Brakes, jumping to conclusions and maths

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russ price

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Jul 26, 2020, 5:54:36 AM7/26/20
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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 ......."

Bill Allison

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Jul 27, 2020, 3:36:46 AM7/27/20
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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)


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.



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russ price

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Jul 27, 2020, 4:24:27 AM7/27/20
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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.
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Jim Hearne

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Jul 27, 2020, 4:37:18 AM7/27/20
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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
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Tom Walker

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Jul 27, 2020, 10:20:41 AM7/27/20
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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


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Jim Hearne

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Jul 27, 2020, 10:21:27 AM7/27/20
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Hi Toms,
Whose maths are you querying ?

Jim

Tom Walker

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Jul 27, 2020, 12:42:12 PM7/27/20
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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
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Bill Allison

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Jul 27, 2020, 2:23:24 PM7/27/20
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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)


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flob...@gmail.com

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Aug 28, 2020, 4:55:36 AM8/28/20
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I've now fitted a 19mm bore master cylinder from on Citroen Saxo and a servo from a Mondeo, adjusted sevo to m/cylinder gap and eliminated as much play from linkage as I am able. Brake bleeding was a 'mare. After trying several times and methods I disconnected the 4 pipes to the m/cylinder and bled just that with no problems, I then reconnected and bled each wheel in turn and all was ok until I reached the O/SR which  still had bubbles after nearly a litre of fluid had been drawn through,   I renewed joints in the pipework as I'd convinced myself air was been drawn into the the system somehow, how I have no idea. However after remaking joints and renewing pipework which was now too short it bled easily and the pedal feels fine. This may not be the end of the story however as there is a lot of flex in the m/cylider/servo bracket which needs sorting at some point and i'm not sure if the servo i too powerful but i want to bed the pads/shoes in and maybe just get used to the feel before deciding if that needs attention.

Jim Hearne

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Aug 28, 2020, 5:17:09 AM8/28/20
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I’ve used both a pressure bleeder (puts pressure in the master cylinder) and a vacuum bleeder.
They are easier but you have to be careful or you can flip the seals in the master cylinder.
It’s also very messy with the pressure bleeder if it pops off the top of the reservoir, brake fluid everywhere, BTDT
 
 
Is there a bracket from your servo brackets to the bulkhead ?
It usually goes down to the steering rack bolt.
 
Also, on the pedal box, there should be a fixing going up vertically from the top of the pedal box into the scuttle area (a length off 10mm studding works well).
 
Both of these don’t seem to get fitted to a lot of the Saloons and 2+2’s
 
Ideally there should be a reinforcing steel plate on the bulkhead around the servo bracket fixings , transfer bar fixings and pedal box fixings to stop the fibreglass flexing.
I covered the whole bulkhead in the engine bay between the chassis rails with a stainless sheet (about 1.5mm i think), bonded on with sikaflex.
Apart from spreading the load it meant that all the original grommets fitted properly in the thin sheet. (make the whole in the fibreglass behind a little bigger).
As well as plates on the inside of the car.
 
On my 2+2 i put two 3mm x 25mm stainless straps from the top 2 servo mounts up to the chassis rail behind and above the servo.
This made quite a difference.
 
I also tried boxing in the 2 servo brackets but that didn’t make any difference.
 
Jim
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