[s-cars] [urq] S-Car Brake system conversion- Read- no more PSpumpintegration <question>

[racingiron at comcast.net]

Mark Strangways wrote: 
> I don't agree with your way of thinking. 

Doesn't bother me, but mother nature might have a bone to pick... 

> I have been in hydraulics for about 20 years now, and pressure 
> apllied by opposing piston on a clamping surface is the sum of 
> each side. If fact you may say that on a sliding caliper, the 
> effective clamping force is shared between each side. 

I said nothing about pressure/force. The whole discussion is about pedal travel and piston sizes. 

> The reason the pedal travel is less with a fixed caliper is that 
> each piston moves less than that of its G60 conterpart, 

Roughly half the distance, actually. But if you have 2x the pistons moving 1/2 the distance, isn't that the same amount of fluid? The difference in pedal travel has to be accounted for somewhere. My position is that we have something slightly less than 2x the piston area. 

> while imposing twice the clamping force. 

Sounds like you're getting something for nothing here. Pedal effort doesn't change (certainly not double!). If you've got a given pressure (psi) from the MC applied to a larger area (2x the pistons) won't it be half the force on each piston? Besides, we're talking about pedal and piston travel here, so we can effectively ignore force/pressure until the pads get to work on the rotor. 

> Result less pedal travel, twice the pedal resistance... And that 
> is exactly what I found going from G60's to 996 TT calipers. 

Less pedal travel, but the pads are moving (roughly) the same distance as before, no? How do you explain that without less fluid movement (which implies lower effective piston diameter)? If you got TWICE the pedal resistance, I submit that something is very wrong. The lower travel can make the effort seem greater, as can the stiffer feel from new fluid, less caliper flex, new rubber lines, etc. 

> (Flame suit on) 

No flames from me, just a desire to understand. I'm always happy to correct my opinion. 

Eric R. 
'86 urq 
'92 urS 
'93 urS