Lead Foot Equall/Not Equall Need Brake Job

October 15, 2009 3:35 PM

Theoretical brake wear: Do brake pads wear out faster, slower, or at the same rate if you brake later and harder than normal?

Let's say you have two drivers, Alice and Bill, each with their own identical car.

Let's say Alice is a professional race car driver and Bill is a normal vanilla defensive driver. Alice drives her regular car at legal speeds on the open road, but because of her professional skill and training, tends to brake much later and with greater foot pressure than Bill would making the same journey.

For instance, while traveling 60 MPH, they both see an approaching red light. Bill applies the break gradually and takes 1000 ft to come to a stop. Alice doesn't apply the break until she is closer to the light, and applies more foot pressure, coming to a stop in 500 ft.

Over the long run, does Alice need new brake pads before Bill, or do they both need new brake pads at roughly the same time?

I can see it going either way. Either Alice needs pads sooner because she brakes harder, applying more force to the pads, or Alice and Bill both wear pads at the same rate, because they are doing the same amount brake use, only with different short-term usage patterns.

Or maybe I'm missing something entirely and you can help me understand.

posted by Liver to travel & transportation (17 answers total)

According to people in Formula One, late brakers will go through more pads.
posted by Zambrano at 3:43 PM on October 15, 2009

Well Bill, if he is indeed a normal defensive driver, will be having much greater stretches of having his foot on neither the gas or the break. At these times the rolling resistance of the car will be slowing the vehicle not the brakes.
that's much less kinetic energy his brakes have to convert into heat than the breaks on Alice's car.
posted by French Fry at 3:48 PM on October 15, 2009

Alice wears out her breaks sooner, and more importantly and expensively, gets worse mileage. She also wears out tires and engine faster and will need oil changes more frequently, all other things being equal.
posted by idiopath at 3:54 PM on October 15, 2009

Alice wears out her breaks sooner, and more importantly and expensively, gets worse mileage. She also wears out tires and engine faster and will need oil changes more frequently, all other things being equal.
posted by idiopath at 6:54 PM on October 15 [+] [!]

This is absolutely untrue. Your braking has nothing to do with gas mileage or engine wear. Only acceleration and miles travelled effect those two, respectively. Hard braking does wear tires (slightly, in a street environment) more rapidly, but that's outside of the scope of the question as posed.

To actually answer the question, Alice will suffer more brake wear. The reason for this is simple. Brakes work by using friction to turn inertia into heat, and friction is dependent upon force applied. The more friction generated, the hotter the brakes get, and the more brake pad molecules make contact with the rotor material, heat up, and are shorn from the mass of the pad.
posted by TheNewWazoo at 4:09 PM on October 15, 2009

TheNewWazoo: "Your braking has nothing to do with gas mileage or engine wear."

I said that with the assumption she was maintaining speed up until she starts braking. If she is coasting up until that point, yes, point taken.

What the asker describes is part of a familiar and all too common driving style, quick accelerations, achieving the maximum speed you can get away with as soon as possible and maintaining that maximum speed for as long as possible before braking. If we are talking strictly about the thought experiment, the asker did not specify if speed is maintained or if on the other hand Alice starts coasting when she sees the red ahead of her.

I presumed the latter, because in terms of real world driving, people do drive like this, and it definitely causes accelerated engine and tire wear and poor gas mileage.
posted by idiopath at 4:37 PM on October 15, 2009

I meant "I presumed the former" in my final paragraph above.
posted by idiopath at 4:38 PM on October 15, 2009

The physics major will tell you that it is equal, because it takes an equal amount of force to bring the same two cars to a stop if they start at the same speed.

Unfortunately, he will not be accounting for the nature of the brakes themselves, which have ablative properties inversely proportional to their temperature, as TheNewWazoo correctly points out. Hotter brakes = more pad loss.
posted by Cool Papa Bell at 4:52 PM on October 15, 2009

idiopath is correct. the "harder" you drive the faster things wear out. Ask any mechanic.
posted by patnok at 4:52 PM on October 15, 2009

Bill will also presumably do more engine breaking. Unless they foolishly drive Automatic cars...

*ducks*

:)
posted by lundman at 5:28 PM on October 15, 2009

Bill will also presumably do more engine breaking. Unless they foolishly drive Automatic cars...

Nothing about an automatic car prevents you using engine braking.

As for the OP - Cool Papa Bell has the most complete reasoning. The higher pad temperature attained in the harsher braking will promote pad wear and cause more frequent pad changes.

Hard braking does wear tires (slightly, in a street environment) more rapidly,

It's a little more than slightly. It's certainly comparable with the rise in pad wear.
posted by Brockles at 5:45 PM on October 15, 2009

Your braking has nothing to do with gas mileage or engine wear.

This is absolutely untrue, in a theoretical and practical sense. If you're stepping on the brake, you're not stepping on the gas (hopefully, heel-toe shifting excepted). If Alice is braking later, that means she's on the gas for longer periods.

Also, by coasting and braking longer, the light might change before you reach a complete stop, so you don't have to accelerate back from 0 MPH.
posted by hwyengr at 5:52 PM on October 15, 2009

hwyengr, and others, you are making a lot of assumptions about alice's driving style. Just because she brakes late does not necessarily mean she is accelerating up to the point of braking (and therefore getting worse gas mileage). In fact, one could argue that a late braker could achieve better gas mileage, because if traffic suddenly clears up, she is already at a higher speed and has less accelerating to do.

