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PostPosted: September 19, 2016, 10:03 am 
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I have a two master cylinder brake setup using a bias bar. But I am still getting to much rear braking even with the bias bar set to full front. I am getting more brake dust on the rear wheels then the front, plus under heavy braking the back end is very loose, floating around. So I need to play with the master cylinder sizes to get less pressure going to the rear brakes. So this is the question: To reduce the brake pressure to the rear brakes I make the rear master cylinder LARGER?

Thanks

Graham


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PostPosted: September 19, 2016, 10:08 am 
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Location: Duxbury, MA USA
Yep.
making it larger reduces the force per unit area in the master, which in turn reduces the pressure in the wheel cylinder.

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PostPosted: September 19, 2016, 10:18 am 
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Yes, larger. The key to understanding what's going on is to always think: force = brake pressure * area. For the same pedal pressure, a larger master cylinder increases the area, decreasing total force.
[edit] Sigh, I goofed, fixed. Thanks Driven5.

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Last edited by KB58 on September 19, 2016, 1:09 pm, edited 1 time in total.

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PostPosted: September 19, 2016, 11:29 am 
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KB58 wrote:
force = brake pressure * area
FTFY

The way I look at it though is pressure = force / area. This is because the pressure will always be the same on both ends of the braking system. So what you get is:

master cylinder force / master cylinder area = caliper piston force / caliper piston area

So if you hold any two constant, it makes it easier to understand what raising the 3rd will do to the 4th as variables. Making the variables both on top or both on bottom will have a direct relationship, while having one variable on top and the other on bottom will have an inverse relationship.

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PostPosted: September 19, 2016, 12:30 pm 
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Just like everything we do there are some practical implications and tradeoffs to be made. Everything above is correct.

Can you tell us the current sizes of your master cylinders? The size and number of slave pistons in your calipers would be nice to, but maybe we do't need hard numbers.

The flip side of the above explanations involves the amount of brake fluid that is pumped when you use the pedal. This comes into play in taking up the slack etc. in the system until the pads actually touch the rotors. Even then things like brake calipers and also brake lines have a springiness to them and they bend in use a little bit.

So you can use a larger rear master cylinder or a smaller front one to get where you want. You have a tradeoff between how firm you want your pedal to be, how far down it travels before the brakes start to work and how much effort you want to put on the pedal to stop at your desired rate. Oh, they also only make certain sizes - no 1/8" bores or 2" bores.

The good news here is you can really get what you want! Your car is very light and there will be no problems getting a good solution for you.

I like a hard pedal, my race car basically feels like there is a brick under the pedal when you use it. People are used to power brakes which are often calibrated for even elderly and infirm people to be able to use easily. Things is your body is good at carefully applying large forces to your feet. Every step you take you support your full body weight on one foot.

I don't have numbers off the top of my head but you probably shouldn't be afraid of needing say 100 lbs. of force to lock your wheels up ( if that seems too much use 50 instead or whatever - I'm just picking a round number ). That would scare someone used to only power brakes, but heck you stood on one foot just getting into the car. What you get here is that that pedal will feel solid - when you modulate the pressure it will happen right away because the pedal is not spongy. If the pedal is spongy your foot will need to actually move up and down to change the pressure.

An important point on the setup of a dual master cylinder is that the master piston rods should be close to snug against the balance bar. Send us a picture of your unit so you get this right. The point here is that if one circuit fails and that piston rod just travels downwards uselessly, the adjustable part will bid up and still push down on the good circuit. The thingies the master piston rod threads onto should have a shoulder on them to cause this, but they can be installed backwards if you don't realize this. I learned these things by screwing it up...

If your pedal does not already feel rock solid perhaps there is just a little air in the front circuit? That may give the same symptoms. You would have a really soft pedal though...

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PostPosted: September 19, 2016, 1:11 pm 
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They are Wilwood calipers front and rear. Tomorrow I will get wheels off and find the part number, I am sure we can find the piston sizes from the Wilwood web site.

Graham


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PostPosted: September 19, 2016, 2:58 pm 
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To add a little more to the information.
The balance bar should be parallel to the balance bar tube with the pedal pressed fairly hard. This means the push rods may NOT be adjusted evenly! Be sure all the air is out of the system.
The blocks on either side of the balance bar should allow the bar to rock back and forth the full extent of the tube but no more. Too much clearance can change the adjustment due to float and too tight defeats the purpose of the balance bar.
A good written description is here -->
http://www.wilwood.com/TechTip/TechPedalTip.aspx

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PostPosted: September 19, 2016, 3:16 pm 
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Wow, I did not know you had to adjust the bias bar! You learn something every day.

Graham


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PostPosted: September 20, 2016, 10:57 am 
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There is a brake calculator posted in a thread on this forum that takes the parameters mentioned previously into account and lets you play with different size master cylinders, etc. I found it very helpful. I'm traveling so can't dig it out easily, but search on Brake Calculator.
Kartracer, do you still have the thread link?

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PostPosted: September 20, 2016, 11:48 am 
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Thread name is Brake Components Calculator. Can't copy the link on this mobile device but a subject search will get you there.

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PostPosted: September 20, 2016, 12:19 pm 
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This one: http://www.locostusa.com/forums/viewtopic.php?f=5&t=14424

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PostPosted: September 20, 2016, 6:25 pm 
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Well after checking the calipers and master cylinders and plugging them into the spreadsheet. I have a 70% rear brake balance, oops. Just swapping the front and the rear master gets me a lot closer, but I will have to check the Wilwood web site for my master cylinder options.

Thanks everyone, I will tell you what the outcome is.

Graham


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PostPosted: September 20, 2016, 7:32 pm 
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Maybe if you lean forward while you're driving? :)

Maybe your adjustment will be enough after you swap the cylinders... Good luck.

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PostPosted: September 20, 2016, 8:45 pm 
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I was going to say how much pedal effort?

You might want to make the front smaller (instead of making the rear larger), this will reduce pedal effort.

Making the rear larger will likely increase pedal effort.

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PostPosted: September 21, 2016, 12:17 am 
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I am currently running .700 front and '526 rear. I cannot just go smaller on the front, I think .500 is as small as they go. So I think I will go .526 front and .700 rear, or .750 rear. I am running a hydraulic emergency brake and I think that is a .750 so that would also be a simple swap.

Graham


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