Disc brake - how...???

Nowadays they do, they sell their services commercially.

Most of the retraction is from the seals, there's a reasonable clearance betwixt brake piston & cylinder

The message from "Mike G" contains these words:

Think again...

measureable

Thank you for confirming my original supposition. Particularly the point about the pads not needing to move away from the disc. I arrived at my conclusion by logic, as I am not familiar enough with the laws of physics, except in a practical way, to explain it in a scientific way. I'm just an uneducated bum basically.:-) Mike.

The message from snipped-for-privacy@aol.comNoEmails (Dave Baker) contains these words:

Sorry, Dave, but you can't reduce the applied force without some movement.

Of course you're both right. Dave is right in the practical sense - brake pads aren't that springy, so there isn't much movement involved. Guy is right in the real sense - there will definitely be some movement, it may just be too small to easily measure.

cheers, clive

I feel I must point out that only the most expensive calipers can generate an infinite force :-) But I have seen quite cheap pads that can be made to generate close to zero friction :-(

Steve

The message from "sro" contains these words:

I managed that in a rather hurried bit of driving in the Montego estate the other day. Repeated heavy braking round country lanes when heavily loaded dunnarf make the brakes smell. Any more of it and my pants would've started to smell as well.

Since no real object is infinitely incompressible I agree. The direction of that movement might not be such as to take the pads off the disks though. I suspect the greatest amount of flex is in the caliper itself. As you apply pedal pressure the caliper loses some of the intended movement by springing out. As you release the pedal it will spring back in and push the pads toward the disk. Everything relies on their being no residual pressure in the brake lines to cause a force holding the pads onto the disks and hopefully within a revolution or so the pads will have reached an equilibrium position where they just touch or are maybe just a micron clear of touching and all the sprung or compressed components have reverted to their load free state.

IME when you jack up a car wheel and spin it by hand you usually get a tiny amount of drag from the pads. You can hear it and feel it. I therefore conclude that most braking systems have a tiny amount of residual contact force from either line pressure or spring in the piston seals, stiction of various components etc. In practice it makes not a jot of difference because the forces acting are too small to cause retardation or wear.

I think the bottom line is that it is not necessary for the pads to be pulled back from the disks, I doubt any part of the system (piston seals or otherwise) are designed to do this or actually do it, and it makes no difference as long as there is no contact pressure to create friction. The premise that piston seals achieve this by design or accident appears to me to be unsound.

Short of taking the issue to a braking system design engineer for an opinion I think we've covered this pretty thoroughly now.

The message from snipped-for-privacy@aol.comNoEmails (Dave Baker) contains these words:

Fair enough - but next time you have a copy of Hillier's Motor Vehicle Technology to hand have a look and you'll see what I mean.

IMHO, any flexible seal between a piston and bore will result in some setback of the piston when the pressure applied to that piston is relaxed. I've also read this explanation somewhere - of course that's not to say it's correct.

Of course, run out on the disc - and there will be *some* - or clearance on the wheel bearing - will tend to push the pads back a little too.