master cylinder volume ratios

AngryCorvair (Forum Supporter) said:

OK, so your old front calipers were single 54mm bore. piston area = pi * (r squared) = 3.14 * 54 * 54 = 9156 mm^2

Your new front calipers are 4-piston opposed with bores of 40 and 36 mm, so total piston area = area 1 + area 2

area 1 = 3.14 * 40 * 40 = 5024 mm^2; and area 2 = 3.14 * 36 * 36 = 4069 mm^2, total = 5024 + 4069 = 9093 mm^2

Oops.  You used diameter squared, not radius squared.

Fronts:
Original: 3.141 * (54/2)^2  = 2290 mm^2
New: 3.141*2*(40/2)^2 + 3.141*2*(36/2)^2 = 4548 mm^2

Rear:
Original: 3.141*(34/2)^2 = 908 mm^2

You also used the diameter instead of the radius for the center of brake pad torque.

Honsch said:

AngryCorvair (Forum Supporter) said:

OK, so your old front calipers were single 54mm bore. piston area = pi * (r squared) = 3.14 * 54 * 54 = 9156 mm^2

Your new front calipers are 4-piston opposed with bores of 40 and 36 mm, so total piston area = area 1 + area 2

area 1 = 3.14 * 40 * 40 = 5024 mm^2; and area 2 = 3.14 * 36 * 36 = 4069 mm^2, total = 5024 + 4069 = 9093 mm^2

Oops.  You used diameter squared, not radius squared.

Fronts:
Original: 3.141 * (54/2)^2  = 2290 mm^2
New: 3.141*2*(40/2)^2 + 3.141*2*(36/2)^2 = 4548 mm^2

Rear:
Original: 3.141*(34/2)^2 = 908 mm^2

Ah yes, so I did.

And as math would have it, it doesn't matter, because old diameter squared divided by new diameter squared equals old radius squared divided by new radius squared. Diameter equals radius times 2, and you can trust me when I tell you the 4's cross out. 

On a different note, one thing you have to remember when comparing sliding calipers to fixed calipers is that you only count the pistons on one side of the rotor when calculating effective piston area. On a slider, the piston pushes the inboard pad against the rotor with a force equal to line pressure times piston area, the caliper body slides inboard, and the outer pad is pushed against the rotor with the equal and opposite force. On a fixed mount, both the inboard and outboard pads push against the rotor with a force equal to the line pressure times the piston area.

I mention this because in your correction of my diameter vs radius, you used the area of all four pistons in your calculation of effective area for the fixed mount caliper, which gives a result that is 2x actual.

Honsch said:

You also used the diameter instead of the radius for the center of brake pad torque.

Yes, because we are given rotor OD and piston OD. I subtracted piston OD from rotor OD, then divided the result by 2. If you do it both ways, ie half of 280 minus half of 54, I promise the result is the same as half of (280 minus 54). I think it's the distributive property or something like that, where a times b plus a times c is equal to a times (b plus c). technically, both methods use diameter. 

so im guessing, pretty much I was on the wrong track. Thinking Im for some reason not moving enough fluid for the front calipers? 

Something to consider... your pad compounds.

I've somehow never driven Hawks, but from what I understand some of their compounds can be very "grabby".

I have Gloc R10's on the front of my Escort, and I've noticed their cold performance is a bit lackluster, certainly not grabby at all.  I assume the R12 is similar/worse as far as cold bite.

Is it possible that your rear pads are serving up their full brake torque before the front pads are warmed up, resulting in what presents as too much rear bias?

In reply to therieldeal :

I have thought about this.  I'm actually trying parts store pads in the rear to see what type of change I get.  Other then that, also considered buying a pressure gauge to see what #s the calipers are seeing for pressure.