I suspect that I have a problem with the front disc brakes on our car. That is the brake pads are not 'clearing' the brake disc, they thus 'rub' and are heating up the wheel.
One thing I do not understand. When the hydraulic pressure is released from the caliper , what causes the pads to move away from the disc? On 'old' shoes type brakes there were springs that pulled the shoes away from the drum. What pulls the pads off the disc? John Hewitt Malaga Spain
The message from John Hewitt contains these words:
The seals are springy and drag on the piston. When you press the pedal the piston distorts the seal, pulling it from it's rectangular cross-section into a parallelogram shape. When you release the pressure, the seal springs back into shape pulling the piston back a tad.
Obviously, if there's more movement because of wear than the springyness of the seals can accomodate, the piston moves past the seal - but it does so grudginly and keeps the seal extended.
Just the springiness of the seal. If the whole lot gets gunged up, this isn't strong enough.
I think you will find that the disk is slightly warped and that is what moves the pistons back. If you had ever changed a 2cv brake pad where the caliper is so stiff that you need to force the pad back with enormous force and will bend a large screwdriver you would realise that it has nothing to do with seals. HTH Phil
Sounds f***ed to me. If I had calipers that needed that much force to retract, I'd fix them. But then I'd not buy a Citroen in the first place.
The message from snipped-for-privacy@yahoo.co.uk (Phil) contains these words:
That's 'cos you're pushing the fluid back through a very small hole with a very (compared to the master) large piston - which of course makes things harder.
Anyway - if what you say is true then once the wheel's stopped you'd never get it moving again 'cos it'd not be moving so it couldn't push the pads back to release the brakes. In reality, however hard you press the pedal, once you release it you can turn the wheel easily.
Have a look at
Vin.
Nope, not on any car I've ever owned, you're only requiring the seals to retract it a fraction of a mm so the small hole you have to push the fluid through is pretty irrelevant.
That would occur even if the seals did not retract the pistons. Which incidently I have my doubts about, as the seal is not distorted in the way suggested, only compressed. IOW it retains it's rectangular form, if somewhat slightly thinner under pressure. I don't believe that small compression is enough to move the pistons when the pressure is released.
Just my opinion based on experience with hydraulic pistons and seals in industry.I don't claim to 'know'.
The pads only act on the discs when pressure is applied. Release the pressure and friction between the two is lost. Therefore the pads do not need to move back from the disc to allow it to rotate freely.
Of course you are right in your explaination of why pistons are not easily pushed back Although not entirely as there is also the resistance of pushing a few feet (in length) of fluid through a small bore pipe. Even without the restriction caused by the master cyl it is still quite significant. Mike.
The message from "Mike G" contains these words:
How is the friction lost if the pads do not retract slightly?
And what on earth are you on about with "the seal is not distorted in the way suggested, only compressed."? The seal isn't behind the piston - it's round it and the piston slides through it.
Anyway, what books I've read say it's caused by springyness of the seals, and my own experience leads me to believe this is the case - and I really CBA to argue.
Think about it. Imagine holding two pads to a disc by one hand. Squeeze them onto the disc like a caliper, and turn the disc with the other hand. Resistance will be felt. Now relax your grip, still holding the pads in contact without any gripping pressure. The disc will now rotate freely, yet the pads are still in contact and haven't moved away from the disc.
I know it's not behind the piston. It sits in rectangular groove in the cyl bore. It doesn't have to be to be behind the piston to behave in the way I suggest.
Pressure reaches the seal by travelling between the cyl and the piston, acting on the entire face of the seal, pressing it against the side of the square groove, the wall of the piston, and the outside diameter of the groove. Three faces at rightangles to each other. Given these conditions, how can it assume a parallelogram shape?
Fair enough, but you did ask the questions. I'm not interested in arguing either, and didn't post with that intention. Mike.
The message from "Mike G" contains these words:
Oh yes they have! How else could any change be made? For there to be no travel involved you'd have a situation where force is applied and has an effect with no work done - 'cos work is proportional to force x distance travelled. If one of these terms is zero then no work has been done - and there's all sorts of laws of thermodynamics which forbid that. /Everything/ moves when a force is applied to it, and of course, it springs back when the force is removed - except of course if it doesn't
- but then it'd have to move forward again when the force is next applied and so on.
