I’m afraid it looks like my calculations disappeared in one of my PC upgrades.
I know on my final setup i drilled a new hole in the brake pedal further up
from the existing XR2 one.
I think that was to give me more pressure in the brake system at the expense of
a bit more brake travel.
As Bill says, you need to upgrade or replace all the bushes and pins in the
linkage.
You can also make sure that the pushrod in the servo is adjusted correctly so
that there is no play between it and the piston in the master cylinder.
Most people don’t know that is adjustable.
Be careful though, the pistons in the master cylinder have to return to the
full idle position to allow any pressure in the system to fully disappear when
you release the pedal.
If you leave pressure on the pistons in the master cylinder via the servo then
when the brakes get hot the expanding brake fluid has no where to go and
gradually holds the brakes on, BTDT
Jim
From: russ price
Sent: Monday, July 27, 2020 9:18 AM
To: Quantum Owners Group
Subject: Re: [Quantum Owners] Brakes, jumping to conclusions and maths
Thanks Bill your reply certainly does help. I have fitted braided braided lines
but the linkage could do with a bit work. So all i need to do now is find a
servo + mastercylinder the same dimensions or as close to the Fiesta items as
possible, there is plenty of room and fabrication of the servo tower/bracket if
it needs changing should not be problematic. That is of course unless someone
has a mk2 servo they want to sell.
On Monday, July 27, 2020 at 8:36:46 AM UTC+1, Bill Allison wrote:
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 <flob...@gmail.com> 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 25.4
Which give an original set up m'cylinder/caliper ratio ~ 1:23, original
m'cylinder + new calipers ~ 1:20, mondeo m'cylinder + new caliper ~ 1:13
So 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 ~ 1:23
I 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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