Mk2 front brake upgrade and master cylinder capacity


(tapped) #1

A few months back I purchased a pair of Wilwood 120-6818 four piston calipers for the front of my Mk2 which is currently being restored. I will be using the original front Dunlop calipers on the rear. The Wilwood calipers are 1.75 inch pistons. I learned recently that the upgrade kit sold by XKs uses the 1.375 inch Wilwood calipers. I’m curious if anyone is using four piston 1.75 calipers and how these work with the stock master cylinder. My concern is that the larger calipers will require too much master cylinder (and hence pedal) travel, especially when combined with the larger rears.

Tom


(Robin O'Connor) #2

To my mind you are going to have to increase the M/c size, not only have you doubled the front calipers voluume but the rears have also raised the requirement.


(Tigger) #3

Wilwood 4 pot fitted to front of our car, standard m/c used. I may be showing my ignorance, but it makes no difference? Once system is bled / full it is a tiny amount of movement required at the piston to activate the brakes, and the difference in the column area of fluid behind a 1.375 inch piston or 1.75 is negligible surely? I have no sensation of extended pedal travel over the much smaller original single piston set up.


#4

My car came with an old XKS conversion kit using 1.375" Wilwoods.
I replaced these with 1.62" Wilwoods #120-6813.
I fitted EBC RK271 rotors and EBC DP4038R pads.
I also ultimately fitted S/S braided lines to the front only.
I ran a trial between the OEM style rubber lines and the S/S ones and have attached a document showing my test results.
The takeaways are:-

  1. Yes braided hoses do indeed stiffen up the pedal as everybody says.
  2. 1.62" Wilwoods work fine with either rubber or S/S hoses.
  3. I could lock the wheels on dry road.
  4. I am sure that you could go to the maximum diameter Wilwoods in this Dynalite range, 1.75" 120-6818, and still have proper operation in terms of master cylinder travel. - I wish I had done that to get more sensitive braking but really what I have now is fine for me.

BruceBrake Braided Hose Testing.pdf (2.9 MB)


(tapped) #5

Thanks for the responses. I may be over thinking this solution, but with time and knowledge that seems to be what happens. I’ll upload a spreadsheet tonight looking at MC volume and caliper piston volumes. Based on my initial assessment my 1.75 x 4 fronts and 2.125 x 2 rears requires ~60% more MC travel than stock to move the pistons the same amount. No idea what this means to pedal travel or real-world setup.


(TheoSoares) #6

I have the XK’s wilwood setup with stock M/C and SS lines and my pedal actually requires a lot less travel than before, perhaps with a fresh bleed and stronger braking power it requires less force to stop the car? Just personal experience.


(tapped) #7

Theo – I assume those are the 1.375 Wilwoods from XKs? Yeah, a thorough bleed is a requirement for any setup. Good to hear about your experience.

Tom


(tapped) #8

Bruce – excellent test protocol and great writeup. Finally had a chance to look at that. Braided stainless seems to be the way to go.

Attached is the comparison I put together of the three scenarios. Assumption around piston travel is a SWAG, but doesn’t change the % increase in MC travel required. I need to check how much travel the MC has. That will have to wait due to upcoming vacation.

Brake capacity.pdf (103.4 KB)


#9

Hello tapped:
Thanks for the comment.
My rear brakes were standard and not the 2.125" you have.
I looked at your spreadsheet and think that the effective piston areas are double what you list since each Wilwood caliper has four pistons (2 each side) and each must move to clamp the disc. Of course the amount of travel at the pad is a SWAG as you say.

