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Discussion Starter #1
I am finished with my race car (105) and found that I have extremely hard pedals. The setup is as follows:

- Tilton floor pedals
- Dual master cylinders for front and rear
- Rear with proportioning valve
- BMW 2002 Tii 4pod front callipers
- Bias adjuster

I read that guys would bore the master cylinder on the stock Alfa MC to 28mm for better pedal pressure for racing. So I think this is where I might have gone wrong. I wanted to stay with this idea so: the Tilton setup now has two 29mm master cylinders (one for front and one rear - with the remote bias adjust). Also worth mentioning: there is currently no brake booster installed.

Whiles bleeding the brakes I started feeling that the pedal travel is at a minimum and is way hard. Was my assumption wrong when I bought two 29mm master cylinders or should I have gone with a smaller bore, now that there are two? Is it wrong to think that because the two MC's work independently on separate lines, they would be the same size as when there is only one?

My options are:
1) I could put the brake booster back on the front line? Is this in any ones opinion even worth trying? And will this possibly give me enough brake.

I am taking the car out on the track this weekend and I am scared I am not going to have enough brakes. I want to (if I had too) at least get the wheels to lock up. I am itching to go like one won’t believe

2) Install smaller bore masters? (Meaning the car will have to wait for the track)
 

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Shane,

I think you've got a mess and need to see if you can still get your money back for the track day.

1. You didn't specify the model of the Tilton pedal kit so we can't tell what the mechanical advantage in the pedal assembly is.

2. If you have a bias bar, why are you running a rear proportioning valve? Remove it for race use.

3. As this is an Alfa BB, we don't know the diameter of the BMW calipers. Getting in the ballpark with sizing the MCs is straightforward math, but we need more info to be able to help.

4. Because of different caliper piston sizes and weight transfer, front and rear MCs won't be the same size. FWIW, Paul Spruell sells a kit to accomplish what you're trying to do. He includes 5/8" front and .700" rear MCs for Alfa calipers. Much smaller than your roughly 1.125" MCs.

5. Sizing is fairly basic hydraulic design. You probably need to do some reading and number crunching to understand what (and why) you're doing.

6. Given your setup, I doubt anyone on the BB can say "from my experience, you need this and that". And in this litigious age, I'm too chicken to tell you what size MCs you need for liability reasons.

I hope some of this points you in the right direction.

Paul
 

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Discussion Starter #3 (Edited)
1. You didn't specify the model of the Tilton pedal kit so we can't tell what the mechanical advantage in the pedal assembly is.
Hi Paul

Thank you! I had a look on your site and printed the "Master Selection Selection Guide" and Ill do the number crunching and see what I come up with. B.T.W I have the Series 600 pedals and 75 Mc's

These ones with 75 Master cylinders:
 

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Shane,

FWIW, I'm not that Paul, but glad you found the proper Tilton documentation.

Good luck, and let us know how it works out.
 

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Discussion Starter #5 (Edited)
sorry paul

If at all possible is there maybe someone that is au fait with this calculation to check with me.
I’ve read the “MASTER CYLINDER SELECTION GUIDE” from the paulspreull.com site. There are some assumptions made that will only be possible to make one’s own if you had the equipment to make your own readings and or could confirm them with experience.
Assumptions:
  • FRONT Callipers will need 700 PSI line pressure to lock the wheels
  • REAR Callipers will need about 600 PSI
  • 120 lbs. was used in their example to calculate the estimates with the pedal ratio. (It does not mention why in the piece)
Calculations:
  • Calliper piston area in A=πr^2 (EACH PISTON on ONE SIDE) = 1.3273229 〖IN〗^2
  • Both pistons = 2.6546458〖IN〗^2
  • Brake pedal ratio: Y/X=6/(1.365 )= 4.395604 ͘
  • So … 120 lbs. X 4.395604 ͘ = 527.472527' ͘lbs.
  • So ... again: Per side\calliper: 527.472527' ͘ ÷ 2 = 263.736263 ͘lbs.
  • Taking the table examples of Master Cylinder sizes in consideration for the FRONT callipers to have a close 700 PSI.
  • These were my estimates:
((b) being the closes)
  1. 263.736263 ͘ ÷ .307 = 859.07577 PSI
  2. 263.736263 ͘ ÷ .385 = 685.02926 PSI (7/10 in² MC area)
  3. 263.736263 ͘ ÷ .442 = 596.68838 PSI
I will have to get the b) (7.10")Master Cyl. for front and one or two sizes bigger for the back???
 

