[T3] Brake Caliper etc.

[Jim Adney]

On 23 Jul 2013 at 14:13, J. Jonik wrote:

> What exactly does the Piston Retainer Plate do?

Imagine pushing a hockey puck across a table, assuming the puck was 
soft, like a rubber pencil eraser. You're pushing down on the top 
center of the eraser with a single finger as you push it across, so 
the puck will tend to tilt, due to the frictional force of it's 
bottom face against the table. If you keep pushing in the same 
direction, the puck will wear at an angle.   

To keep the puck from wearing at an angle, and developing a taper, 
you can move your finger toward the rear. This will even out the wear 
and prevent the taper.

The OE pistons have cutouts in their faces that cause their force to 
be applied somewhat "behind" the incoming rotor. The retaining plate 
makes sure that the cutout stays in the correct orientation. The 
cutout must be rotated 20 deg from the orientation that you might 
first assume would be correct, but I assume that's what ATe (Alfred 
Teves GMBH) has determined empirically gives the most even wear on 
the pad. On our calipers, where the calipers are mounted to the rear 
of the rotor, the cutout is on the bottom, with the inside edge 
rotated 20 deg up.

On cars where the calipers are mounted in front of the rotor, the 
cutout will be on the top, rotated 20 deg inward.

The correct orientation is well covered in the Bentley, although the 
reason for doing so is not.

I believe current replacement calipers have pistons without cutouts 
and dual bleed valves. This is not as good a caliper, but it allows 
the same caliper to serve either side, thus reducing inventory costs. 

I've also seen pads which have odd cutouts in the backing plates. 
These would make the full circle pistons work okay, but you have to 
be careful, because with these sets there are R & L pads, one of each 
side in each caliper. You have to get this right. These pads will 
also work correctly with pistons that have cutouts.  

It's fairly common for me to get calipers that are assembled wrong, 
even from commercial rebuilders. Sometimes the retaining plates are 
inserted wrong, sometimes they're just not there and the cutouts are 
randomly oriented.

>   As a temporary "fix" of some odd braking shakes at front right
> wheel, I pulled the brake pads and checked the disk.  One pad was
> thick enough but, along the side with the pin holes, the pad sloped up
> slightly...a shape I haven't seen.  The other pad, again nicely thick,
> sloped down noticeably...not drastically...from the pin hole side to
> the other.  Working on the street with rain on the way, I just
> switched the pads, left one stubborn Piston Retainer Plate out, and
> took it for a successful test spin. Braking now fine and shake-free. 

I misread this the first time thru, but now I think I understand what 
you had. A taper in this direction seems odd. It would make me wonder 
if something is bent that keeps the caliper and rotor from being 
parallel. Next time you're in there, look from the top or bottom to 
see if the rotor is parallel to the slot between the caliper halves.

If the slot and caliper are not parallel, you'll want to figure out 
why. It could be that there is some dirt or rust where the caliper 
mounts, or the axle could be bent.

It's also possible that the pads have not been free to move in the 
caliper. They should go in by hand. If you're tempted to hammer them 
in, you need to take a file to the edges to remove excess material. 
They should not be loose in there, but they should not be tight, 
either. If you pay big bucks for a set of pads, they will fit 
perfectly, otherwise you'll have to do what I do whenever I fit a new 
set of pads: file the top and bottom edges.

If the pads have been tight, it's possible that this has caused the 
taper, or maybe the rotor is actually worn to a taper.

I agree that the retaining plates should not be necessary. I've never 
done any tests to see if the pistons tend to rotate without them. The 
cutouts tend to make an impression on the back of the pads, however, 
and I've never seen any sign that a piston has been in any position 
other than how I found it. Nevertheless, I like to see the plates 
installed, if only as a sign that the job was done carefully and 
correctly. It's also a check on whether I got the 20 deg offset 
right, as the plates won't go in if the rotation is wrong.

I also find retaining plates that fit poorly or have been mangled. I 
finally made a die so I could straighten and reuse them as long as 
they hadn't rusted excessively. This service is included with my 
caliper rebuilds.

-- 
*******************************
Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
*******************************