Originally Posted by redlude97

On an atomic level, the atoms valences are "crashing" into each other, and energy is transfered in that case. If you don't think energy is being transferred, I don't know how else to explain it to you. Work is defined as a force*distance, while the coefficient of friction doesn't change, the frictional force does depending the pressure applied by the brakes, frictional force is mu*normal force, the normal force is the brake piston pressing on the pad, so braking lighter causes a smaller normal force and thus less friction force. Thus the distance the force applied must increase to get the same amount of work that is required to stop your car. By increasing the distance over which you. Becuase the friction force is smaller at any given time, the contact between the two surfaces on a microscopic level see less shear stress on a per molecule basis, so individual molecules are less likely to shear off, when you increase the normal force, you increase the shear stress on each molecule making it more likely to shear off, thus causing more wear on your brakes

Originally Posted by redlude97

I can guarantee a stop from 100mph in 10seconds will cause the brakes to be hotter than if you slowed from 100mph in 60seconds, you just have more time for heat to dissipate over the period of braking

Originally Posted by avs007

I don't think the difference is going to be as great as you think it is. Assuming, both create the same amount of heat..... Case one, generates the heat in 10 seconds, than has 50 seconds to cool at rest. The other generates the heat over 60 seconds.

Unless you are comparing the heat levels after 10 seconds in case one versus after 60 seconds in case two? Otherwise we need to see which allows more heat to dissapate. We could spend forever arguing this, as it's like comparing heating a pot of water on high heat for 10 seconds, or medium heat for 20 seconds, etc. Too many variables at this point, like rotor design, airflow, etc.

Originally Posted by avs007

Ok, well...
I know what you mean by valences crashing into each other, but I don't think that in this scenario, you are going to get electrons to jump from one shell to another, if that's what you mean by transfer energy... Sounds like cold fusion to me Yeah, I know it's not even close, but I haven't said those words in so long

It's been a while since I took thermodynamics and chemistry so I could be off base, but I don't remember studying electrons jumping from one shell to another in this fashion in these circumstances. You'd be creating isotopes if they were doing that.

I understand what you are talking about sheer stress and such and frictional force. However, you are forgetting that even though you are applying more force in the hard-brake scenario, you are braking for a much shorter duration. Whearas in the easy-brake scenario, force is less, but distance is greater. From a statistics point of view, it's almost a moot point, because sheer force is greater, but time is smaller. So for example, if in that one second, the sheer force was great enough to sheer one particle off... In the other brake scenario, there is only a 50% chance of a particle sheering off, because of reduced force, but because you are braking for 3 seconds, you have higher probability. I'm not saying it works out to be the same, just saying that argument isn't as sound as you think it is.

I could be completely wrong, but it's kinda fun to think back to my college days Except now I feel old

Originally Posted by redlude97

I am comparing the maximum temperature at any time. Since in the case of hard braking, all the work that is transfered to thermal heat, but only has 10 seconds of dissipation time, while in the case of braking gently, the same amount of thermal energy is transferred, but you have 60 seconds of dissipation time. So while the brakes brakes will essentially be at the same temperature after 60 seconds, their maximum temperatures will be differents, with the case of hard braking being much higher

Originally Posted by MechEE

To say that stopping in 10 seconds versus stopping in 20 seconds has no net effect on brake wear makes the assumption that (neglecting temperature effects) brake wear is linearly proportional to brake force. Is it? I don't think that's clear, and will be a function of the materials, and likely some higher order function of relative surface speeds (between brake and rotor) and temperature to name a few.

Originally Posted by redlude97

I just finished up a grad thermo class last term so its fresh in my memory.

Originally Posted by avs007

That I agree with... I just don't think there's a correlation between max temp and brake wear. I'm thinking the wear comes from the act of conversion, which we both agreed are converting the same amount of energy.

Maybe I'm just looking at it from the wrong perspective, I don't know... I view the brake pad as having enough friction material to convert a fixed amount of kinetic energy. How hard you brake would than be irrelavent, as at the end of the day, how much energy you converted is what's important, and what dictates wear. So in that case whether you brake from 100 to 0 in 10 seconds or 60 seconds wouldn't matter, because at the end of the day, you converted the same amount of energy in both cases.