brake line pressure

Hi al, I am not so familiar with lbs/psi.

I have made some calculations to check brake line pressure, I presume 75lbs (leg) pedal pressure is correct? times 5 for the pedal ratio gives me 375lbs on the MC. I would like to use a 5/8 MC to start testing (small pedal ratio and no brake booster) but if my calculations are correct this will give me 1221psi on the brake line and the willwoods take 1200 max. the question is, are my calculations correct? or do I miss something?

( 5/8 MC= 0.625x0.625= 0.3906 SqIn. X 0.7854 (phi/4) makes 0.307 SqIn. so 375lbs divided by 0.307 gives 1221 psi) am I forced to use a 0.7" MC to lower the brake line pressure because the calculations are correct or did I do something wrong? please, feedback required (ps a 0.7MC gives 974 psi with the same calculations)

thanks, Rick
 
You are assuming a single 5/8" master cylinder to operate both front and rear brakes. Typically, there are two masters on a balance bar system. The balance bar geometry decreases the force into each master cylinder by roughly 50%.
 
@ Bill, I know there is much more into it but for now I just want to now if the max. brake line pressure that can be achieved with my pedal/MC combo does not exceed the factory safety limit for the calipers. because if it does I don't even consider using the 5/8MC for testing.

@Bob, this is very good information I was not thinking about! I do have a balance bar to split between F and R, so yes the force will be a lot less just because of that. a very big thank you for your reply!
 

Ian Anderson

Lifetime Supporter
Supporter
Great the numbers worked the same
And yes the 2 masters will half the force approx
But still amazing how little margin there is So have a moment and BIG push on pedal and you will beat their number!

Ian
 
Ian, think I am going to place a bathroom scale against the wall and see what I push......:) (just to be safe)
 

Howard Jones

Supporter
I have used this calculator a lot and I fine it to be pretty good at making comparisons from one master size to another for example.


With a balance bar set at equal bias to front and back and two equal size masters you do indeed 1/2 what would be the single master calculated line pressure and then apply that new result to each end of the car. So I am getting 610 psi line presure applied to the calipers at both ends of the car with 75 leg pounds, 2 ea 5/8 masters, and 5 to 1 peddle ratio. That is well within caliper max pressure ratings and I am in fact using 5/8 masters with a standard Tilton peddle box (5.6 to 1 I believe) in my SLC .

75 pounds of leg pressure is a good number to use in this calculator. And I fact I have read else ware that a common good leg pressure depending on driving style is anything from 50 pounds to 100 pounds in a un boosted brake system.

When you try the bathroom scale remember that the other foot is being used to work the clutch and the one on the scale is also being used to modulate the throttle, hang it (left foot) in the air and off the ground and use only the right one. When I did this I found that it wasn't a very good test but in fact 50,60,70 felt about right for me.
 
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Randy V

Admin
Lifetime Supporter
I had pressure gauges on my Cobra racecar. I would dial in (via balance bar) 1000 psi front / 800 psi rear at the beginning of a session with new pads and/or tires. I had a 6.5 pedal ratio and was giving the pedal all I had to give it. According to my videos which I could see the gauges, I would periodically approach 800 with the front under extreme hard threshold braking.
Edit - 3/4” MC front and rear as I recall.
 
Rick i have used a 5/8 front and 3/4 rear master cyls on my race cars with a 5-1 pedal ratio for god knows how long, and found it to be extremely good, both in pedal feel and braking performance..
cheers John
 
John. I installed a 0.7 F and an 0.75 R this weekend, gives little less then 2" pedal travel (very hard) so this weekend going back to 5/8 F and 0.7 R just to see the difference.
 

Randy V

Admin
Lifetime Supporter
If you install brake pressure residual valves, you’ll have a much taller pedal and less travel. 2# valves work wonders.
 
Randy, I have one in the front brake line already, (2psi) helped a bit, I was testing without the balance bar and a single MC first just to see how each on their own would feel. so I noticed an improvement after installing a residual valve.
 
I take a simplistic approach to this, I have had numerous duel master pedal boxes and have sold over 30.
Most have ended up with 5/8 front and rear and just balance it off on the bar.
If the pedal is to hard the cylinder or cylinders are to large, if low to small, when correct the feel is some ware between boosted and non boosted.
I have driven cars with to larger cylinders and they are always a mission to stop, airing on the smaller size reduces the pedal effort.
I apply on the brakes in a straight line keep adjusting the bias till I can stand on it and just feel it having a slight weaving in the rear end then add a small amount to the rear, after that I generally never touch it again.

Jim
 
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