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Shane and all other interested parties,

It would be a good idea to save a copy of Paul Spruell's reprint of the Tilton MC sizing documents that Shane cited. These were written by the best in the business and are no longer available on the Tilton site.

Thank you, Paul, for maintaining them.

Shane:

1. If you want us to check your math, please post the diameter, not the area, of the BMW caliper pistons.

2. According to the installation instructions for your pedal assembly, the pedal ratio is laser scribed on the back of the pedals. I remembered those pedals as having around a 6:1 ratio, instuctions show ~5.15 - 5.75. What do your pedals say? I'm pretty sure it isn't 4.4.

Sorry, but there's room for my misinterpretation of your math (e.g., does n= pi?). If you post the requested info, we'll double check your work. And, btw, Tilton juxtaposed the area and bore diameter headings in that table. ;)
 

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Discussion Starter #7 (Edited)
Shane and all other interested parties,
... please post the diameter, not the area, of the BMW caliper pistons.

Sorry, but there's room for my misinterpretation of your math (e.g., does n= pi?).
- Yes n = pi, and the diameter of 1 calliper piston is 1.29 IN.
Ok, As I currently have my foot rest set, the ratio (as enscribes on the pedal) is 5.29

this changes the calculations as follow:

  • .307 = 1033.89
  • .385 = 824.41
  • .443 = 718.099 :D
  • .518 = 612.741
 

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Alrighty Shane,

Looks like you're closing in on it. Don't forget the rears. However, a few observations:

1. We have no idea what kind of car Tilton was using in his example (mid-engined, aero, dirt?). I wouldn't base my sizing on the line pressures he quoted.

2. Spruell sells a setup for a car similar to yours. He specs a 5/8" front MC, which is similar to the size Tilton indicates in his table for your BMW caliper area. I'd have more confidence in that size than chasing his line pressures since we have no empirical data. Check front/rear sizing split of your BMW/Alfa caliper areas vs. stock Alfa calipers and adjust Spruell's sizing accordingly.

3. I didn't look it up, but I bet the hanging pedal ratio is probably different from yours. May or may not need compensating for, but front/rear spread will remain similar.

4. Tilton indicates that sizing by his method gives a starting point that may need adjustment based on personal preference as to pedal feel, etc. Depending on how hard you use the brakes, how sticky your tire, the coefficient of friction of your brake pads, and pavement type, your weight transfer, and thus optimal MC sizing, will vary.

5. Bias bar is most efficient when centered between MCs. Make the effort (based on your answers to 4) to size the MCs correctly. Remember, in the rain or on the street, because of reduced weight transfer under braking, you will run more rear brake at the limits of adhesion, even while still locking the fronts before the rears, as you must for stability.

6. In any event, I think you've figured out why you weren't happy with your 1 1/8" MCs.;)

This should probably get you pointed in the right direction.

HTH,
Paul
 

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Discussion Starter #9
MC's on its way

I've ordered the new Master Cylinders and its on its way. I went with the 5/8 " as per the Paul Spreull application and then one size up for the rears.

I guess I'll never have it perfect anyways, but I hope to possibly install it before the weekend so I can try it out again on the track. This was the one thing I really wanted to get right on the car and now with everything else sorted is the one thing still not right.

Thanks for everyone's input and help !
 
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