OT - But engineering related at least!

Posted here where I'm more confident of an engineering debate than car forums where there seems little understanding or the ability to express an argument in words.

I have a Ford Cmax and amongst the web forums for this car it is acknowledged that the back disc brakes wear more quickly than the front ones. This appears to be caused by the caliper not releasing fully and there is always a silght drag on the rear wheel when jacked up. The handbrake does release fully and the cables are slack at the caliper so not related to the wear.

This has started me thinking about disc brakes in general. How do they release to give clearance for the disc once hydraulic pressure is removed? There is no return spring as there is in a drum brake. I can only think that a small amount of disc runout is desirable to push the pad away from the majority of the disc surface.

I'm changing pads soon so I almost wonder if it is worth using some shim stock to induce slight run out (still within spec) as an experiment.

Can anyone perhaps shed some light on this please?

Bob

Reply to
Bob Minchin
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the square seals in the caliper cylinders twist slightly as the pistons move towards the disk. when the pressure is released, the seals tend to pull the piston back a thou or so.

This applies to proper Girling twin cylinder calipers. I'm buggered if I know how the lopsided abortions that they fit to most cars and modern bikes are supposed to work :-|

Mark Rand RTFM

Reply to
Mark Rand

Just a guess. Perhaps the caliper pistons are drawn back hydraulically by the master cylinder return spring.

Cliff.

Reply to
Cliff Coggin

You were quicker with your answer. :-)

Nick

Reply to
Nick Mueller

Full details here,

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But essentially, and to quote,

"Since there is no spring to pull the pads away from the disc, the pads always stay in light contact with the rotor (the rubber piston seal and any wobble in the rotor may actually pull the pads a small distance away from the rotor)"

MH

Reply to
max

As others have said, it's due to the square seals deforming, and then pulling the piston back in, once hydraulic pressure is released.

As for the wear issue, it's due to how modern braking systems are set-up. Very few modern vehicles have load compensating valves, so instead, the rear brakes always see full hydraulic pressure, and rely on EBD (Electronic Brakeforce Distribution) to prevent the rear wheels from locking when the vehicles unladen. EBD is essentially just a marketing term for the fact the ABS is being relied on to stop the back end from locking up.

The usual cause for rear brake pad wear, are the pads stuck in the calipers. With the caliper off, you should be able to move the pads in the calipers with your hands. If you can't, then they're needing stripped out, the rust/dirt cleaned off, and put back in.

The suggestion of using shim stock won't work. If you use it to induce run-out in the disk, you'll get a vibration when braking, and also increasing the brake drag/binding. Very slight scuffing is normal with any disc brake, but there should be no noticeable drag with the wheel on.

Reply to
moray

And don't put grease under the rubber cover. It requires some special "grease".

Nick

Reply to
Nick Mueller

Silicone brake grease IIRC.

Peter

Reply to
Peter Neill

There has to be some light rubbing to keep the disc clean and free of surface water etc, that goes with the design, as far back as the early Dunlop ones for the Jaguar XK series.

Modern brake pad materials are so fast-wearing that it does become an issue on cars (and vans, our Renault has rear discs as well) where the replacement rate is far higher than expected.

If you do a lot of daily miles, then the disc will stay brighter and won't abrade the pad as much. Local or infrequent trips mean that the discs rust and the pads get worn faster.

I don't mind the pad wear as much as the discs, our older Renault had to have new rear discs long before the front ones, dealer had no reason for it.

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

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Reply to
Peter A Forbes

(12.5k miles)!! I seem to be quite lucky and only need a rear pad change after 28k miles. However the front pads, originally 12mm when new are only down to 6mm. Seems cock eyed to me especially with a heavy diesel engine in the front.

Bob

Reply to
Bob Minchin

No there doesn't. Water, or anything else, will get removed once the brake is actually applied. The correct answer has been given several times already. The rubber piston seal deforms and withdraws the piston slightly once hydraulic pressure is released. If the mechanism is really free then the pads will not be in conctact with the disk at all. In other cases there may be light contact but no force acting so no wear takes place. Only once the piston or pad is partially seized in its bore/housing does a problem occur.

Reply to
Dave Baker

Good god no. Any disk runout will create the most appalling juddering. Cure the piston or pad seizure if such exists and leave the rest as the designer intended it.

Reply to
Dave Baker

I once fitted some secondhand replacement discs which had significant runout. This may have reduced the drag but it made putting the brakes on pretty scary as the first pump just pushed the pads nearer the disc and they only started working on the second pumps.

I believe that gravity racers favour drum brakes because of the lower drag.

Russell

Reply to
Russell

I never said anything about grease! But having just re-read my post, I can see where the confusion has come from. You should be able to move the pads in the carrier, not the caliper. You just need to make sure the contact points between the pads and carrier are clean (if shims are fitted, take them off and clean under them) You can put some grease on the contact points if you want, but first time you drive in the rain, it'll be washed of anyway.

Given the age of vehicle, the slides should be fine, but you should use a non-petroleum based grease on any bits near rubber seals, such as the slides. Silicon grease is the norm for such applications.

Reply to
moray

On or around Sun, 14 Oct 2007 21:21:03 +0100, "Dave Baker" enlightened us thusly:

unforgetting that sometimes the designer is an arse. I've had self-adjusting brakes that don't take the brakes up enough, leaving more pedal travel than a) is needed and b) I like. I've also had ones that even with new parts fitted don't really adjust at all.

not discs, I grant you, but examples of design that can be improved. The ones that didn't adjust enough were considerably improved by the addition of about 1mm of metal at a particular point.

Reply to
Austin Shackles

Dave:

I have NEVER seen any vehicle with disc brakes that had no rubbing contact.

YMMV

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

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Reply to
Peter A Forbes

Never seen proper Girling calipers?

:-)

Mark Rand RTFM

Reply to
Mark Rand

I didn't say that you said that. It was meant as an addition.

Nick

Reply to
Nick Mueller

The disk brakes on my Mountain Bike dont run (at least when they are working properly)

Dave

Reply to
dave sanderson

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