I have my master cylinder under the floor due to space limitations it looks like the best pedal ratio i will have is 4 to 1 ratio for manual brakes I have disc/drum setup I know a ratio of 6 to 1 or better would be more desirable however is it possible to use a different size master cylinder to compensate for the pedal
Maybe, but.... I guess the final ratio is what is important here, how much petal movement to how much fluid is moved. But here comes the but part, so to get that ratio you would need a smaller dia master cylinder or the pedal would be too hard to press. And this would move less fluid, maybe not enough to apply all 4 wheel cylinders. Is the ratio on a power brake system less? maybe look into that and see if you can go that route, with a booster it may make the pedal easy enough to push. If the problem is that you can't have a taller pedal in the car, can you make the distance from the pivot to the point that the master cylinder push rod connects shorter?
Good to see facility for a grease nipple, said the guy who's brakes only today stopped releasing due to lack of lubrication, or lubrication facility! Not so bad after 25 years and about 70k including Boneville etc. Make the pivot to master connection shorter. Might need to move the master upwards though to get the straightest shot into it. Only needs about 1.5" of travel to max the master out. Make sure the swing / clocking of the pedal to master pushrod connection is nicely spaced around center ( side view) to get the travel right. Chris
The area circled in red is the pivot to master cylinder pushrod connection it currently is approximately 3 in Center to Center Taking Happydaze suggestion I was thinking I could just drill a new hole and decrease the distance to 2 inches Center to Center and it would allow the push rod to have a fairly straight entry onto the master
It's never going to stop well if you don't get the ratio around 6:1 So either lengthen the brake arm to the pedal, or shorten the arm to the master to get the right ratio. The arm to the pedal can be cut and angle changed to raise the pedal if you have room inside the car to raise it.
If the current ratio is 4:1 and the length from the pivot to the push rod hole is 3", that means the straight line length from the pivot point to the pedal mount is 12", so moving the push hole to 2" should give you a 6:1 ratio. One thing to check is the length of travel the push rod hole would have. It needs to be slightly longer than distance it takes to bottom out the master cylinder. Also you need to be able to bottom out the master cylinder before the brake pedal hits the floor board. Ideally the line from the pivot point to the pushrod hole should be 90* to the master cylinder at the mid point of normal travel. As to master cylinder size, what size did the OEM use for those calipers? Going as small as practical will create the most force.
Good to see you have looked into this and get the basics! I'll say that the rules of thumb are pretty accurate. Trying to bend the rules will mean something gets compromised. Changing the ratio will lower the amount of force needed to actuate the brakes to a reasonable force, but you also need to consider pedal travel and amount of fluid displaced. The pedal has to travel further to push the same amount of fluid. That amount of fluid must be enough to operate the brakes with a decent safety margin. Here is a site with all the math. It's Not as fun as a new go-fast part, but brakes allow you to go fast more than once! https://motionraceworks.com/pages/brake-system-setup-and-calculations
If you stay with 4:1 ratio work on legs at the gym, you’re going to have to push hard for it to stop. A smaller master will certainly help but pedal travel will increase
I've got manual disc/drum with a 5:1 pedal with no room to change it nicely. Car and me are under 2000 pounds and it's a STIFF pedal!
Here is some info on figuring what you might be able to juggle........ I'd get it fixed before you move ahead, no matter what you have to do to get it working properly.
I don't see how you calculated the brake pedal ratio. Here is some help. https://mpbrakes.com/calculating-pedal-ratio/ What ratio do you get using this info? Notice that measurements are made with vertical lines from the pivot point.
I did use that formula I've been calculating and mocking a few things up today I figured out how I can get a five to one ratio but that would be a little tight on my feet I have a rather large foot size 14 My next challenge though is the pushrod travel I only have about an inch and an eighth of travel
On one rod, I raised the lower hole on the pedal toward the pivot. I also redrilled the frame bracket to allow me to also raise the master cylinder. These modifications really improved the manual brakes on that hot rod. You might consider modifying the master cylinder mounting location.
For safety you really need enough pushrod travel to bottom out the master cylinder and that needs to happen before the brake pedal hits the floorboard. Also do you have a better option than those two butt welded joints in the arm?
it looks to me that if the floor was 3/4" thick and you shave a little off and then ad a carpet or mat, the pedal will only move about 2" before hitting the floor. that means with your 4 to 1 ratio the plunger would only move a 1/2". it won't work with any master
The Jalopy Journal