My take is as follows:-
There are three stages of M/C stroke

  1. closure of the internal fill port
  2. displacement of fluid to make the pads contact the discs
  3. build-up of pressure to achieve braking action
    Today I measured the maximum stroke on a spare master cylinder at 1.28"

I took a look at the situation on a spreadsheet and it looks to me that it would be pretty marginal in terms of M/C piston stroke if one went to 1.75" Wilwoods.
My calc’s assume a 0.020" pad clearance when brakes are off.
My test results give an idea of the pressure rise with piston stroke: this being a measure of the elasticity in the brake system.
The ratio of pedal to piston travel is guessed at 7:1 (this needs to be verified by measurement and it is non-linear as well)
Clearly my assumptions drive the conclusion significantly
I have attached a copy of the spreadsheet as a pdf. I am not allowed to attach a raw excel file unfortunately for you to play with.
Anyway I hope this is all food for thought!

Bruce

Master cyl sizing.pdf (45.3 KB)


(tapped) #10

Bruce – I’ve seen several references including the Wilwood tech site that says you only need to count the piston area on one side of the caliper which would equal the 4.8 square inches per front caliper (9.6 for entire front axle). That’s counter-intuitive to me…but I’m not a fluid dynamics engineer!

Need to do some more research on this. I either keep the calipers I have and give it a try or I sell them and buy smaller ones assuming the 1.75" calipers won’t work. The car won’t be on the road until probably next year. I already had the original 2.125" fronts re-sleeved in anticipation of putting on the rear…so those will likely stay unless I want a complete do over!

Below is from: https://www.wilwood.com/Tech/TechTips


#11

Well I am no expert on brake systems but I think that the piston area defined on the Wilwood page is used when you need to calculate the force being applied to the pads. Not the fluid volume needed.

Consider a sliding caliper with one piston versus a fixed caliper with two opposed pistons.
Assuming same dia pistons, at a given pressure they will exert the same force on the pads. So indeed just one side area is used to calculate force.
But to displace fluid to get the pads in contact with the disc, you must consider all the piston areas.

I just found this site that provides methods for designing brakes.

https://www.engineeringinspiration.co.uk/brakecalcs.html

Check the paragraph
HYDRAULIC BRAKES
Brake Fluid Volume Requirements

Segue
I did not realise you could fit front calipers to the rear - interesting idea.

The deeper you get into this the more complicated it all becomes.

Bruce


(tapped) #12

You may be correct and that’s why I said the advice seems counter-intuitive. However, I believe we ended up at the same place because I doubled the required piston travel in my model. I assumed each piston needs to travel 1/64" (x 2 is .03125). I have no idea if this is accurate. Here’s a link to my Excel file on OneDrive: https://1drv.ms/x/s!AnpOM7cFWalQ0IQjx8LWdn0N56Jh7g

Yes, stock front calipers are a direct bolt-in to the original rear carriers.

Still not concluding anything, but I do believe your assessment that my brake setup is at the very limits of the MC is probably correct. I’m not close to putting it together yet, so some more research and fine-tuning before I make any real conclusion. On the other hand I could just put it all together (which will be in a ~year) and see how it works…but I try to avoid rework scenarios whenever possible.

Excellent article you found, btw. Will have to study that. Wrt volume requirements, it still comes down to running clearance (which I called piston travel). I haven’t found a definitive source for what that should be. I also like the reference to housing stiffness which ties exactly to your analysis on braided stainless lines. At minimum these will be part of my solution and would seem like a requirement.

Tom


#13

Tom
Well if its any consolation, I have dug deeper and I think that normal running clearances can be expected to be as little as 0.015". What drives the clearance is the elastic response of the piston seals and the unevenness of the disc. I have redone my calculation and see that this gap is a major factor.

Here is my excel file:-
https://drive.google.com/file/d/1qSFhAleq2TycNmgkGNg5HNaDtQPMIAS6/view?usp=sharing

This conversation makes me think I should re-run my tests and measure the M/C piston motion directly as well as measuring the pad clearances. I would like to know how near I am to bottoming the M/C piston.
Bruce


(Robin O'Connor) #14

Just to throw another thought into the equation, what’s the diametre/stroke of the booster ?
I have just had seals manufactured for the VH40 booster that I have in my ‘S’ and that is only .750”


(tapped) #15

Good question. I’ve been assuming the MC is the constraint in the system…if it were the booster I’d be surprised…but who knows.