, bursting over-heated inner tubes and rim failures because of worn brake tracks.
— Horst

Dear Horst,
It is true that I did not mention some of the great advantages of road discs and perhaps should have. One is certainly that the rim and tire and tube don’t heat up from braking. Joseba Beloki should have been so lucky. Another is certainly braking in the wet. Yet another (albeit less of a big deal than a few years ago), is braking performance in all conditions with carbon rims, and so is eliminating the concern of softened rim walls with carbon clinchers. Yes, if someone does not pay attention to the wear indicators on aluminum brake tracks, it prevents blown-out sidewalls on them, too.

I also regret saying that braking demands are higher on road bikes due to the higher speeds. Yes, there are situations on mountain bikes that are at least as stressful on the brakes as ripping down a mountainside at 100 kph and slamming on the brakes for a sharp switchback, and then doing it repeatedly through many more of them. On the other hand, the bigger caliper, rotor and lever on the mountain bike all have the capacity to shed heat far better than tiny road ones do.

There is another issue I discovered at Eurobike, however, namely that handmade tubulars and open tubulars that have hand-glued tread are not made to deal with the amount of heat that some manufacturers are concerned might be produced by road disc brakes. I still am not sure what they mean, since you’re either skidding or not, but there’s another concern.
— Lennard

Dear Lennard,
I like the article on the road discs. I agree with most of what you said, but hydraulic lines/hoses can expand and therefore longer lines can feel more mushy than short ones. There are steel braided hydraulic lines that eliminate this hose expansion, but they are… heavier.

I don’t think road bikes need more powerful braking than mountain bikes (especially a DH bike), but I do agree that 140mm rotors, on a long mountain descent, would probably overheat fairly easily on a road bike. Hydraulic discs for road bikes sounds great on paper, but they have lots of hurdles to overcome for a lightweight road bike. I would guess you would need to add close to a pound for a properly working system (which isn’t terrible).
— Dave

FILED UNDER: Bikes and Tech / Technical FAQ TAGS:

Lennard Zinn

Lennard Zinn

Our longtime technical writer joined VeloNews in 1987. He is also a framebuilder, a former U.S. National Team rider, and author of many bicycle books, including Zinn and the Art of Mountain Bike Maintenance and Zinn and the Art of Road Bike Maintenance, as well as Zinn and the Art of Triathlon Bikes and Zinn's Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a Bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to Ask LZ.

Stay Up to Date on Everything Cycling

Subscribe to the FREE VeloNews weekly newsletter