The Disc Brake Naughty Corner Thread

[Brucey]

One observation on modern hydraulic disk brakes on bicycles: if you don't mess with them they don't normally give any trouble for the lifetime of the product. Its only when people attempt to change the fluid that problems begin.

As I see it there is so little fluid in there to begin with that they have to be set up at the factory under very clean conditions with all air removed from the fluid that your normal garage DIY enthusiast cant hope to replicate. changing the pads is all one should ever do.

except that any system will -sooner or later- become contaminated with water, this is inevitable. If it is a system running DOT fluid and you don't change it regularly, the system will fail by internal corrosion, and may well boil up in service before then. Water gets past the seals into mineral oil systems too, although I think it doesn't happen as quickly.

Bleeding hydraulic systems is not that difficult to do, it just takes a methodical approach and a reasonable level of skill. But it isn't really on by the roadside, and it isn't cost-effective on a £50 brake at current workshop labour rates, either.

The outcome of this is that your typical inexpensive hydraulic brake is effectively bin-fodder as soon as it starts to misbehave, or when it is a few years old and needs the fluid changed, unless you can fix/maintain it yourself.

Cable operated brakes are a lot more practical to deal with when they need work (which might be more often), and generally don't fail without any warning at all; things like frayed cables and loose bolts are things that ought to be found during routine inspections. By contrast a small percentage of (even quite new) hydraulic brakes will fail to a leak with no warning; factories that assemble these things are not quite as clean and as efficient as you might like them to be.

FWIW I use hydraulic discs on an MTB (and I really like the way they modulate most of the time) but I wouldn't have them on a road bike.

cheers

[jb]

I'm not sure modern bicycle brakes have the potential for leakage or water ingress, with such small areas and tiny movements I doubt they would give any more trouble than a thermostat or clutch cylinder. I would admit though that the perceived manufactures life time of the component may be some what less than your idea of a lifetimes worth of use.

[Vorpal]

Any hydraulic brakes have the potential for leaks or water ingress. No seal is perfect, and the life is limited. I'm not sure that the likelihood is as high as Brucey seems to think.

[Brucey]

in the case of brakes with DOT fluid, even if the system is 'sealed' water from the atmosphere will diffuse into the system and contaminate it, whether you use the bike or not.

There is always water hanging around the seals in bikes that are used, and the system sees negative pressure every time the pistons retract. Brake piston seals only seal properly when they are under positive pressure; that is how they are designed to work. The rest of the time they seal rather badly and can let all kinds of rubbish past.

Seals wear, too, and if you don't change the fluid the debris compromises the system integrity.

Trust me, brake fluids of all types do get contaminated in service, and should be renewed at regular intervals.

Ignore this fact at your peril.

cheers

edited for typo

[reohn2]

I often wonder that if my first disc brake experience hadn't been with BB7's,whether my disc brake outlook wouldn't have been as positive .
A friend who has the exact same brakes on the exact same C/dale tandem as ours maintains that "the more you abuse these brakes the better they seem to be!" he's not wrong.
Other than a clean of the rotor and calliper now and again and a couple of clicks of the adjuster knobs once in a while,they just work and work very well indeed,in all weathers.
Of course the pads need changing once in a.....let me see now... ....that's it! Once in a Blue moon!
I Avidly(sorry) recommend them

[Brucey]

I often wonder that if my first disc brake experience hadn't been with BB7's,whether my disc brake outlook wouldn't have been as positive .

I am sure you are right. There are very many brakes out there which don't work so well and are a PITA to live with by comparison. Even so, BB7s have their own list of known faults, several of which are likely to catch you out if you use these brakes for (say) winter commuting.

Of course the pads need changing once in a.....let me see now... ....that's it! Once in a Blue moon!...

I think this comes with the caveat if you use sintered pads. Most other pad types wear faster than that, often many times faster. However it isn't a good idea to fit sintered pads in some systems at all, and even where this is possible, not everyone is happy with the noise they make (which varies BTW).

If you can't or won't use sintered pads, the next best choice (for longevity) often seems to be what are described as 'semi-metallic' compounds. However this description covers a multitude of possibilities, so 'YMMV' applies here.

cheers

[andrewjoseph]

Thanks for the prompt replies, guys: I guess you don't agree with John Allen:

'•A front disc brake stresses one blade of the front fork very heavily, requiring a stronger, heavier fork, resulting in a bumpier ride with a non-suspension fork, and if a fork isn't quite rigid enough, producing 'brake steer".
•A front disc brake caliper behind the fork blade generates a powerful force tending to loosen a quick release and pull the wheel out of the fork. Special hub and fork designs are needed to surmount this problem.
•Disc brakes are generally heavier than rim brakes.
•Disc brakes are more complicated, expensive and difficult to maintain than rim brakes or drum brakes.
•Some disc brakes are grabby. This problem is likely if dirt gets trapped between the calipers.
•Disc brakes require special fittings on the frame and fork, and special hubs.
•Disc brakes can interfere with baggage racks and fender stays.
•The disc is vulnerable and easily bent. Other hub brakes do not have this weakness.
•Hub flange spacing is often reduced, resulting in a weaker wheel.
•Adjustment may be difficult to avoid calipers' rubbing on the rotor when the brake is not in use.
•The disc gets extremely hot and can cause injury if touched, and melt nearby plastic or cloth items. Care must be taken to use the correct brake fluid with a hydraulically-actuated disc brake, so it does not boil, resulting in loss of braking'

I should stress that I've never ridden a bike with disc brakes and am simply weighing up the pros and cons

many of these are easily surmountable. e.g. mount the calliper on front of fork (as i had justin burls do on our tourers), then braking forces pull the axle into the fork.