If you're stepping on the brake, you're not stepping on the gas

Wrong. Ever hear of coasting?

To answer the question, Alice will indeed wear out her brake pads faster because of the higher heat generated.

Also: it's brake, not break.
posted by lohmannn at 6:20 PM on October 15, 2009

If you're stepping on the brake, you're not stepping on the gas

Wait, I misread. The point still stands, though.
posted by lohmannn at 6:21 PM on October 15, 2009

By increasing brake pressure, you're generating more friction (and therefore heat) between the pad and the rotor. That's how you stop faster. And more heat and friction = faster stopping = accelerated pad wear.

But, that's why disk brakes are such an advancement over drums. A disk brake can dissipate a great deal more heat than a drum, thereby increasing the amount of pressure and heat allowable in the system and decreasing potential stopping distance.
And carbon-ceramic brakes, used on super-high performance cars and race cars have a much greater tolerance for heat and wear to compliment their greater friction properties, resulting in potentially very short stopping distances for some extremely heavy cars.
posted by Jon-o at 6:54 PM on October 15, 2009

Wrong. Ever hear of coasting?

Coasting is never implied. Alice is a trained racing professional. Race drivers don't coast, it's wasted time on the track. You'd gas longer and brake later, which is what I consider the spirit of the question to be.
posted by hwyengr at 6:59 PM on October 15, 2009

One thing that helps keep everyday road brakes functioning properly and wearing evenly is the ability to dissipate heat from the site of contact between pad and disc. If temperatures there rise higher than usual, greater pad wear will result, not to mention other undesirable wear effects which are not really part of your question.
One way discs help dissipate the heat is by simply soaking it up. Metal discs are beefy as they are partly in order to increase their heat capacity. Another way is to pass it off into the surrounding air (this is aided greatly by vented disc designs).

Trying to guess at the spirit of your question behind all the numbers, the two drivers will be converting roughly the same amount of kinetic energy into heat using their brakes. You hint at this when you say in your possible explanation "they are doing the same amount brake use, only with different short-term usage patterns". What will kill Alice's brakes relative to Bob's is the fact that Bob's brakes will be able to better keep pace with the heat generated by venting it to the moving, turbulent air while slowly coming to a stop, whereas Alice will dump all her kinetic energy into the discs in the form of heat, creating a much higher temperature spike, which will then stay in the discs for longer as the discs which have now come to a stop must rely on convection to get rid of all the energy.

Sorry if that was too long-winded.
posted by tigrrrlily at 9:31 PM on October 15, 2009

Coasting is never implied. Alice is a trained racing professional. Race drivers don't coast, it's wasted time on the track.

Absolutely. Any driver that coasts will get a slap around their damn head when the data is downloaded, that's for sure. Not only is it wasted track time in a 'you could be on the power and going faster' sense, but also leaves the transmission and suspension as unloaded as it can get, which is very much an undesirable condition. The shunt of driveline, engine and suspension loading when the everything is connected back up just as corner entry occurs will unsettle the car enormously and be slow as Christmas.

Coasting is a definite no-no.

Metal discs are beefy as they are partly in order to increase their heat capacity.

Not so much. The size is more to allow the disc to dissipate high temperatures into the surrounding components or air without too high a temperature gradient that would result in warping or cracking - thermal shock prevented through thermal inertia, basically. The fact that the greater mass can absorb a small amount of heat itself is a happy coincidence, but not the driving factor. All discs would be vented and grooved if money was no object (higher cost and lower service life with these types increases initial and servicing costs), more massive discs are an acceptable cost compromise that gives a small amount of the required properties of the system. Any rotating mass that is unnecessary is a bad thing in road or track cars - you wouldn't use a heavy disc if you had any sort of alternative that fitted your design/cost parameters.

What will kill Alice's brakes relative to Bob's is the fact that Bob's brakes will be able to better keep pace with the heat generated by venting it to the moving, turbulent air while slowly coming to a stop, whereas Alice will dump all her kinetic energy into the discs in the form of heat, creating a much higher temperature spike, which will then stay in the discs for longer as the discs which have now come to a stop must rely on convection to get rid of all the energy.

Um. This isn't quite the issue. The higher temperatures in the pads and disc will produce a fundamentally different wear rate. The heat dissipation rates you refer to are just a side effect, really, and something that needs to be 'dealt with' as a result of the braking. A pad that slows a car without getting too hot is not able to generate the same sort of stopping power as if it were hotter (for the same compound). The material of the pad works differently at different temperatures, and will wear faster (but work better) at hotter temperatures. It's not just about heat dissipation, but the material properties of the pad and disc combination at higher temperatures. The heat dissipation is far more important to controlling the maximum temperatures to within the range where the pad and disc operating temperatures are within reasonable parameters - ie no warping and no physical breaking up of the pad material. Overheated pads can chunk off or wear in an astonishingly short period - I've seen road cars taken on track days that go through a set of (totally unsuitable!) pads in a few hours of running - something like 300 miles of driving as opposed to 30,000 more in their normal applications.
posted by Brockles at 7:56 AM on October 16, 2009