Disc Brakes

[DerbyBorn]

Why don't they make the pads just a bit bigger so that they don't allow a lip to develop on the periphery of the disc?

[Jim GM4DHJ ...]

so mercedes etc can sell you discs ......

[Walter]

Fuckwit.

When the pads wear enough, the lips will meet and prevent the pad from contacting the disc.

[alan_m]

The lip is the indication that the disks need replacing. If the pads were larger you wouldn't get an easily seen indication of the thinness of the disks.

[The Natural Philosopher]

cos then they'd put a lip on the pads instead

[The Natural Philosopher]

f****it. The pads would be down to metal before they met

[newshound]

The right answer, IMHO

[Roger Hayter]

I am sure you are right. Secondly it would produce an axial force on the pad, thirdly it would render part of the pad unused. Fourthly, and I think most importantly, movements of the pad against the edge of the disc would shatter the outer few millimeters of the pad material (which has little tensile strength) so you would be back to the bearing surface of the pad being inside the perimeter of the disc quite quickly.

[DerbyBorn]

[e-mail] (Roger Hayter) wrote in news:1n5mgjn.zwsb101eenncwN% [e-mail]:

Sounds good to me. A micrometer and the specification is the proper way of establishing wear surely though.

[Roger Mills]

Sorry, I don't agree! You often get some rust on the outer bit of the disk, outside where the pad rubs, making any lip look a lot thicker than it really is. Besides which, disks are allowed to wear quite a lot before they need replacing. There's quite a substantial lip on mine, but the disks are still within spec when measured with a micrometer.

[Dave Plowman (News)]

The entire pad needs to be in contact with the disc. If it overlapped it, the differential in heat would cause it to break up.

[alan_m]

But unlikely to be the way the MOT tester "advises" a replacement.

[The Natural Philosopher]

Indeed. Unless you do a huge mileage, uneven disc wear and scoring are likely to be more of a reason to replace that simple disk thickness..

[Brian Gaff]

I am given to understand its to stop the material breaking up around the outside because its still thick enough to hold the rest together.

Not that I know a lotabout cars, but that seems to be the reason people are given. Brian

[Dave Plowman (News)]

At least some cars have the minimum thickness stamped on the disc hub.

Generally, discs need replacing every second set of pads.

[Roger Hayter]

This is partly because by the time you have taken them off it costs relatively little more buy new ones than to skim the old ones. Probably a lot less if you haven't got your own suitable lathe.

[Dave Plowman (News)]

Not really - they've worn down below the minimum thickness. Mainly applies to the vented sort.

Not really understood this US thing of 'turning rotors' Even quite noticeably grooved ones seem to work ok.

[The Natural Philosopher]

well that's bollocks of course.

The metal thickness is important - especially in unvented discs, as the disc is a heatsink that absorbs braking energy.

A skimmed slender disc will get hot quickly and the pads will fade possibly before a 70mph emergency stop is complete.

[DerbyBorn]

"Brian Gaff" wrote in news:oemlrc$c2s$ [e-mail]:

Mmmm! Hoop Stress.

[Andy Burns]

That seems to be the way, though the garage generally tells me I've been "lucky" as modern discs and pads are intended to need replacing at the same time.