[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [New Search]

Re: [T3] Master cylinder question

On 11 Nov 2001, at 19:46, Everett Barnes wrote:

> Everett said:
> >> 19mm vs. 20mm master cylinder

> Jim said:

> >If you switch to the larger MC you will reduce the stroke necessary
> >to apply the brakes, and make the brakes feel "firmer" but you will
> >have to press harder to get the same amount of braking action,
> >because you have actually reduced the hydraulic mechanical
> >advantage by 15%. This strikes me as a lot, so I wouldn't do it.

> Since the 20mm was stock on the Squarebacks, wouldn't this mean the
> Squarebacks had  brakes with the attributes you mentioned?  This seems
> disadvantageous, I had assumed the 20mm m/c gave you improved braking power
> somehow to allow for the hauling capacity of a Squareback.  I wonder why VW
> would do that...

I'm not that familiar with the early braking systems, but I suspect that when 
VW changed the MC diameter they also changed the wheel cyl dias. You 
could certainly change both dias to keep the hydraulic advantage 
approximately constant, but sticking with the 19mm MC is the easiest, and 
best, way to go.

The thinking that increased dia makes for better braking is widespread in the 
automotive public. For years, aftermarket suppliers for the Porsch 914 have 
sold the 914/6 19mm MC as a high performance upgrade for the 914/4 which 
came with a 17mm MC. This is completely backwards and results in exactly 
the opposite effect.

The mistake is made because people think that any part from the 914/6 
must be better than its 914/4 counterpart. What they have not bothered to 
look at is what the diameters of the pistons in the calipers were. You see, 
Porsche changed those, too, so it is necessary to look at the whole system 
before you can get a complete understanding.

BTW, at least in our era, MCs and drum cyls were actually in fractional inch 
sizes, while calipers were in mm. For most of us the rear wheel cyls are 7/8" 
(22.2mm) with 3/4" (19.05mm) MC and 42mm calipers.

I think some of the early squares came with 15/16" (23.8mm) rear wheel 
cyls. It was probably these cars that came with the 13/16" (20.64mm") MC.

On all of these, you will often, depending on the manufacturer, find the 
diameter cast into the outside of the part to the nearest mm, even though a 
careful measurement will make it clear that the bore is an exact number of 
sisteenth's of an inch, and not any integral number of millimeters. If you read 
the ATe literature that comes with your MC kits you will find the bore called 
out as 19.05 which seems like a strange number until you realize that this is 
exactly 3/4". ATe casts a 19 on the side of this MC.

Jim Adney
Madison, WI 53711-3054

Too much? Digest! mailto:type3-d-request@vwtype3.org Subj=subscribe

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [New Search]