Brake pedal question

[Gacannon]

I design everything on CAD before fabricating to save time and materials. Getting to old to do things more than once. I am good at CAD.

Mustang II disc's and drums with Maverick MC 15/16 bore 1.25 stroke for non boosted disc and drum.

I am not a brake specialist but understand hydraulics very well. Process Engineer for over 30 years. Have bought and read several good books on brake systems but that does not make me a know it all whatsoever.

I read that 6.1:1 is about where you want to start with a non boosted system with pedal ratio.

I read that I need a 15/16' or 1" bore MC to provide the flow requirements for disc and drum application.

I read that 100 lbs foot pressure is a good number to use for normal driving.

100 x 6.1 / bore surface area of .6902 = 883 lbs hyd. pressure.

I am ok with that.

I read that you must design for a full stroke of the MC.

This is where things get big.

Using the small arm and 30 deg of pivot point rotation which a good bit, I find that it takes a small arm of 2.2" in length to provide this kind of horizontal movement with 30 deg of rotation. Any more and it would be through the firewall or floorboards. Now as for the long arm, we are now up to 14 inches in length to satisfy the 6.1 ratio requirement.

I look at the short arm as the key to MC stroke needs and the long arm as the resultant to get the 6.1 ratio to develop the necessary pressure.

Mine is an under the floorboard installation.

If 6.1 is the right ratio to use, it has to be these dimensions to get a full stroke on my MC.

A smaller MC bore seems not to meet my system needs on volume.

Any comments on this.

I have seen more than one installation which DID NOT full stroke the MC. I understand the severe safety consequences of not energizing the secondary circuit if the pedal were to go down with one circuit failure. Most of us have experienced this before. Most systems I have worked with were boosted systems.

Am I correct in my thinking here. If so, I will have to move some things around to get 14 inches of the long arm in there. I can do it, but the fire wrench needs to come out to accomplish it.

wanted to ask some forum talent before the torch comes out.

The key issues here are pivot arm ratio and full stroke of the MC as fundamental design criteria. Is 6.1 ratio right for non boosted system pressure and is full stroke critical?

Bob in Ga. 28 original ford closed cab PU rod project.

 

[Toad]

Although 6:1 is the guideline, I fudged on that because it made it awkward in my truck cab. Can't quite remember what my final ratio was but it wasn't too far off. Mine was non-boosted and under the floor too.

Toad

 

[Gacannon]

Pedal ratio

My frame is a 28, but it saw duty as a rod evidently for quite a few years. It had a weldment setup on the left side for a master cylinder. I positioned the engine a little more forward to get it out from under the firewall so I have a little more room than would be normal.

I cut out some of the old weldment and fabricated a pedal assembly. I can almost get 14" on the long part of the lever arm so I am going with that. Some of the pedal assemblies I see for under the floor mounting don't appear to have a 2" plus short arm on them. They look more like one inch. I don't see how they can get a displacement of over 1/2" with anything like a normal pedal travel in side the cab.

Looking at the original weight of the car these brakes came out of, my weight will be considerably less so a little less brake torque should not be a problem. ITs not going to be a race car.

Wish they made a 4" booster, that would be enough to boost a manual system and fit under the floor board just fine, but the 7" units would require too much work to install and would stick down a good ways.

I appreciate the comment. I build everything from scratch. Just finished the steering column also. My work is good. I have built several experimental aircraft and flown them so I trust my work. Some people say I am crazy to make everything, but I enjoy machine work and have the time.

thanks

Bob in Ga.

 

[zzrodder]

Here's the CP unit I'm going to put in my Caddy project. I've used them a couple times with great results though the measurements on the arm defy the 6:1 rule - the small arm is 2" long and the long arm is 10 3/4" long - and yet it works without extreme pedal pressure.