[T3] Brake Calipers
Jim Adney
jadney at vwtype3.org
Fri Aug 12 07:06:08 PDT 2011
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.
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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