XJ_ranger said:
interesting...
though - going from a small one - which your MC was designed to move the piston we'll say only 0.005" and then putting in a system that requires .010" would create a huge difference at least in my mind....
where is my logic wrong?
are all pistons pretty much the same then?
does the size of the piston have anything to do with how much fluid it requires to be moved? It seems to me that you would have to fill a gap behind the piston with hydraulic (brake) fluid, and the larger the piston is, the more fluid needs to be there...
pi*r^2 gets bigger real quick...
i am working on my d44/ford 9" swap and was planning on swaping MC's to solve the problem that i thought i was going to have (that and get rid of ABS), but i might not need to?
sorry to hijack - but thanks for any answers...
The logic is hard for me to follow, but I'll try.
Both D30 and D44 (and D60) disc brakes only need about 0.010" of piston travel, from rest position to full pad contact. The only difference is piston bore area and how it relates to the consumed volume of fluid. larger bore caliper pistons (or multiple piston calipers) have a greater net pressure area and need more fluid to fill the consumed volume (we agree, I think).
The issue is one of relative volume.
The stock MC volume is ~1" diameter (IIRC) with over an inch of stroke for each circuit (0.785 cu in volume), and the stock caliper piston is ~2" diameter with 0.010" stroke (0.063 cu in, for two calipers). The reserve volume in the MC is much greater than the consumed volume in the caliper piston bore (in this example a 1,250% reserve capacity of MC volume).
Add a caliper with a net 4" diameter bore (a whopping 4x increase in applied force) and the consumed volume increases to 0.251 cu in, leaving the MC with only a 200% reserve capacity. This is a little extreme (an extremely large caliper area) but it reflects the factors of scale.
In reality, the tandem MC does not apply pressure to either circuit unless both circuits are pressurized or one of the pistons runs out of travel (leaving the worst case circuit to determine the travel consumption). Rear wheel cylinders are usually the more critical volume consideration. The available working piston travel of both circuits in the MC is usually compromised, sometimes as much a 50%, by the volume consumed by poorly arc'd and adjusted rear circuit brake shoes (one reason why most competition vehicles use two MC's).
Keeping all this in mind it's a wise decision to keep the reserve volume in the MC somewhat above 400% of the expected consumption of the rear wheel cylinders (do the math for either front or rear circuits to be safe). There is actually a minimum travel/volume after circuit failure that needs to be considered when using a tandem MC (but I have never read published testing data). This reserve volume also helps to add safety for line flex and expansion and compression of trapped air.
FWIW, the E350 MC is (IIRC) and 1.25" bore, with a swept volume of 1.277 cu in through a 1" stroke.
