Wednesday, 7 March 2012

The big debate

So it all started innocuously enough - the most poorly-guarded tech secret of the year came out with this post on . So after the cross revolution caused by discs, the industry was keep to get early-adopters on them on the road as well. Whilst superficially similar in appearance, the differences between a cross bike and a road bike go well beyond some extra rubber on the tyres, and whilst i can see the sense, to some degree, in disc brakes for cross, the idea of using them on the road had me scratching my head.

Being a physicist by training, i don't just scratch my head, i like to start drawing pictures and thinking about forces. This is where the putative "disc brake revolution" really falls down for me. So let's look at why...

Firstly, a bit of history. Disc brakes appeared on the mountain bike scene in the mid-90s, it's 20 years since Hope released their first disc brake for mountain biking, and it's fair to say that almost no mtb, whether intended for XC, Downhill, Enduro, Freeride, Gnar-Core etc etc comes with the previous incarnations of technology, the V-brake or the cantilever. So how did they come into the sport, and why are they so universal? The early-adopters, as is so often the case, were the DH crowd - they were tired of struggling with stiff v-brake levers that were unable to apply sufficient force to the rim of the wheel to slow them down, in spite of using wide tyres with well-designed treads. The difficulty of applying sufficient force compounded the all-too-common issue of "arm pump" where the muscles in the forearm become painful from pushing on the muscle sheath, and would cause a further reduction in control. In short, the technical sport of downhill was becoming more about who could cope with the arm-pain!

When disc brake technology crossed over from motorbikes through motorcross, at much the same time as oil damped (as opposed to elastomer-damped) forks, downhill riders were keen to adopt it. They could now leave braking later, relying on applying less force at the lever to generate the same braking force at the tyre, and had the added advantage that disc brakes were away from the wheel rims, meaning that in foul conditions they didn't suffer from the same fade and unpredictability that bedevilled v-brakes. Within a few years, discs were everywhere, the extra weight they added to bikes (they were significantly heavier than v-brakes at this time) more than compensated by the improved performance they allowed. With the improved performance of the brakes, the limiting factor became the tyres riders were using, the narrow 2" rubber of the 90s was too narrow to cope with the extra braking forces, and it was soon the case that DH racers were up-sizing to 2.5" tyres that could once again handle the job.

It took a little longer for XC riders, and especially racers to embrace the new technology, which initially required significant extra weight, not just for the levers, hoses and calipers, but also because frames had to be built much more robustly around the anchoring points for the brakes (a simple bit of physics, rim brakes apply their force approximately 30cm from the centre of the hub, whereas disc brakes are around 8cm from the axle, and so apply ~4 times the force of rim brakes to the frame where they are anchored). When i started mtbing again as an adult (back in the heady days of 2003), discs were pretty ubiquitous even on
lower-end models. There were still design issues however, as people discovered that the shape of fork dropouts and placement of the caliper were vital to keeping the wheel in the fork when braking hard - there were a couple of high-profile accidents in 2003 in fact that highlighted this problem, and made us as a community realise that even the mature technology of disc brakes required extra development and thought to become as trusted as v-brakes had been. As they became more universal, people discovered that there were potential problems that had not been noted on the race course, where by definition, descents tended to be short, and racers extremely competent. Particular problems seemed to arise from your average rider descending for long periods of time in mountainous areas - the heat build-up in brakes that are in constant use for a long time was too great, and was causing the fluid to boil

You might very well ask, what was the point of that wander down history lane? Well, given that there is all this history of the development of discs for offroad use, we might viably ask, what can we learn? Well, there are a number of points that are relevant, where the brakes have superceded previous technologies, what changes in technology they have elicited, and what their limitations are. I think factually, it's pretty uncontroversial to say that over the course of an XC race on dry tracks, it makes little difference if you're riding discs or not. Hydraulic disc brakes come into their own where the rider requires:
(1) repeated or continual hard braking (better tranfer of force from lever to point of application - note this mechanical advantage is lessened by using cable discs) - this lessens rider arm fatigue.
(2) resilience to foul conditions - the hydraulic lines are unaffected by mud (not so cable discs, which by their nature have many problems in common with vees), all discs are further from the tyre and not as likely to be coated with mud and sludge, and allow better frame clearances (although these are often limited by other factors anyway!).

Like anything though, they have their limitations. These are particularly
(1) They are less good at dissipating heat than rim brakes - the brake rotors are generally made of steel, which conducts heat better and has a higher specific heat than aluminium rims, but there is also many times less thermal mass in a rotor than in a rim. If you have problems with aluminium or carbon rims overheating on descents, then discs are not the answer, especially given that based on the design of the system, overheating can result in two possible outcomes - either the brakes clamp on (closed hyrdaulics) or the fluid overheats and they cease to work (open system). Either is pretty bad!
(2) The braking force you can apply is limited by the width of the tyre - if you're able to apply more force you're more likely to run out of traction, and given that disc brakes work better in the wet than v-brakes, where tyre traction is lower, this is a particular worry scenario.
(3) They apply significantly more force to the frame requiring much stronger frame designs, which means extra weight in the short term, whilst designers get their finite-element work done in the longer term. There is also the potential for front wheels "popping out" under hard braking with fork designs are based around much weaker caliper brakes.

Bearing these plusses and minuses in mind, it seems hard to justify the need for disc brakes on road bikes - they're just not necessary, and in the wrong situations could even be detrimental to rider safety. The plusses do not address problems that are common amongst road riders and racers, who rarely have to cope with repeated hard braking(and live to tell the tale) or muddy tracks, and the minuses seem to be too great for the same people. The only conclusion i can come to is that this is a way to sell us, the consumers, something we don't want or need. I am prepared to change my opinions in the face of well-reasoned, physical arguments, but i have so far heard nothing to make me deviate from my opinion above. It will be interesting to watch the adoption or not of disc brakes on the road...

Next week, i'll piss off the 29er consortium - stay tuned :).

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