[T3] Late Type 3 Calipers

[Jim Adney]

On 21 Jun 2017 at 0:27, Dave Hall wrote:

> I think these calipers may need a bit more investigation, Jim. At
> least with only one bleed valve, there's no doubt which side they need
> to be fitted, however the piston orientation may not be correct for a
> Type 3.

Everything that comes here gets taken apart and rebuilt. After that, 
I can put it back together correctly for whereever it's needed.

With a single bleed valve, the piston positions will indeed be wrong 
for switching between Type 3 and Type 4. One advantage of dual bleed 
valves is that it makes a given caliper work on either model. Once 
it's set up as a "right" it works as a right for either model; the 
upper bleed valve just becomes the lower. 

To answer Gary's question, in addition to making the caliper castings 
"universal", a bottom bleed valve lets us bleed off brake fluid from 
the bottom of the caliper, where dirt tends to collect. It's also 
possible that "wet" brake fluid is more dense than dry fluid, so 
bleeding from the bottom may be more effective. At any rate, with a 
bleed valve on the bottom it's possible to completely drain the 
system. A bottom bleed valve is also a huge advantage when switching 
to DoT-5 brake fluid, which is lighter than standard fluid, so 
bleeding from the bottom lets us get out the last traces of the old, 
heavier fluid.  

> Genuine calipers need the piston retaining clips and piston orientation set
> correctly for a Type 3.  If not, the pads can wear unevenly. 

I rebuild a LOT of calipers, and I've seen every possible combination 
of incorrect piston installation. Most of the calipers I do are for 
Type 3s, so I've gotten very used to how those go together. I have to 
admit that I have to stop and think, and get annoyed, when I'm doing 
calipers for a car where the calipers are mounted in front of the 
hub.

> I've not seen any late calipers with single bleed valves.
> I think the Type 3 had the disc runout ccmpensating pins in the cylinders.
> Do these have them, or perhaps it was only the ones through '71 with the
> thinner discs?

All the Type 3, Type 4, and 914/4 calipers I've seen appear to have 
come with the compensating pins, although many of them have clearly 
been broken out, I assume by some previous commercial rebuilder. I 
consider the pins to be useful and much prefer to see them installed. 
Unfortunately, I don't have any way to replace pins that have been 
broken out.

> I've forwarded Jim separately a page from our Club Technical manual with the
> Type 3 piston orientation, and some comments from a member. 

Your pdf file speculates on the why of the cutouts and their 
positioning with respect to the rotation of the rotor. This 
positioning is common to all ATe and most other caliper makes. I 
agree that putting the cutout on the side facing the incoming rotor 
(when moving forward) evens out the pad wear.

Why this cutout is angled 20 deg is harder to understand. I'm not 
sure, but I have a SSWAG (slightly scientific wild ass guess):

We know that static friction is larger than dynamic friction. I'm 
willing to guess that it might also be true that lower velocity 
friction is larger than higher velocity friction. Since the tangental 
velocity is larger at the outside of the rotor, it's possible that 
less wear compensation is necessary out there.

Rotors in different cars vary a lot in diameter, as do the proportion 
of the circle that is covered by the pads. Braking at different 
speeds also varies the rotor speed. All of this  would seem to 
require different amounts of tilt to compensate for the radial 
variation of friction, although it's possible that calipers for 
different cars get different amounts of their faces cut away. 
Nevertheless, I suspect that the universal 20 deg tilt is a 
compromise.

I've never worked on a caliper with 4 pistons. Does anyone know if 
there's anything similar done with them?

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