The Great Disc Brake Swindle?

Hmmm…things just started getting interesting. A recent post on the Cyclist magazine website may just have cracked open Pandora’s box and let the genie out of the bottle (bar-keeper, mix me another metaphor, this one’s gone flat!). Because (whisper it) disc brakes on road bikes may not be all that.

Here are some quotes from Cyclist’s review of the Trek Domane 6.9:

“The Domane 6.9 Disc is the first disc braked bike I’ve ridden with zero brake rub, even when making exaggerated movements to try to induce it. On this point, the thru-axles are a significant benefit, banishing the constant ‘zinging’ of the disc rotors against the pads I’ve experienced on so many other disc bikes.

It’s a matter of opinion, but I think the dropouts and axle arrangement, especially at the front, look ugly and over-built, certainly not befitting of an otherwise stealthy carbon road bike.

In the wet it required an enormous amount of care not to lock a wheel. Some might say powerful, reliable brakes are a great feature for safety, but I would say you can have too much of a good thing. Dialling down the power, perhaps in this case by fitting a smaller 140mm rotor (which would also look considerably neater) would allow a greater opportunity to tap into the modulation and the ‘feel’ that hydraulic disc brakes can undoubtedly provide. With the set-up as it was on this bike I was wary of pulling on the levers hard enough to feel the modulation, for fear of locking and skidding the wheels, even in the dry.”

OMG! Heretic! Blasphemer! Burn him!

So, to be clear, Stu Bowers (an experienced road-tester of bikes) has experienced brake-rub on every other disc-braked bike he has tested. And the brakes on this bike were so fearsome that he feared wheel lock-up even in the dry.

Oh dear…the industry is not going to like this. What about the fabled modulation improvements? What about the performance and safety gains? What about the Emperor’s news clothes? It’s actually a fairly shocking admission when you think about it. Here’s a £6000 bike that could have you chewing tarmac on your first ride. One that could scare the living crap out of you every time it rains. Is it so over-braked as to not be fit for purpose? I haven’t ridden it, so I can’t tell you. But Bowers has, in addition to loads of other bikes, so you have to assume he’s not incompetent (the accusation normally leveled at people who have braking issues).

Interestingly, this is the subject of a really excellent article in Cycle, the magazine that goes out to all CTC members. In the October/November issue is an piece by Richard Hallett, and it’s probably the most interesting and well argued article about disc brakes on bikes that I’ve read. In it he points out the things that no other mags dare, namely that disc brakes may not be suitable for road use.

This is something that has been bugging me for a while…we’re being sold a pup. The industry wants us to buy new stuff, and to buy new stuff that we can’t easily maintain ourselves. If the bike shops aren’t going to make much profit on sales (thanks to the interweb), at least they can make some money on replacing internal cables, bleeding brakes, replacing disc pads, etc.

Hallett, too, has experienced brake rub on many disc-braked bikes he’s ridden. In an industry obsessed with marginal gains, disc brakes seem to offer more marginal losses than gains. The tiny clearances necessary mean that brake rub is almost inevitable, leading to increased pad wear, especially when road grit gets onto the discs.

Heavy use, and tiny components (relative to a motorcycle or car disc brake setup), means the disc and caliper get very hot, even on relatively short descents. Hydraulic brake fluid is hygroscopic (it absorbs water), so when the fluid gets really hot the water molecules within it turn to vapour, the lever comes right back to the handlebar, and the brakes become spongy and fail. Sometimes the bonding agents in the brake pads overheat and release gasses that prevent proper pad-to-disc contact. This results in a very wooden feeling at the lever and greatly reduced braking capacity.

Bowers criticises the Domane 6.9 for looking over-engineered, but the fact is that for safety reasons it needs to be. To cope with the increased braking forces being nearer the end of the fork leg, and to one side, it’s necessary to build a stronger, tangentially-laced wheel and much beefier forks. That adds even more weight, on top of the already weightier disc and caliper. It’s physics, innit.

Unfortunately the cycling press doesn’t want to address any of this. Except for Cycle, which is funded primarily by CTC subs rather than advertising from the bike manufacturers. By all means embrace disc brakes, but how much of the gains from your aero bike, helmet and clothing are being negated by dragging your brakes the whole time? And how much of the savings you’re making on lack of rim wear are being devoured by increased pad wear and replacement costs? I don’t know.

But what I do know if that your favourite cycling magazine is unlikely to tell you any time soon.