What holds you up when you stand on the floor? The floor flexing, of course.
The site suggests that 'wobble' of the disc may move the pads away from the disc. Assume that the pads are "just" touching the disc, shouldn't the wobble move them away, and not contunue to "caress" them. It seems in our case that the pads continued to rub, and rub, and.... Guess we must have corroded wheel cylinders.
Oh s***t
John Hewitt Malaga Spain
You're right You are obviously more familiar with physics than I am. I consider myself to be a good practical engineer, but academically an ignoramus.:-) Nevertheless I am genuinly interested in what you're saying. If you think I'm just trying to start an argument, ignore my remarks, and we'll leave it at that. I have no wish to annoy you, or anyone else come to that.
Correct me if I'm wrong by all means. You quote 'work is proportional to force times distance travelled'. As a simple practical engineer, I understand the theory, but in many cases the distance involved is immeasurable, and for all practical purposes can be discounted. Even with the sort of equipment they might have at the NPL. I very much doubt they could 'measure' the movement caused by the weight of a feather resting on a solid mass. Yet in theory movement must take place. So yes, in theory the pads do move, but not necessarily by any measureable amount. At least as far as can be measured using instruments found in the average w/s.
I still can't see the seals alone causing that movement. The very small movement the pads need to make to free the discs could be caused by a number of factors, play in the wheel bearings, vibration, stresses on the disc, pads, caliper itself, runout of the discs. No disc is going to run absolutely true. It could be close enough for all practical purposes, but it cannot be 'absolutely' true.
Maybe your book does say different, but personally I need a little more convincing. I've found that you can't always believe what you read in books. :-)
As I said. I'm not posting to annoy. They're just my opinions. Whether or not they make sense is for others to decide. Mike.
A combination of that and the seal distortion would give a very small running clearance. Sufficient not to be a problem. The force generated by the seal trying to revert to its static state, when applied to the relatively large piston, would be sufficient to push enough fluid back into the system to make clearance.
Which is why your car can fail the MOT for binding brakes or unequal braking performance. To be binding enough to heat up a wheel things must have reached a pretty bad state. Time to get them fixed and worry about how they work later.
Vin.
Just as an afterthought to my previous response...
If your brake calipers are of the single piston type ( I think most are now ) then if the brake is binding it could be the caliper slides sticking rather than the pistons themselves. I had this a couple of times on my old Carlton. Cheaper to fix than new calipers.
Vin
The message from "Mike G" contains these words:
Quite easily, apparently. Scanning Tunneling microscopes and atomic force microscopes can resolve amazingly small movements. For example, you can see a plane surface distort as you pull a carbon nanotube off the surface where it was stuck by Vandervaals force.
Lots of confusion about physics and forces in this thread. The answer is straightforward. Frictional force generated is a function of the coefficient of friction (mu) and the vertical component of the contact force. Regardless of the values of either if one is zero then no frictional force is generated. An infinite force clamping two objects with zero coefficient of friction will generate no friction. A zero force clamping two objects with infinite coefficient of friction will generate no friction. If there is no friction then there is no retardation or wear.
As soon as the brake pedal is released then the clamping force drops to zero barring any residual force generated by stiction, seals, blockages in brake hoses etc. The pad does not need to move away from the disk. All it needs to do is not be pressed against it by any force. Disk runout or piston seal distortion may well cause the pad to move an infinitesimal distance away from the disk but it is really immaterial. There is no longer any clamping force so there is no longer any braking force, friction force or wear taking place.
If a caliper slide is seized or there is a blockage in a brake hose then a clamping force may continue after the brake pedal has been released. In this case there will be friction and wear taking place continuously. This is not a normal mode of operation though. It's just a fault and can be corrected.
Slightly better than I can get with my trusty 6"/150mm rule then. :-)
Serously I find that amazing. I bet the NPL haven't that sort of equipment though, unless they've come into a lot of money recently. Possibly they hire their use from Co's rich enough to afford them. I remember years ago when they didn't have a set of quartz or ceramic calibration grade slip guages. They used to borrow a quartz set from a m/c tool Co in Walton-on-Thames. The only set I've ever seen. Mike.
More likely to be the pistons. If they're chrome plated, they may have rusted. But seals seem to swell over time too. Often a strip and clean with new seals and pistons as required will restore them - but you may be able to buy exchange ones as cheaply.
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