Brake Pad Contact

[prplefilipinoyam]

Are there any readings on whether or not Pad Contact with the rotor is very or not significant? It makes sense that if you have a pencil eraser sized pad, won't stop anything. But is there any equations or readings that prove a "perfect" size?

[prplefilipinoyam]

Are there any readings on whether or not Pad Contact with the rotor is very or not significant? It makes sense that if you have a pencil eraser sized pad, won't stop anything. But is there any equations or readings that prove a "perfect" size?

[TorqueWrench]

Not being a brake guy, but it seems like basic physics to me. If you know your coefficient of friction, the required torque to lock up your wheels, the diameter of the rotor and the pressure the pad presses on the rotor with, it seems like it should be easy to calculate the required contact surface area.

I know Carroll Smith goes over a lot of information about brakes in his books and talks about pad contact area for a bit in at least one.

[Tim.Wright]

I'm not a brakes guy either. But simple friction theory (F=Fn x mu) does not take into account an area.

So theoretically you could have a eraser sized pad but then you are asking this small volume of material to absorb all the braking energy. So the limiting factor on size will probably be a thermal one rather than something related to classic friction theory.

Tim

[Adambomb]

Check out Wilwood's caliper tech tips here, specifically the section on weight reduction:

Wilwood caliper tech tips (http://www.wilwood.com/TechCaliperTip.aspx)

[STRETCH]

reiterating what Tim said, the pad area doesn't enter into the torque calcs etc... it's a matter of thermal capacity, wear rate and pad life. The pad size will be dictated by your choice of caliper though, which has to dissipate this heat (unless you start modding pads), so if you buy an appropriate caliper for the work you intend to put into the system, then the pad size will likely be suitable.

[TorqueWrench]

I was more thinking in terms of F=PA and then basic friction. I know you need a certain size pad/rotor to absorb the heat and that the coefficient of friction varies with temperature, but I would assume you also have to be able to put down a certain amount of force to lock the wheels.

[STRETCH]

That's probably a common misconception Chris. But if you think about how the brake line pressure gets transferred to the rotor... This pressure acts on the piston behind the pad, which is in the bore diameter of the caliper. Hence, the pressure acts over an area related to the bore diameter, giving you a force on each piston. The sum of these forces (for a multibore caliper) is what the pad exerts on the rotor, no matter what its size. The size of the pad simply effects the pressure that IT experiences.

Hope that's clear,

[TorqueWrench]

I learn something new everyday. Thanks for clearing that up.

[Mikey Antonakakis]

All this makes sense. Let's say you were trying to calculate braking force (or torque I guess). Would it makes sense to find the centroid of the brake pad, a coefficient of friction, apply at the radius of the centroid from the hub, and then you have a torque? I'm assuming the friction coefficient depends on temperature. Then you have a torque on the wheel, can transfer that to the contact patch, do whatever you want with it. Does that approach make sense?

[Adambomb]

Yep! It really is that easy. Then with all those parameters, picking a pedal gain in lbf per g of braking and max stopping acceleration, and taking into account weight transfer, it would be possible to, say, select master cylinder size, caliper size, rotor size, brake pedal dimensions, or any other arbitrary question related to braking systems parameters that is asked here on a daily basis.

As for temperature effects on friction, I know Wilwood used to have plots of mu vs. temperature posted on their website (haven't looked for them in a while...I just DL'd them and tucked them away in a special place http://fsae.com/groupee_common/emoticons/icon_wink.gif ).