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Re: [T3] Converting Single MC to Dual MC

On 11 Dec 2002 at 19:02, Keith Park wrote:

> But.. Jim, since you brake  about 2/3 on the front and the rest rear... how
> does this proportion stay the same with such a different type brake
> (caliper vs Drum).  Drum brakes are much less effecient... I would think
> youd need at the least a much different brake fluid pressure with them.

The proportioning is done in the design stage of the car, by choosing what 
diameters of slave pistons you use AND at what radius the braking force is 
applied. Thus we got 42mm F caliper pistons and 3/4" (~19mm) rear pistons. 
These can't be compared directly since they are disks/drums. I'm sure the 
manufacturers have ways of making good educated guesses, however, and they 
certainly have the option to adjust those sizes in the prototype stage.

On the Golf, where the rear wheels really have very little braking to do, you 
can see extremely small diameter drums, inside of which there are very small 
diameter pistons.

In a car with 4 wheel disks, there is a practical lower limit on the piston 
size, so on the 914, for example, they got the same diameter front pistons and 
the smallest rear pistons they could do, then they added the proportioning 
valve. Frankly I think they would have been better off with rear drums, but 
there are plenty of people who think that 4 wheel disks MUST be better.

BTW, I'm not sure exactly what the advantages of disk brakes are (maybe it's 
unsprung weight,) but I don't think its accurate to say that drums are less 

> As to amount of fluid used... I could swear there were differences... but I
> bow to the master as he has hard numbers to back him up ;)

I agree, there certainly must be differences, but this is just a matter of 
available MC stroke. VW adjusted the F/R stroke on our cars in 72, I assume 
just to make sure that they seldom ran out of travel. This is most likely to 
come up with drums, which take more fluid displacement as they wear, until they 
get adjusted. Disks just happen to be inherently self adjusting.

If you think about it, you will see that in a tandem MC each internal piston 
will move until all the "free travel" has been taken out of BOTH circuits. Only 
then will the fluid pressure start to rise, and it will riss equally in both 

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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