How to unseize a disc caliper piston?

[Cubist]

[QUOTE 3747988, member: 9609"]you're in ambiguous mood today - No to my question or No it doesn't slide[/QUOTE]
The majority of modern hydraulic disc brakes have two opposing pistons, both of which move in order to put the pads on the rotor. Some have four pots, two on either side. These tend to be more downhill oriented. Hope had some four pot versions with elongated pads. They were a royal pain to set up as the caliper had to be perfectly aligned.

Unlike the cable or BB operated mechanical brakes which have a fixed side and one moving piston, current hydraulic brakes don't need to be adjusted as the pads wear, and caliper/reservoir design means they stay at an optimum distance from the rotor once they have been bedded in. The hatred for Elixir and many other Avid hydraulic designs, stems from the fact that they are very difficult to setup without rubbing, singing or simply losing power. They had a complicated system of conical washers on the mounting boltsto align them correctly, and these were not very home mechanic friendly. They are difficult to bleed .

Modern Shimano versions, Deore, SLX, XT and XTR are easier to bleed but most importantly employ a servo-wave lever, which means that the pads can sit further away from the rotor, virtually eliminating unwanted rub, even with a fair bit of rotor runout. The first touch of the lever on a well set up system brings the pads close in, then the rest of the lever travel modulates very nicely.

 

[Profpointy]

No, the caliper does not slide. It is static. The single piston from one side contacts the disc and flexes it over to the waiting static pad on the other side. The static pad does have some adjustment by way of a dummy piston that sits behind it and can be adjusted with a screw mechanism so that it sits closer or further away from the disc.

Not saying you're wrong, but with single piston car disc callipers, the calliper body used to slide on a couple of pins: "floating calliper" was the term I recall. They were a nuisance as they tended not to slide /float and then pressed the disc only on one side or rubbed. Hey, even my old Cortina had a piston each side and that worked fine without hassle. A single piston rigidly fixed calliper sounds like a bad idea to me, but if that's how they are...

 

[Yellow Saddle]

Not saying you're wrong, but with single piston car disc callipers, the calliper body used to slide on a couple of pins: "floating calliper" was the term I recall. They were a nuisance as they tended not to slide /float and then pressed the disc only on one side or rubbed. Hey, even my old Cortina had a piston each side and that worked fine without hassle. A single piston rigidly fixed calliper sounds like a bad idea to me, but if that's how they are...

That's how they are. The discs are so flexible that it requires less engineering to make the disc move than the caliper. But, as you say, it is a bad idea and they were noisy and required constant adjustment. I can't even call it a floating disc design because the disc doesn't float, it flexes.

They were all cable actuated and frequently found on tandem rear wheels.

 

[Profpointy]

That's how they are. The discs are so flexible that it requires less engineering to make the disc move than the caliper. But, as you say, it is a bad idea and they were noisy and required constant adjustment. I can't even call it a floating disc design because the disc doesn't float, it flexes.

They were all cable actuated and frequently found on tandem rear wheels.

well I'll not be buying one of them then! Are all cable operated disc systems of this type or are some double-side (if that be the term)? My continually evolving dream bike * has discs, but I might revert to rim brakes after all.


* dream bike meaning exists only in my dreams

 

[Yellow Saddle]

well I'll not be buying one of them then! Are all cable operated disc systems of this type or are some double-side (if that be the term)? My continually evolving dream bike * has discs, but I might revert to rim brakes after all.


* dream bike meaning exists only in my dreams

Some are double-sided, but no cable-actuated disc brake is worth the effort. Hydraulic is just so much better, especially now that we've found the cure for Elixir. Is that a pun or something? A cure for a cure?

 

[Yellow Saddle]

In my experience they have one massive advantage, they are more tolerant of being left unused than hydraulic ones.

I'd like to hear why you say that but my view on cables left unused in a damp environment like the UK is that they quickly oxidise and become gritty when operating. The very high mechanical advantage (short travel in the cable) of cable discs means that this leads to poor modulation. I would concede that DOT fluid brakes could potentially degrade over time but the oil-based brakes seem to survive time quite well.