Mystery A Calliper pad shims...

[911hillclimber]

I bought my A type callipers (same pad size as the aluminium S calliper) from an early Husbourne Crawley Porsche autojumble in around 1993 to fit the 3.5" mounting Bilstien struts found in the same field.

With the callipers (not new) came the shims shown below.

There must have been for anti-squeal, match the piston angle of 20 degrees and are directional.
They are 0.6mm thick which is less than the step in the piston (that is aligned to 20 degree step).

My car has the original M type callipers on the rear from 1973 and these too have the same design of shim, but smaller and 20 degrees/0.6mm/ directional.

I have just replaced my discs and pads for Brembo all round and have for the first time left these shims out.
Why?

There is no description of these shims in the Haynes manual, so I thought I would see if the brake are quiet for the first time in 30+ years.

I do wonder if these shims are just there to stop the piston rotating round in use from the 20 deg position and nothing to do with noise suppression.

So, any thoughts/comments please?

[911hillclimber]

Just shuffled this to the top to get a bit more exposure.

I still cannot find anything about these factory made shims.
Going to ask on the PCGB forum too.

[Lightweight_911]

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Maybe they were included just so that you could set the piston orientation ...

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[Nine One One]

Google A.I. gives exactly the same response as Design 911.


ABOUT BRAKE PAD ANTI-VIBRATION SHIMS
Porsche Brake Pad Anti-Vibration Shims | Design911 Precision Noise Reduction & Performance Enhancing Components

Brake pad anti-vibration shims play an essential role in maintaining quiet, smooth, and consistent braking performance. Fitted between the brake pads and calipers, these components absorb vibration and dampen noise caused by friction and movement under load. By reducing brake squeal and ensuring even pressure distribution, they enhance comfort, braking precision, and component longevity — making them an important part of every Porsche braking system.

At Design911, we offer a premium range of brake pad anti-vibration shims, manufactured from high-quality composite and stainless-steel materials. Each shim is designed to deliver maximum noise reduction, thermal stability, and perfect fitment across Porsche model ranges. Ideal for servicing, restorations, or performance upgrades, these shims ensure your Porsche brakes perform with the refinement and control expected of the marque.

What Do Brake Pad Anti-Vibration Shims Do?

Anti-vibration shims are thin layers designed to prevent metal-on-metal contact between brake pads and calipers, reducing noise and improving braking efficiency.

Core Functions:
• Absorb vibration and resonance during braking.
• Reduce or eliminate brake squeal and high-frequency noise.
• Ensure consistent pad pressure and even braking.
• Protect caliper pistons and pads from heat transfer.
• Improve pedal feel and driver comfort.

By maintaining quiet, smooth braking, shims help preserve the driving experience and mechanical precision of your Porsche.

Why Do Brake Pad Shims Fail or Degrade?

Brake pad shims are subjected to extreme heat, pressure, and environmental exposure, which can cause deterioration or loss of performance over time.


https://www.design911.co.uk/porsche/911 ... efficiency.

[911hillclimber]

Thanks for the responses.
If used to set the '20 deg' piston orientation why have a shim for each pad when one will do the lot, but also Design911 do not sell these shims, or Type911. and I have never (so far) seen them on any google search or YouTube video.
They are thinner than the step in the piston/pad contact face.

This is the first time I've left them out of the pad changes I've done (6) on these brakes on this car.

Did your M callipers have them Andy?

[KS]

My 914 had shims like these and it seems the sole purpose was to align the pistons. They were pretty beaten up, so I just chucked them with no obvious ill effects...

[911hillclimber]

Interesting, and can see why they are there for the piston alignment and not the noise suppression.

I'll leave them out and see how the brakes perform.
Weather against a road test/run-in of the pads for now, maybe next week.

I still find it strange that the shims are nowhere to be seen on the internet.

[Nine One One]

This explains it for you............apparently clamping forces?

https://www.thesamba.com/vw/forum/viewt ... p?t=731122

[911hillclimber]

Interesting as ever!

So the recess allows the pad to tip during the pressure being applied stopping grabbing etc and so a smooth braking effort when the pressure is fully applied.

Also stops the piston rotating over time/use.

All reads as 'important stuff' but nobody supplies the plates/shims with new pads or new callipers or re-furbished callipers!

I have yet to find a picture of these shims anywhere on the internet, and indeed, the works diagrams do not show the parts.

Yet when new the shims were fitted. The 914 shims are quite a different design.

Thank you for unearthing the above. If I can find some I will get them and fit to my new brakes, but it will be a long search.

Just might buy some shim steel and make some as I have the old ones as a pattern.

These kind of things bug me!

[neilbardsley]

Where do the shims fit? Between the pads and pistons?

Sent from my 22081212UG using Tapatalk

[911hillclimber]

Yes, the 20 deg slope matches the angle the pistons have to be set to when in the calliper.
They are not pinched between the piston annular ring and the pad backing steel plate but rest in the gap, thus the function id more keeping the 20 deg angle in place over the life of the pad.

As said in the video etc from 9-1-1 above they are keeper plates, not sure a plate is made from shim steel 60 thou thick, but I do think he is correct.

My huge tripple pot Brembo brakes on the LOLA have none of this, just simple annual pistons and steel pads, no judder, no noise.

The Boxster have quite an elaborate shim damper to each pad / piston contact faces to kill noise and smooth things out, very effective.

In my case, Porsche used these shims or rather ATE sold them thee callipers with these shims presumably for a good reason?

[hot66]

might not have been for a performance reason, maybe a customer complaint reason of squeeling brakes ?

[Lightweight_911]

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A quick search found these similar-looking shims for a Rover P4:

[Lightweight_911]

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- & these for a TR3A:

[Nine One One]

I think you are going to answer your own question?

PMB replied to one customer.........Their answer :

""They do not. Pistons don't "rotate" so we find those plates are generally unneeded in all applications.""

Then the other post in that link........
The reason for the cut-out is to spread the clamping force towards the rear (trailing edge) of the brake pad. If the piston were flat (no cut-out), the caliper would apply more clamping force to the leading edge of the brake pad. This would cause the pads to wear unevently, with more wear
on the pad's leading edge.

So perhaps over a period of time, they discovered that once the piston was set to that 20°, it did not rotate whilst in use, so there was no necessity to keep those shims, or continue manufacturing them?

Like everything, pad technology has moved on over the years. Most modern pads now come with a chamfered edges on both up and lower parts of the pad. Older pads always came completely flat. I think both yourself and, others have on, say, Carbone Lorraine pads, filed/sanded down a leading edge on pads to stop squealing?

I never knew about this 20° angle of the piston, until someone (maybe you) raised it here years ago, and have just replaced pads when needed. None have had adverse wear on one face or the other and no squealing either. I have never checked the piston angle.

It makes you think, if this cut out was such a necessity why it has never been continued since? Nearly all modern cars have a flat piston face, use ABS, which must have an impact on piston movement in the calliper and, on the rears of most, you now need a ‘wind back tool’ or VCDS or other software on electronic callipers to wind in the piston to fit new pads.

You may be over thinking this? You are probably more engineering/mechanically minded than most, but perhaps there is no need to fit these shims anymore. If they were such a safety issue Porsche would still be manufacturing them, as would brake pad companies?