Hi Russ
I hope these comments will be useful:
Your standard setup: Caliper piston area 2 x 1810mm2 = 3620mm2
Hi-Spec setup: Pair of twin piston calipers (a pair of opposing pistons exerts
the same clamping force as a sliding caliper with a single piston)
4 x 765mm2 = 3060mm2
Difference = (3620-3060)/3620 = 85%
So on the face of it you need a 15% smaller (area-wise) than standard master
cylinder to get the same braking effort, BUT, you need also to take account of
the increase in leverage due to the larger discs: 265/225 (the effective
diameters) implies a 17.5% increase (area-wise) in m/cyl size.
The two as near as dammit cancel out, so you actually need to not change master
cylinder size!
I have not incorporated rear slave cylinder size in the calcs because the rear
pressure limiting valve takes them out of the equation at quite low pedal
pressures.
I hope that helps.
Regarding pedal feel, it's pretty much essential to replace the rubber flexible
hoses front and rear with braided teflon lines, I found also that pedal feel
can be further transformed by removing play at every point in the longish route
from your foot to the master cylinder, including setting the m/cyl pushrod
clearance.
Best regards
Bill
Bill Allison
Borders Group of Advanced Motorists
National Observer (Cars)
07702 739474
01750 22550 (before 21:00 please)
www.bgam.org.uk and www.bgam.org.uk/ydi www.iamroadsmart.com
www.facebook.com/BordersAdvancedMotorists
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On Sunday, 26 July 2020, 10:54:37 BST, russ price <[email protected]>
wrote:
After fitting Hispec 4 pot calipers and 280mm discs i thought i should do
something about a probable leak from the master cylinder, the clue being rust
and bubling paint on the servo. The trouble began when i sheared off a stud
holding the m'cylinder to the servo, no problem i thought i have a Mondeo
m'cylinder/servo which i have heard is used as an upgrade and a bigger
m'cylinder is probably a good idea after going from 2 to 8 pistons on the
front. This i now believe was an olympic standard jump to the wrong
conclusion.If i have done the maths right, ( I would be grateful if someone
checked my results) i need to go down in m'cylinder bore size. Now i know
calculations of braking forces are really complex but this just about the
"feel" of the brakes.Here are the figures i used, original calpers single 48mm
piston, new calipers four 31.2 mm pistons, rear slave cylinder 17.5mm, original
m,cylinder 20.6mm, mondeo m'cylinder 25.4Which give an original set up
m'cylinder/caliper ratio ~ 1:23, original m'cylinder + new calipers ~ 1:20,
mondeo m'cylinder + new caliper ~ 1:13So given Fiesta mk2 servo's are not
exactly thick on the ground i thought about a set up similar to that recently
used on 003, ie. landrover servo + Mk1 19mm m'cylinder which should give me a
ratio ~ 1:23I would really appreciate it if anyone can say to me " Your are
talking nonsense and here's why......" or " Yes that should work but be aware
of ......."
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