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I notice when buying brake pads that often shims are no longer supplied. Is this because they are not deemed necessary anymore? I always fit the old ones. Is this correct? had to put Citroen because computer asked for a make but this is a general question. {edited - DD}
Edited by Dynamic Dave on 24/10/2008 at 14:37
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Carl,
Shims are, on the whole, a thing of the past. The reason for this is due to the special coating that is used these days on the brake pad backplates. (I't a bit like a soft glue.)
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Is it still a good idea to apply a thin coat of Coppagrease to the backs of brake pads ?
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the shims are to help stop heat transfer to the piston so although the material on the back of the pad will do this if there is room for the shim as well it will do some good if also fitted
why do you put coppa grease on the back of the pads ?surely it will melt with the heat and drip off
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Copper loaded anti-seize compound won't melt and drip off - ordinary grease would.
The use of anti-seize on the backs of disk pads was an anti-squeal measure, now made obsolete by the gungey coating on replacement pads.
Metal shims will not reduce heat transfer materially (saucepans get hot on solid hotplates) but the hollow piston used in most calipers does this job quite effectively.
659.
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Metal shims will not reduce heat transfer materially (saucepans get hot on solid hotplates) but the hollow piston used in most calipers does this job quite effectively.
659 with all due respect
the shims help to loose heat by having a bigger surface area than contact area thus absorbing heat very quickly and loosing heat very quickly
what in effect you are saying is that if you put a suacepan on the stove it will boil at the same speed even if you put a large metal plate between pot and stove ,well sorry but it wont
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A metal shim is the same area as the pad backplate, so fitting one just makes the backplate 0.5 mm or so thicker - no significant thermal difference. The saucepan analogy is the same - put a 0.5 mm flat metal plate over your solid hotplate and there will be very little difference in heating time.
Shims were fitted to reduce the incidence of brake squeal.
659.
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>>no significant thermal difference
There is the matter of the thermal conduction contact resistance between pad and shim, and shim and piston - it's definitely not an insignificant effect, and the shim effectively adds an interface, without adding much mass or thickness.
On my W124, there is a thin metal shield mounted on the piston nose. It doesn't touch the pad, and the only action it can possibly have is to reduce radiative heat transfer between the pad/disc and the outer piston seal.
>>Shims were fitted to reduce the incidence of brake squeal.
I agree with that - very carefully worded!
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The piston nose shim is indeed a heat shield - to protect the dust boot. The traditional backplate-sized shim has negligible thermal resistance at the operating pressures encountered during brake application.
Careful wording - used to to work for company which owned Girling.
659.
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>>negligible thermal resistance
Not negligible I suspect, when compared with the thermal resistance of the shim itself.
Incidentally, I haven't found many reliable sources to enable contact resistances to be estimated - how do you estimate them 659?
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One useful source for contact resistance is a semiconductor data book.
Bolting a metal semiconductor device case to a plane heatsink is a similar situation, thermally. For example, a TO-3 cased metal device might typically have a thermal resistance of 1.0 deg C/W junction to case. The case to heatsink interface (bolted tight) is quoted as only 0.3 deg C/W. (Source, Philips data book).
In brake development, a fully instrumented rig is used to ensure that critical components such as rubber seals and boots, slider pins and of course the fluid itself all operate within acceptable thermal limits. Thermocouples would be embedded in pads and attached to other parts of the assembly and all data logged during trials.
659.
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>>One useful source for contact resistance is a semiconductor data book.
Yes, my all too limited sources are very similar in their derivation - which is fine when estimating the temperature at a semiconductor junction - but not too helpful in other, more mechanical, situations.
>>In brake development, a fully instrumented rig is used...
Yes, I'm sure that most parts which are designated as safety critical will have to be proved by test - although, perhaps this is changing. Some safety critical aerospace parts are now being qualified for flight purely via analysis.
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659 the flaw in your argument is that the shim is not 100% in contact with both surfaces all the time otherwise it would not work as an anti squeal either ,there would be no point in it being there at all
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Patent application for anti-squeal shim ........... tinyurl.com/57brzp
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Shims have always been "anti squeal" shims.
It was quite acceptable to use pads without shims if noise was not an issue; excess heat that they dissipated (or not) was never an issue. (Advice from Lockheed and Girling, many years ago)!
If the heat was an issue, they would still be used. Most pads from point of sale are only fitted with the "squashy" type backing on.
Argue about the the heat and performance all day long but its not actually relevant ?
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