This is more a physics question... I think but I'm sure someone here is better at math than myself. I am building a hot rod and it's time to do the brakes, or master cylinder that is. Pedal ratio is the ratio between the length of the pedal arm to the fulcrum or pivot point compared to the length from the same fulcrum to the point the master cylinder is connected. I am looking for a 7-1 ratio and that makes the long arm 14 inches long as the other is 2 inches. The end of the long arm is 3 inches from the floor so my question is, can I weld an extension to the lower end of the long arm bringing a contact point upward off the floor and maintain the effective 14 inch length and have the pedal pad higher from the floor pan? A perfect reason for paying attention in high school math. The car is a 26 T turtle back roadster with a 406 small block so I need the brake calcs. to be exact. The ideal master cylinder dia is 12- 14 % of the total dia. of all other driven pistons and that makes it a 1 inch piston. I have installed a .75 cylinder to up the effective %. Thanks for any help with this one
Bill

 

Your ratio would be determined by measuring from the pedal you're using as a contact pad for your foot to the pivot, then from that point to the master. This is in a direct line from the pedal to the pivot and not through the path of the pedal arm/extension.

I should add that you may be able to do what you're suggesting if you were to imagine a circle surrounding the pivot with a radius of 14 inches and the contact pedal for your foot fell on the arc of that circle.

 

So moving the pedal upward towards the pivot point would diminish the effective advantage directly based on linear measurement to the fulcrum? I was hoping the overall length would be realized as mechanical advantage.... poop
Thanks, Bill

 

I dont quite get the question.
You want the pedal off the floor more so you want to weld an extention.
Does this mean that you would weld an extention that is 90 degrees to the brake arm toward the drivers body?
If not, in what direction would the extention be put on and for what objective?

 

Yup, that is the way it would work. This talking in terms of absolute leverage only = the force being applied and what comes out the other end. However, because of the positioning of the foot pedal relative to your body position you may very well be able to exert more force or exert the necessary force more easily by putting the pedal in a different location and overcome the loss of the desired 7 : 1 ratio.

 

My thought was to attach an arm at the longest point of the arm returning upward away from the floor while leaving the point of effort or force at the longest point of the arm. I have done a few other set ups moving the pedal o a different location for the purpose of clearance and had it fly but as stated earlier math wasn't my strongest suit, Thanks to all for the ideas.
Bill

 

I drive trucks and itty bitty gas saving small cars. And there is a LARGE difference in brake pedals and clutch pedals as to how much pressure you need to apply to get them to work.

My old truck you had to really cram on the brakes to get them to work. Then I went to buy a newer truck with power brakes and I nearly threw the salesman through the windshield the first time I applied those brakes! (Very touchy brakes.)

So judging by the different vehicles I have driven, I would think the brakes don't *have* to be an exact specification so far as this goes, rather it seems there is a "range" of ratios which would be acceptable.

If you know the brands/models of vehicles that master cylinder is used on, might want to "cheat" and go to the wrecking yard. Take some measurements of the pedals on those vehicles. Might be interesting what you find!

 

I am actually trying to build a car that will be sold, I enjoy building and if I can cover my costs for my hobby it will be all good as I am otherwise retired. As for the brakes I want the car to be truly a driver so it needs to be really comfy to drive for some one. I worked on Mercedes Benz for 40 years so my target is sort of high. The other part is it will likely weigh around 2000 lbs maybe a bit more and the engine I am building will deliver near 500 hp with about 520 lbs torque so it will have a potential for being scarry by itself. If I do it right it wont have to done again to correct an oversight on my part. I am using an adjustable proportioning valve so I can tune the brake function to the chassis. The hard part is I wake in the middle of the night with design thoughts and get on the net to research stuff, I need a rest.... LOL Thanks again
Bill

 

I don't think that looping the long arm upwards to raise the pedal pad will affect the leverage of the 14"arm providing that the angle of downward pressure is not significantly changed. Would the pedal pad be of an additional width from top to bottom to still cover the lower view of the long arm from the driver position, example making the pad 2" wider than what you see today? In regards to the leverage concern, I can still remember driving old Chevy and Econoline Vans from the very early 60's that required what felt at times like straight downward force as the brake pedal arms went through the floor boards with only a very small curve. Maybe that is where the term 'standing on the brakes' came from. We have come a long way since those days. It sounds like you are having a lot of fun, and truly making a performance ride that you can be proud of. What an amazing power to weight ratio. Kind of like dropping 500hp into an old VW Beetle.

 

In that case, I read something about the average pressure on a brake being 100 ft. lbs. So here is a google search with that figure in it. Might find some interesting stuff...

Google

Here is another with the word engineering in it...
Google

Here is an automotive engineering forum...
Eng-Tips = Automotive Engineers Area

Then some engineers hang out at the following. They have been more political with their discussions lately. This is google groups or "Usenet" alt.autos.gm
Discussions - alt.autos.gm | Google Groups

 

Hey, thanks for the links. I actually joined the automotive engineering site, tons of cool stuff. The required pressure for a brake system is between 1000- 1200 psi. Applying the leverage formulas it should be around 120 lbs, an average male should be able to muster 300 lbs briefly but if less muscular folks are to drive it a more advantage should be designed into it. At least thats what I have gleaned from my late night searching...... instead of sleeping.
Bill

 

Just another wild thought you might be able to use a smaller bore M cyl to increase the hyd advantage if the pressure required is too high.