Tom
Google provides a couple of wordings which might be clearer than mine:
"Fixed calipers have one or more pistons on each side of the disc which each
apply an equal force to the disc. Sliding calipers have pistons on one side of
the disc only and rely on Newton’s Third Law to allow an equal reaction on the
back of the disc by letting the whole housing slide on one or more guide pins.
Ignoring frictional effects both types will produce similar amounts of total
clamp force"
>From an online piston area calculator comparing floating in which the total
>number of pistons is used in the calculation - "REMINDER: For floating
>calipers, multiply the result of the calculator by 2".
It is useful to remember that pedal feel is significantly improved,
irrespective of cylinder sizes, by replacing floating calipers which are
literally a "rattling good fit" with fixed calipers which are a more rigid
system and which in addition typically are made to higher standards (closer
piston/cylinder fit, stiffer seals, etc).
Pedal in mine is right at the top, virtually no softness. Brakes build is in
here somewhere.
HTH
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
The contents of this message, and any attachments, may include information that
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AL7 3BT.
On Monday, 27 July 2020, 17:42:13 BST, Tom Walker <[email protected]>
wrote:
I'm not sure what I'm doing wrong here - I thought by 'reply all' it went to
the group. I'll try again - apologies if I've done it again.
I was querying Bills calculations above (or below - I don't know) because even
though the pistons are opposed you still need to use the full volume of the
slave cylinders.
Tom
On Monday, 27 July 2020 15:21:27 UTC+1, Jim Hearne wrote:
Hi Toms,
Whose maths are you querying ?
Jim From: Tom Walker Sent: Monday, July 27, 2020 3:12 PMTo:
[email protected] Subject: Re: [Quantum Owners] Brakes, jumping to
conclusions and maths Are you sure about the maths there? With the 4-pots
you've got a total 3060mm2 of cylinder per wheel (regardless of which side of
the disk they are on) vs 1810mm2 for the original calipers - which is about 40%
more piston per wheel. So for a given pressure of fluid pushed (N/mm2) from
the master cylinder, the slaves will exert 40% more pressure (F2=F1xA2/A1) but
your foot will have to move 40% further (F1d1=F2d2). If you can live with the
pedal travel, then for the same leg work you will be applying 40% more pressure
to the disks. Add to this the fact that with the four pots you have that
pressure over a slightly larger pad area, and of more significance at a greater
distance from the axis giving greater braking torque - the braking efficiency
increase will be more than the simple 40% calculation. If you fit a bigger
master cylinder (for argument's sake let's say you find a cylinder which is 40%
bigger) then you will negate the need to push the pedal so far, but at the
expense of physical effort as the pedal pressure will need to be higher. As it
happens I've got the HiSpec 4-pots on my 2+2 but with 305mm rotors (you need
16" wheels). On the back it's the Fiesta Centre rear-disk conversion (I think
they are Escort disks with a Granada caliper, but I may be wrong). Under the
foot it's got an adjustable front/rear brake bias pedal box with 0.75" pistons
for both front and back and most recently, about 4 years ago, I fitted a remote
vacuum assistance unit (designed for Robin Reliants - I think?!?) but to be
honest for various family reasons, I've not driven it yet, so I can't tell
whether it's brilliant or a death-trap. I can tell you it's really really
really dusty. Tom On Mon, 27 Jul 2020 at 09:37, Jim Hearne
<[email protected]> wrote:
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
The contents of this message, and any attachments, may include information that
is private and confidential and should not be read by persons other than the
intended addressee(s). IAM RoadSmart nor the sender accepts any responsibility
for viruses and it is your responsibility to check the email and any
attachments. If you have received this email in error, please inform the sender
and delete the message from your computer. IAM RoadSmart is a trading name. IAM
RoadSmart is a trademark. Charity number: 249002 (England and Wales) SC041201
(Scotland). Registered Address: 1 Albany Place, Hyde Way, Welwyn Garden City
AL7 3BT.
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 25.4 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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