[T3] Brake Calipers

[Dave Hall]

I think most VWs after the Type 3s got aome sort of brake proportioning
valve.  Type 4s had a prssure limiting valve on the rear, and our 74 Camper
did, plus all the watercooled VWs we've had since.

I appreciated that the piston retaining plate isn't the full thickness of
the step in the piston.  The proper VW pads have a cork (?) layer on the
back which I presume allows an amount of variation in the pressure across
the pad.  I suppose the flexibility of the steel backing just makes sure the
greater circumference of the outer area of the disc  doesn't wear the pad
more than on the inner area.   Pretty careful engineering work by the brake
designers, with the anti-run-out compensation mechanism in the caliper too.


The uneven wear I find is more to do with the inner and outer pads, and the
last set wore out the inner pads while the outer were still only about
half-worn (I think it was that way).  I concede this may not have anything
to do with the step.  The first set in the calipers wore out quite quickly
too.  I can't be sure when I fitted them, but I bought them when Tom
Siligato from Ford Detroit came over and stayed a couple of days.  We
visited a local VW event, and I bought the calipers there.

Dave.
UK VW Type 3&4 Club
===================

-----Original Message-----
From: type3-vwtype3.org-bounces at lists.vwtype3.org
[mailto:type3-vwtype3.org-bounces at lists.vwtype3.org] On Behalf Of Jim Adney
Sent: 12 August 2011 15:06
To: type3 at vwtype3.org
Subject: Re: [T3] Brake Calipers

On 12 Aug 2011 at 13:42, Dave Hall wrote:

> I would have thought some sort of plate is essential in there, if only 
> to ensure the pad is pressed on evenly.  I suspect it's not necessary 
> to have the inner grip teeth.  A simple shape with a hole for the pad 
> pin would be enough, I'd have thought.

The point of the plate is to provide exactly the right degree of unevenness
to offset the tendency for the pads to wear unevenly if the pressure is
even. The OE plates are about half the thickness of the step in the pistons,
so no pressure gets transferred thru the plates.

I was thinking the same thing about a simple triangular plate. That might be
something I could get made at reasonable cost.

> For the effect of bigger pistons:-
> 50mm instead of 42 is 8/42 times bigger diameter, which is about 19%
extra.
> When you square that up for area you double the %, so 50mm would be 
> 38% MORE braking force for the same pressure in the brake fluid.

That's close enough to get the right idea, but you really have to do the
50/42 math, which gives you 1.4172, then subtract 1, giving .4172, or almost
42%.

> As you say, the braking balance would be very different.  With the 10% 
> difference of 40mm compared with 42mm diameter, the brake balance 
> change isn't all that problematic, particularly when it's quite hard 
> to lock up the rears anyway, and the fronts still tend to lock up under
heavy braking.

Right, and brake balance is always a matter of compromise anyway, because
the right choice changes with load, load distribution, and slope of the
road. That's why some cars have a proportioning valve to adjust the pressure
being transmitted to the rear brakes to alter that pressure depending on the
state of the rear suspension.

> The slightly smaller pistons generally just mean you need to press a 
> bit harder for the same braking force, as you would need to produce a 
> bit more fluid pressure.  Although sold as 311 parts, the thinking is 
> that my replacement calipers are smaller pistons.  I don't know if 
> that's true, and haven't pulled the piston out to check - nor will I.  
> They don't wear as evenly as the original calipers did, but with the 
> fairly low mileages I do in the Type 3 (shame on me!) I'm not that 
> bothered about that.  Usually they will brake far better than calipers
that need a rebuild!

The uneven wear is due to the full face (evenly applied pressure) by the
full circle piston. They did this, and added the second bleeder valve, so
that they could be used on either side of the car.

Have you actually noticed uneven wear of the pads? I have very little
experience with the full circle pistons, but I got in a set of OG calipers
last week that had been assembled with the cutouts oriented randomly. Two of
those pads were badly worn at an angle. 
Unfortunately, I hadn't noted all the relationships before I took everything
apart.

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

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