What size master cylinder to go?

[myw1fe]

Hi all,

Im in the process in fitting a wilwood reverse mount pedal box in my RHD 02 and now so confused to what size Brake MC and Clutch MCs to run.

Im running the balance bar (so 1 MC for front and another MC running rears)

It will be a booster-less setup for 80% track 20%street. Stock Calipers at the moment but hopefully will be up graded soon the the girlfriend lets me spend money again

what size bore to go?

3/4 - 58 - 34 - 78

All numbers to me, i tried to find a calculator and that went over my head.

Any help would be greatly appreciated

[FB73tii]

Size of front & rear masters affects both brake bias and pedal feel. Friction coefficient of front pads and rear shoes (assume you are running drums) and other parameters also affect bias. Do you know what range of front/rear bias you are trying to achieve? For example, a stock tii has roughly 85% braking in front, 15% in rear, and stock boosted pedal pressure is 88lb.

I put together my own calculator for static bias calcs. In doing so I did a lot of research and found errors in some of the online calculators. I could send my spreadsheet to you or am happy to run the numbers for you.

I sized my masters with the overhung Tilton pedal and full Wilwood disc setup to be able to adjust rear bias from roughly 13% to 33% rear bias at 83lb pedal pressure with the short (5.5 ratio) brake pedal. With this setup the pedal is fairly stiff and brake feel is great. It is almost like having my toes right on the rotors, brake modulation is much better than with a boosted setup.

For track you want want to be able to run higher rear bias than stock. You can run more rear bias in the rain than in the dry (less weight transfer to front under braking in rain, so allows rear to do a bit more work). What you do want to avoid is the rears locking before the front.

What brakes are you running now? Stock 2002, stock tii, a mix, some form of big brake kit in front? What size rear wheel cylinders? What track pads will you use front and rear? What is the total travel of the clevis on the bias bar (the Tilton bias bar I use goes from 3/8" left of center to 3/8" right of center, for a total travel of 3/4") and the distance between front and rear master cylinder rods is 2.62".

BTW, used 3/4" master for clutch, same as stock bore (19.05mm).

Best, Fred '74tii & '69GT3

[gracer]

Go to Tiltons website. They have a section where you can fill in all the data on your system and they will send you a recomendation of the correct size. G

[myw1fe]

Thanks Guys,

I will eventually be going big brakes setup but for now Running stock 2002 atm with drums on the rear. the pedal box is a 6.25:1

I do have the adjustable bias, but would be looking to go similar to your with 15-25ish% Rear (Balance bar on pedal box 3/8-24 threads).

Kristian

[FB73tii]

Kristian--You can try contacting Tilton, but some time ago it was posted that they come back with specs for a pretty hard pedal. I did a few calculations for your stock 2002 brake system. Here I assume you are using the same brake pad compound front and rear.

Input parameters are:

1.34" front piston diameter

2.0" front pad radial hieght (estimated)

9.45" front disc diameter

0.625" rear wheel cylinder

9.06" rear drum dia

pedal ratio 6.25

distance between MC rods 2.62" (Tilton overhung pedal spec)

Larger MC's will give you a stiffer pedal with less travel, smaller MC's will give you a softer pedal with more travel. I ran three levels of stiffness for your consideration, each shows rear bias for bias adjuster set at minimum, mid, and maximum rear bias. For reference purposes, assume you press on the hard pedal with 80 lbs force. You will get the same brake torque with 71 lbs on the medium pedal and 62 lbs on the soft pedal. Perhaps someone with your exact setup can chime in and give real-world data on what works.

------------------

Harder pedal

front MC = 1.000"

rear MC = 0.750"

min rear = 11.5%

mid rear = 19.0%

max rear = 29.8%

---------------------

Medium pedal

front MC = 0.9375"

rear MC = 0.700"

min rear = 11.6%

mid rear = 19.2%

max rear = 30.0%

------------------

Softer pedal

front MC = 0.875"

rear MC = 0.625"

min rear = 12.6%

mid rear = 20.6%

max rear = 32.0%

------------------

Let us know what you end up with and how it works out.

Best regards, Fred

[TobyB]

My advice would be to go to the Volvo/525 calipers now.

And the 320 rear drums if you possibly can.

It's pretty darned cheap, they work well, and then you won't be

completely rebiasing everything when you do change. And pads last

lots longer, so in the long term, it's probably cheaper.

The stock fronts DO work for a track car- but they need ducting,

and the ducting for a stock front is very different than the vented front.

So I'd do it all at once.

But that personal opinion aside-

Email Tilton your numbers. They'll get back to you in a day or 2

with their recommendations, and you'll find that they get the bias bar

within about 5 turns of center for aggressive braking. They will set you up 70% front, 30% rear,

and that, from my experience, is great with the balance bar. (so you can

spin a bit of brake off the rears when you need to)

Good luck- it's worth it- it was one of the best things I ever did for the race car.

t

[myw1fe]

Thanks for the advice guys. Making sense now !

Once I Get the pedals mounted ill get the MCs. Ill keep you posted.

Thanks again

[myw1fe]

Just spoke with wilwood and gave them FB73tii specs (thanks again, most helpful)

they came back to me and said run 3/4 front and 7/8 rear.

I do want to go your medium pedal but struggling to find the .9375 in MCs