mgkluft wrote:
Lonnie,
. . . Your problem will easily be fixed with tilting the pedal forward (or more appropriately backward towards the driver), and adjusting your pedal surface angle. . .
There's definitely something to both of those changes, Martin, but there's a little more as well.
It takes a little time to do the analysis on some of these things, but I like to feel confident I've made a good, reasoned choice. Most people wouldn't bother much further than a good guess, and that's OK if you're comfy with doing so. I'm willing to push it a little further to increase my confidence level. It's not that hard if you're willing to put a little time into things.
I took a print out of my pedal design, full size. If you have a modern dot matrix or laser printer, these printouts are very accurate. That I glued to some cardboard. You don't even have to do that to observe gross behavior, but I wanted to see how much the pedal moved forward at the pedal pad, and what the angle of the pedal pad would make with my shoe and the "floorboard." You could just draw out a simple vertical bar shape and carefully mark the distances between things. You'd loose visual information like the angle of the pedal pad, however. Using a small finishing nail, I attached it to a small piece of plywood through the pivot hole center mark.
Attachment:
Cardboard Brake Pedal-Small.jpg
I cut out a rectangular piece of carboard 4-1/2" long by 5/16" wide to act as the plunger rod. With the card plunger stand-in, I put a brad through it into the center of the balance bar pivot point (the center of the tube/rod combo in real life) on my printout and let the loose end ride atop a small, thin, piece of 1/8" thick wood trim that is flat and square that the plunger rod slid along. So the block of wood stood in for the end of the master cylinder, but the cardboard rod was not restrained.
With the rod level at the start, I just rotated the pedal and could easily observe through tick marks on the rod and plywood how far the rod moved forward as the pedal turned. It was also easy to see when the majority of the pedal movement started to go downward as the rigid card rod started to rotate up off the wood block and you could see the pedal end sink below horizontal. I got about 7/16" (+/-) 1/16" of travel before the "over center" behavior started. That's less than half the full travel of the master cylinder. I don't feel like that's enough for safety.
As you can see from the image above, I started moving the rod up the pedal, increasing the distance between the pedal pivot point and the rod pivot point. I move the block of wood each time so that the cardboard rod was level at the start. This was just a "quick and dirty" way of measuring how much more the rod would move forward into the master cylinder before "over center" happened. That gave me a good idea of the target distance I wanted. Given that the master cylinder control rod height is fixed by the chassis (unless I want to bring the master cylinders inside the cabin, which I don't) and I need to keep the brake pad height inside a certain range, that limits my choices too.
Using the free Calc program from Open Office, I set up a simple spreadsheet to try out some different configurations:
Attachment:
Distances Screenshot-Small.jpg
Wanting to keep the pedal ratio at least close to 5:1 for my wife, but realizing I'd have to increase the pedal height some, it looked like a pedal height of 8-1/2" was a good choice. I'd always planned on a heel rest anyway. If I can make an adjustable one, and give my wife an extra "lift" of 1" with that, we'll be effectively back to a 7-1/2" pedal height. Coincidentally, 8-1/2" is just about ideal for me.
I just re-entered the pivot height, increasing it by 1/16" each time and looked at the output:
Attachment:
Final-Distances.jpg
From that I concluded that using either of the 2 circled entries would give a good, all-around solution: significantly more near-linear rod movement before "over center" is reached; a decent pedal ratio; and a pedal height I could work with using a little creativity on the heel rest. The main thing is that I feel I know what is going to happen and that I've balanced out the factors reasonably given the real-world constraints of my build.
Back to martin's points quoted above. I'm rotating the pedal pad downward to increase shoe/pedal contact as the pedal nears the over-center point. I won't be increasing the distance back towards the driver, as the face of the pedal is just where it needs to be based on our heel location and the angle our feet can make comfortably. We developed those measurements from mock-up, wooden pedals we did some months ago when the chassis was on blocks and it had a seat (my old Mustang seat cushions) and floorboards using plywood clamped in place.
Anyway, V3 of the pedal design will be done with much better information and assumptions.
Cheers,
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