# Car brake-pedal force

## Main Question or Discussion Point

Car brake-pedal force....

Hey all,

im posting because i would like suggestions (preferably with proof) on how a relationship between initial velocity of a car, force to brake pedal and desired braking distance can be formulated. Assuming we know the velocity and the desired braking distance from this initial velocity?

so far i have the equations for forces going through the brake system but only once a foot force is generated.

for example,

biasForce = footforce*Ag

where Ag = mechanical advantage of pedal (found by taking moments about the pedal axis)

biasForce = force transmitted to bias bar.

After this I have pressures generated at master cylinder etc, but id like some references to where I can find a suitable equation to calculate the needed foot force. My idea was to take real tests, and set up a table, but I haven’t managed to get hold of a device which I can strap to my foot, while im braking in my car!

any help appreciated,

jOn

## Answers and Replies

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russ_watters
Mentor
I'm not sure braking force is linear with pedal force anymore, since they are hydraulic or drive-by-wire (not sure if there is much of that), but friction on the brakes is directly proportional to the force between the brake pads and the rotors.

...

I'm not sure braking force is linear with pedal force anymore, since they are hydraulic or drive-by-wire (not sure if there is much of that), but friction on the brakes is directly proportional to the force between the brake pads and the rotors.
Im ok with the frictional forces for now,

some sources say average foot force is 100Ibf but it seems a bit vague to use just one force measurement.

if anyone has any suggestions for working this out mathematically please let me know,

otherwise my best bet is to try and find a measuring device which can take readings while im braking and use that data

thanks jOn

Danger
Gold Member
It shouldn't be too difficult to find or make a pressure transducer pad that can fit over the brake pedal. Perhaps an old-style trailer brake controller can be adapted for that purpose. I seem to recall that some of them were proportional (but I'm not sure).

nearly all cars have power brakes
run off engine vacume thru a brake booster

if you want real forces you would need to disconnect the vacume line to the booster SIMPLE AND EAZY BUT THAT KILLS MOST OF THE BRAKEING FORCE
NOTE DONOT TRY THIS ON THE ROAD, MAYBE A EMPTY PARKING LOT
and only at very low speeds BE SAFE

Danger
Gold Member
Good advice, Ray, but I don't know that you'd have to disconnect the boost. Since it's proportional to the pedal pressure, you should be able to compensate for it.

russ_watters
Mentor
I don't know - I've driven cars with touchy brakes and "soft" brakes. It seems to me that though it would make sense to make them proportional, but I don't know that they realistically are.

Danger
Gold Member
Maybe. All of mine have been, but it probably depends upon the manufacturer. Also, none of mine had any ABS or other weird stuff.

...

lol, thanks for the replies,

going a bit off the topic but...

ill probably just use a variety of forces starting at about 100Ibf.

jOn