recumbentpanda wrote:I am interested by two of Brucey’s points and would like to know more:
1. Under what conditions do disc brakes fail completely and without warning? (I really want to know this!)
2. Conventional wisdom among mountain bikers and others has always seemed to be that discs are more consistent in the wet not less. Am interested that you should have had such a different experience.
re 1)
a) Hydro discs rely upon some fairly flimsy rubber seals to retain the fluid. There is only a few microns of rubber keeping the fluid in, and (unlike most motorcycle or car disc brakes) there is no secondary shielding to keep crud out of the caliper piston bores. Dirt ingress and corrosion of caliper bores is not at all uncommon. There is also little reinforcement in the brake hoses that are used on bicycle brakes. The result is that bicycle hydro disc brakes are somewhat vulnerable to damage and quite likely to spring a leak. Since the brake fluid used is a pretty good lubricant, the brakes basically stop working if the pads and disc become contaminated. If the leak is bad enough the brake doesn't even try to work, because there won't be any pressure in the system.
b) brake pads are usually about 4mm total thickness, being 1.6 to 1.8mm backing and 2.2 to 2.4mm friction material. Once the pad is worn, the remaining friction material can part company with the backing in one go. With hydro discs, this can leave you with no brake until you have worked the brake lever a couple of times and allowed the pistons to come out further. In some caliper designs (esp those without pad retaining pins), an offset disc can allow a whole worn pad to escape through the caliper slot.
c) it is very uncommon but it is possible for caliper bodies (which are cast not forged ) to just snap in half
d) discs can wear very quickly when some pad/disc types are used. Many modern 'Road bike' disc designs get powerful braking out of small disc diameters by using a highly abrasive pad compound. In some cases two sets of pads and the disc is worn below the wear limit. If the disc fails suddenly (quite likely with worn sandwich discs) then 'no brakes'....
e) bicycle disc brakes are simply not built to tolerate prolonged heat soak. This means on mechanical disc brakes plastic parts (eg the adjuster knobs on BB5/BB7 can go all Salvadore Dali on you, and in hydro discs the fluid can boil spontaneously, especially if it is contaminated with water. Because of the dynamics of heat soak this can happen when you are off the brakes; the result is that in one corner you have brakes, but for the next one, you don't. [Very probably this is what happened to the Italian fellow a few days ago.] This is most likely to happen if you descend fairly slowly on a very big hill. But such problems can occur on a descent with a drop of just 1000' or so, which is not so much 'Alps' as 'Home Counties'. Unless you have seen this happen with your own eyes you may not believe it to be possible, but it happened to a cycling journalist (who previously had sung the praises of disc brakes) a few years ago, and he was lucky enough to be able to write a more measured appraisal of the brakes from his hospital bed.
2) regarding MTB brakes. I started using hydro discs on MTBs, where some aspects of their performance were quite welcome. However it turns out that there is a fundamental difference between MTB use and road use; in the former case most of the braking is
planned braking but on the road the really important stuff that might save your life
isn't. In planned braking you can give the brake a little squeeze ahead of time and make sure that the pads and disc are clean and the brake will work properly, as you expect, in a few second's time when you really need it. In road use, you can't always do this, with the result that you only have whatever brakes you have in an emergency. If disc brakes are wet, there is always some lag. Since even when it is raining you can't ever be 100% sure that the brakes are wet or dry, you can't afford to go 'full gas' on the brakes because if they happen to be dry, you will be over the bars within half a second. If you only apply the brakes initially so that they don't throw you over the bars should they be dry, when they are in fact wet, they won't work at all for half a second or so until you either realise you are not slowing down and modulate the brakes, or you dry the discs off. Similar issues exist in many other braking systems which is why ABS is used; you can slam on brakes (brakes which might be too powerful and will easily lock the wheels on a wet road, or alternatively be wet through and hardly working at all) and the system will sort itself out. No such device exists for a bicycle.
Proponents of 'powerful brakes are always better' appear not to understand that your feedback loop for
controlling that power is at least half a second long. If you don't want to risk going over the bars, the first half a second of braking is at some lower power, and then you can bring the brake power up. Well that is too long on a bike; you should be aiming to stop dead in a little over one second from normal speeds. This means that a really good set of bicycle brakes is perhaps one that won't quite chuck you over the handlebars no matter what you do, and that is perfectly consistent on first application. Even if such brakes are only 80% the peak power of the most powerful brakes, you will emergency stop consistently quicker with them because they can be 'full on' from the start and you won't have to try and modulate the power between 'not enough' and 'too much'.
Very powerful brakes merely create the illusion of better emergency braking; the reality is that without appropriate control, they can cause more accidents than they prevent, and that when some dozy bugger steps off the pavement in front of you, you might well pull up more quickly using supposedly 'inferior' brakes.
Disc brakes have some good things going for them, sure. But they also have shortcomings too; every brake type has. The 'discs are always best' argument has more holes in it than, er, the average brake disc does.....
cheers
