When coming to a stop should you brake harder or longer in traffic when coming to a stop? When I say hard I don't mean slamming the brakes of course, you get the picture... Which way would wear out the pads faster?

 

Personally, don't worry about which wears out the pads faster, worry about your safety more.

With that being said, I would opt to brake longer.... More can go wrong when you try to make your stops shorter. Hit loose gravel on road, guy in front brakes harder than you think, etc, etc.

 

Well said.

 

just very curious though which would wear the pads faster? and can someone give a good physics explanation?

 

Braking harder will wear pads quicker because more heat is generated. On a microscopic level, braking hard causes larger chunks to dig into the rotor from the pad, which fragment off in larger pieces.

 

Going by straight physics, I would think they wear the brakes the exact same. Miscelaneous things aside, if you are going 60mph, it takes a fixed amount of work/force to stop your car, becuase at a constant velocity, your car has a fixed amount of kinetic energy.

For simplicity sake, lets say it takes 60 watts of work to stop your car from 60mph. It doesn't matter if you apply more force over a shorter duration of time, or less force over a longer duration of time...

You can either apply 60 joules of energy in 1 second, or 10 joules of energy every second for 6 seconds. The end result is the same 60 watts.

 

More heat is not generated. The act of braking converts kinetic energy into heat. If you are going 60mph, and you come to a stop... There is a fixed amount of kinetic energy in the car. Slamming on your brakes, or easing the brakes, will create the same amount of heat.

In a simple sense, saying braking harder will create more heat, is like saying you can control your caloric intake, by controlling how fast you eat your meal.

 

Okay, let me clarify, the temperature will be higher, a longer lighter braking will allow more heat to be dissipated

 

I'd disagree with that too. If you go down a hill, the brakes will cool better (ie not fade), if you brake, then coast, brake, then coast... Rather than lightly riding the brake the whole way down... At least it seems like it anyways...

 

This is not true in the case of friction however, because you can ideally stop something without wearing either surface because energy is transfered on the atomic level and dissipated, a slower brake will allow for more of this to occur. If you jam on the brakes, not only is dissipation occuring on the molecular level, the microscopic rough surface also comes into play more. Its the same thing as on the tires when you slam on the brakes and leave skid marks, but when you slowly apply the brakes, you don't leave as much rubber behind

 

I think you'll agree with me that the same amount of work is done whether you slam on the brakes or come to a stop slowly, but one takes longer so it has a longer period to dissipate heat, so the brakes never reach as high of a temperature. Its like if you went to the track for 1 hour and did alot of hard braking, you could cook your brake fluid it gets so hot, but if you drive around all day, you'll do more work with your brakes, but since its over a longer period of time, you'll never reach the same temperatures

 

What are you talking about? The definition of friction between two surfaces moving relative to each other, is the conversion of kinetic energy into thermal energy.

What do you mean by "transfer" of energy? There is no "transfer" of energy in the case of braking, because the two surfaces (rotor and pad) are moving relative to each other. Transfer of energy would be something like, crashing into another car, and the other car lunging forward as energy is tranfered, while at the same time heat is generated at the impact point, where some kinetic energy is converted into thermal energy.

Skidmarks from a tire is a little different. The coefficient of friction does not change, depending on how hard you brake, so the same energy is still converted if you brake hard or soft. Your tires wearing differently if you skid or not, is because if the force applied to the tire is greater than the amount of force that can be applied to the ground, causes the energy differential to be converted into thermal energy. So kinetic energy can either be converted into thermal energy in the brakes or in the tires, you take your pick. If the tires are locked up and sliding, kinetic energy is being converted into thermal energy in the tires, not the brakes. That's where those skidmarks come from.

 

Not quite... If you go to the track, how often are you braking, and from what speeds? Vs normal traffic in a day?

I'd wager that if you add up the amount of braking that you actually do during a trip to the office, is actually less than when you are tracking your car for an hour.

But I know what you are trying to illustrate, with brake cooling... However, the cooling will come from when you are not using your brakes. On a drive to work, your rotors are dissapating heat while in motion and you aren't braking... But the original poster was talking about just a single brake when coming to a stop. Not the culmination of a bunch of different braking events.

Since it takes the same energy to stop a car from a given speed, and the heat generated is the same, let's look at the cooling.... Riding the brake with a lighter pressure will not result in cooler running brakes. The brakes can only cool if the energy input is less than the output. If you are riding your brake, you are constantly putting energy into the system. It's like putting a pot of water on low-heat on the stove, and saying the water will cool. It won't cool until you turn off the heat.

But in the end, who really cares... I think we're going MUCH deeper than the poster intended

 

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

 

lol I'm having fun reading it. Great arguments.
This question just popped up when I hear people saying that hard braking will wear the brake pads out sooner. It just didn't make sense if the conservation of energy held true.