I've been riding disks for over 15 years now and never bent one. whereas the boardman road bike with rim brakes needed the callipers centring before and after, and sometimes during, every ride.

[andrewjoseph]

One observation on modern hydraulic disk brakes on bicycles: if you don't mess with them they don't normally give any trouble for the lifetime of the product. Its only when people attempt to change the fluid that problems begin.

As I see it there is so little fluid in there to begin with that they have to be set up at the factory under very clean conditions with all air removed from the fluid that your normal garage DIY enthusiast cant hope to replicate. changing the pads is all one should ever do.

i have to disagree with this. i've stripped our mtb hydraulic callipers, replaced pistons and hose, olives, etc. in the kitchen with basic tools like a spanner, a jam jar and aquarium air hose.

[Brucey]

many of these are easily surmountable. e.g. mount the calliper on front of fork (as i had justin burls do on our tourers), then braking forces pull the axle into the fork.

I think JA's point is that, at present, these things are not usually done. Its all very well saying 'you can do this and that if you buy this one special thing at vast cost' but if, as a normal punter, you wander into an ordinary bike shop and look at the ordinary disc braked bikes therein, you don't need to be a towering genius to work out that most of what you are looking at is at the least 'deeply flawed' if not actually a pile of festering ordure....

I've been riding disks for over 15 years now and never bent one.

you've been careful, and/or lucky then. It happens all the time....

... whereas the boardman road bike with rim brakes needed the callipers centring before and after, and sometimes during, every ride.

crap wheels, crap brakes, perchance? ...

cheers

[andrewjoseph]

an ordinary punter is unlikely to notice, i agree. but the industry is working on these issues.

i've had rotors warp quite often, but not bend.

they were indeed crap brakes, but i've noticed the same on all of the road bikes i've looked at/worked on.

[Stevek76]

In the 5 or so years I've commuted/utilitied on discs I've had to true the front rotor perhaps twice and while storage at work is fairly civilised it does also get regularly locked up in the wilds of crowded outdoor racks in town etc.

While I'm sure the super lightweight skeletal ones might be easier to bend a standard rotor is fairly tough, there are plenty of other bits on a normal bike that are going to get damaged first if a bike is taking that much punishment.

As for hydraulics, water has an ability to get everywhere! fluid changes are generally recommended. The impacts do vary depending on the fluid, mineral oil systems (shimano etc) don't naturally absorb water but any that does get in will typically end up in the calliper effectively reducing the boiling point to 100c and can cause corrosion. Dot 4/5.1 fluid (avid/sram etc) is hygroscopic and will absorb water over time gradually reducing the boiling temperature but there's little risk of old fluid causing corrosion, it will just fade at a lower temp.

Bleed kits are hardly expensive though and in the grand list of bike maintenance tasks a bleed or fluid change is less effort than many.

[mig]

what scope is there, if any, to improve the design of a disc brake both in terms of usage and safety?

[Mark1978]

what scope is there, if any, to improve the design of a disc brake both in terms of usage and safety?

I'm sure some sort of cover might be possible, although it's going to look ugly. Problem is we're already decades down the line with their design so retrofitting something and having it standard won't be easy. All for the sake of a few pro riders who have the alternative of rim brakes.

[Dave W]

...Some of those points are out of date. Read about the Cannondale CAAD 12 disc - lighter than it's rim braked sister.....

"it's sister..." yeah, its ugly sister, deliberately made heavier because they want to shift bikes with disc brakes....

Please don't confuse marketing with engineering....

cheers

Of course you are always right. It hadn't occurred to me they'd make a brand new rim braked model too just to make the disc braked model look good! There was me thinking aome people might actually want a rim braked model too. How silly of me and how silly of Cannondale to make it in the first place. What were they thinking? Nobody could be stupid enough to want rim brakes could they?

[Dave W]

...Some of those points are out of date. Read about the Cannondale CAAD 12 disc - lighter than it's rim braked sister.....

"it's sister..." yeah, its ugly sister, deliberately made heavier because they want to shift bikes with disc brakes....

Please don't confuse marketing with engineering....

cheers

Of course you are always right. It hadn't occurred to me they'd make a brand new rim braked model too just to make the disc braked model look good! There was me thinking aome people might actually want a rim braked model too. How silly of me and how silly of Cannondale to make it in the first place. What were they thinking? Nobody could be stupid enough to want rim brakes could they?