If we are exerting one unit of force on a fluid brake system, would it stop faster, slower, or neither, assuming a larger surface area near the brake pad compared to a smaller one?
A friend’s client came in saying that increasing the area at the brake end would make the car stop faster. This seems wrong to me. The only change I can think of is him having to move the brake pedal further or less to achieve the same results.
If you are putting the same amount of force on the brake pad, you’ll get the same deceleration, no matter what the size of the pad. Well, at least approximately: Real friction is more complicated than it is in the textbook. But this has nothing to do with fluid pressure.
Where fluid pressure might come in, is if you have a given pressure instead of a given force, and you’re changing the size of the piston that pushes on the brake pad. In that case, a larger piston will mean more force, which means more deceleration.
Just to be clear, larger piston means a piston with a greater diameter, and the brake pedal will move a bit farther.
Could you please elaborate on the meaning of “if you have a given pressure instead of a given force.” I feel as though our misunderstanding comes from this.
In this example, I have a quick drawing to clarify: Screenshot by Lightshot
In this image the piston presses directly on the brake pad, and what is being changed is the surface area of the piston. The drivers force placed on the pedal will always remain the same.
As a side note for Chronos, I just wanted to say that I have been on this board for years and really appreciate all that you’ve contributed over the many years you’ve been here.
This was our initial understanding. So we are officially not crazy?
Force = Pressure x Area
If you double the diameter of the piston, the area will quadruple, as will the force. However, you will also have to push the pedal in 4 times as far.
Safe stopping distance is a bigger function of the tires (coefficient of friction) and the road conditions. I believe automobile engineers keep a margin of safety considering different road / tire conditions before deciding the braking force. Increasing the braking force by making the pads bigger may result in your car skidding and/or the pads overheating.
“larger surface area/increasing the area at the brake” is a bit ambiguous, but you’ve since clarified it to mean that you’re talking about a larger-diameter piston at the brake caliper.
For a given pressure in the brake fluid, a larger piston means more force at the caliper, resulting in more rapid braking (but also more brake pedal travel to close the gap between pad and rotor).
A larger pad surface area won’t result in faster braking, but it might improve resistance to brake fade (loss of braking due to excessive temperatures achieved after multiple heavy braking events within a short time).
A larger rotor diameter, assuming the caliper is also moved out to a larger radius, will result in more rapid braking for a given pedal force. It will also improve heat tolerance (more rotor mass, also more rotor surface area for cooling). It will also decrease handling performance, since you are adding more unsprung weight to the suspension.
Alternatively, you can achieve your original braking force with less pedal force, which may be desirable for some folks. Years ago I upgraded my motorcycle brakes from the OEM brake pads to aftermarket pads with a greater coefficient of friction. This meant that I needed less force at the brake lever, which was a godsend for my right hand while riding twisty roads.
Ref the OP’s diagram ref’ed in post #4, I only see ads on that page and a where his diagram is supposed to appear.
Where the OP says “If we are exerting one unit of force on a fluid brake system …” do you mean one unit at the pedal end or one unit at the caliper piston end? That changes everything.
Just to clarify since some people can’t see my image:
one unit of force pedal on the pedal. The object changing in size is the piston surface area.
I worded the initial post very poorly, my apologies.
Theoretical mechanically, increasing the brake piston diameter (and leaving the pedal piston diameter the same) is just changing the mechanical advantage. So with the same pedal force, it would result in more force at the brakes (while of course, requiring the pedal to move farther).
In the real world, I’m not sure there are many actual existing cars where the limiting factor in how quickly a car can brake is really the force on the brake pads. On my current car, it doesn’t take that much strength to push the pedal to the point where ABS kicks in (which is basically the car noticing that the wheels are locked up and the tires are starting to skid). I strongly suspect that if the OP’s friend’s client’s car is having trouble braking, the real issue is elsewhere (worn or damaged brake pads, rotors, tires, etc.)
…and the pedal actuates a master cylinder, which in turn actuates a slave cylinder/piston at the caliper. The slave/piston cylinder diameter is what’s changing.