Another Physics Question

[ericwgnsx]

Scenario:
Two identical cars are traveling next to each other at 60 miles per hour. The traffic light 1/4 mile ahead turns red. The driver in Car 1 immediately lifts off of the gas, shifts to neutral, and begins applying his brakes in a smooth, consistent manner, maintaining even pedal pressure until stopping at the intersection. The driver in Car 2 also immediately lifts off of the gas and shifts to neutral, but waits until he is only 100 yards from the red light to begin applying his brakes. He also applies his brakes in a smooth, consistent manner, maintaining even (although necessarily heavier) pedal pressure until stopping even with Car 1 at the intersection.

Question:
Which driver will wear out his brakes faster if they always use their respective braking styles?

 

[NSXFRIEND]

440 yards per 1/4, second car heats the brakes harder less distance and more force is applied, first car uses more brake but keeps the brakes cool with less effort.

I am gona take a guess and say car 2 car which brakes later although he has used wind resistance to slow down so the speed is unknown at the 100 yard.

 

[Vega$ NSX]

Great question but too many assumptions and variables to be able to answer definitively. In my college vehicle dynamics class we had the same question in reverse: If one car gradually accelerates to a speed and reaches a distance, while another car floors it (without losing grip) to a speed so that both cars reach a point in the distance at the exact same time, which car uses more gas.

Depending on which set of assumptions and variables you used, the answer could be either one. Although, for our discussion our given set of assumptions and variables resulted in the car flooring it as the winner.

Some questions/assumptions are, does Car #2 ever break the contact patch force of the tires? If so, then it's brake pads should wear longer, although the tires wouldn't. How warmed up are the breaks and how resistant are they to thermal shock? Sudden rapid temerature increases could damage brake components. What is the wind resistance and internal gearing losses of both cars? If they are high enough, a 1/4 mile is a very, very long way and the driver of Car #1 may not even need to touch his brakes at all.

 

[nsx1164]

Great question but too many assumptions and variables to be able to answer definitively.

+1

Brakepad wear is a pretty straightfoward equation, but the net "momentum energy" being converted to "heat" (and thus pad wear) can vary, based on several variable factor, including several items posted here. Contact Andie at Cobalt for more info - bring a few shots of tequila and hang on :biggrin:

For example, what else is "identical" ? patch of road - same surface, material, contaminant (wet, dry, oil, marbles, etc), inclination, straight-ness, etc

My answer is... the winner is the driver who keeps his eyes in front of him, can see beyond the collision point, and thus plot and execute an escape route. Lifting and shifting to neutral - BAD idea - would suggest that straight-line threshold braking to get you to a target turn-in speed to escape the incident before it happens. That's the one that wins this game.

NSX - the power of dreams
The line for second-best starts here.

 

[4AU4LIFE]

I'm in the position of the first driver. I drove my 95 Integra that way for 12 yrs before needing a brake job.
I would say that #2 wears them out faster.

 

[NSXrunner]

I blew my physics classes :tongue: so I am just guessing that car B uses up the brakes more at the lat min so to speak. (the one who brakes harder) Be interesting to hear what the answer might be though...:smile:

 

[dyee]

To stop the motion of the car (kinetic energy) work is done by the brake pads. Since the speed will be higher in car 2 more force MUST be supplied by the brakes over a shorter distance to stop the car. More force = shorter brake pad + rotor life. :smile:

 

[steveny]

Scenario:
Two identical cars are traveling next to each other at 60 miles per hour. The traffic light 1/4 mile ahead turns red. The driver in Car 1 immediately lifts off of the gas, shifts to neutral, and begins applying his brakes in a smooth, consistent manner, maintaining even pedal pressure until stopping at the intersection. The driver in Car 2 also immediately lifts off of the gas and shifts to neutral, but waits until he is only 100 yards from the red light to begin applying his brakes. He also applies his brakes in a smooth, consistent manner, maintaining even (although necessarily heavier) pedal pressure until stopping even with Car 1 at the intersection.

Question:
Which driver will wear out his brakes faster if they always use their respective braking styles?

Driver 2 will have an excellent set of brakes on a car that the rear end is smashed in.

 

[nsx1164]

assuming reasonable "all things equal" - driver 2 gets better brake wear.

Threshold braking is the efficient way to go... hit the pedal hard as late as possible with max bite (just at the grip threshold), and hold it briefly, then gradually roll off the brake, till you get to your target stop point.

The other method (aka "forward braking"), where you start early, and gradually increase the pressure - this is the more common "street braking" technique - actually is HARDER on the equipment, as you start building up heat in the pad (and boiling the fluid, etc), then you hammer it in the end - not only is brake wear worse (heat applied over a longer time interval), but you run the risk of the equipment failing when you need it.