i tend to over think things sometime. i am putting my brake pedal in my car and was looking at the rod that comes out of the master cylinder and attaches to the bell crank lower arm, what angle should the lower arm swing in. as the pedal is pressed, the arc of the lower arm goes from level then down and if it goes farther it gets level again. i know where the angle is will make the pedal not push in far enough but what angle should it be on. or am i over thinking this.
@flatheadgary Earlier today I was reading a thread here about valve rocker arm geometry that has a lot in common with your question. From what you describe, I’d say your pedal arm geometry is ideal. Here’s why I think that. The initial angle is shallow and the rod travel slight....and brake application is therefore not sudden. Further movement increases the effective length of the lower arm as it swings through it’s arc, increasing rod/ MC piston travel rate, increasing the application rate for a given amount of pedal movement. I would think that is exactly what should happen. It permits brake application modulation. If the brake system is sound, leak free and properly adjusted, more pedal movement probably isn’t needed. However, if the system is deficient in any one or more ways, the pedal may travel farther, even bottoming out the piston(s) in the MC, and you will be back to reduced travel because the arm’s effective length is reduced. But if that is the case, the arm ratio is the least of your problems at that point. Ray
now that's funny Ray. i was looking at that thread too. that's what made me think about my brake. hey thanks for the reply.
You want it pushing as straight as can be along MC bore axis, however there is a slight off axis angle due to the stroke when pedal is depressed and released. I have under floor in my 35 and under dash (Pendulum) in my 46. Under floor arc is on vertical plane (Up and down) whilst under dash arc is on horizontal plane (Side to side); albeit very minimal provided you have the correct pedal ratio. Through firewall is the same as under floor.
thanks for the pic. so from this pic, there is no angle from the mounting bolt to m/c rod. it is straight up and down right?
No angle as rod is on the same axis as MC bore at set up, during braking stroke the rod will push MC piston forward in bore and move slightly off centre to compensate for arc due to pedal movement. Pedal pivot axis is perpendicular to rod, you want uniformity and for the rod to push straight and not be off centre in horizontal or vertical plane. When viewed from above pedal axis pivot should match side schematic and be perpendicular to MC axis. The underdash unit only changes the direction of motion 90 degrees, ratio remains the same. Sorry, not an engineer but lay mans terms. Keep it simple, things work better in a straight line with less stress on associated components. Others may better explain the linear motion rationale ensuring applied forces are equal and constant.
There’s a few things to think about. The whole brake pedal thing is based off a ratio, yet built with rotating arcs creating linear motion. So it’s never going to be a perfect direct translation of the ratio but there will be a range where it’s closest. within this range there’s a first part that gets used quite often, and there’s a second part that gets used only a few times - in mock up, during bleeding, and should you have a system failure. Within This second part its very important to obtain full stroke at systemic failure but not very important to the angularity of approach. Another thing to think about is the available foot room and stroke of the foot pad. Along with that is the ergonomics and comfort. Within these two areas of thinking lies the everyday use and the ability to satisfy the things listed above. So all that being said, I’ve not been fortunate enough to use a brake pedal out of box without cutting and modifying it in some way so all those conditions and concerns are met. What you don’t want is high angularity approach in the most used zone of normal braking. And you don’t want an uncomfortable to use foot pad. All of this is really easy to figure out with cardboard and popsicle sticks and push pins on the bench in full mock up. It’s kinda suck doing it all under the car.
Nothing to add really but – not to give the OP ideas – this got me wondering if there would be any advantage to having rising-rate geometry in a brake pedal linkage?
I think it's pics like that that get folks off on the wrong foot! The pedal to pushrod intersection should be clocked at something like 7 o'clock in that pic. In the real world the straight line travel between say 7 o'clock and five o'clock needs to be about an inch and a quarter, or whatever the travel of the master is. Interesting point made earlier about the changing rate of barking through the pedal travel. Pretty obvious when you think about it but I'd never made that connection! Every day's a school day!! Chris
Best to get it right, I had too much angle going into a Clutch Master and it ovaled the bore at the worst possible time. I was at the HCPU and still had to drive back home to TX. Drove home with basically no clutch pedal, TKO600 did't miss a beat though. I only had to move the hole down 1/4" and it was perfect the second time.
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