Planet 9

hawc · #1 10-04-2009, 12:19 PM

I've got a hypothetical question I'm trying to figure out. And I have no idea what the answer is.

Let's say you're city driving and you come to a stop from 70 km/h (could be any speed really) which will wear your brake pads more a slow gradual stop (but over a longer distance) or a quick hard stop (but over a shorter distance)?

Or is it the exactly same amount of wear because in the end you're braking the same amount of weight from the same speed to a stop.

suhkoi · #2 10-04-2009, 07:13 PM

In my highly informed state ( of confusion) I would venture to say that it takes more energy to enact a shorter stop. So, more energy means more brake pad used.

Now we wait for the truly knowledgeable people to educate us.

bsnively · #3 10-04-2009, 07:29 PM

I'm not more knowledgeable, so we will still wait on them, but I can say that it would take more force to stop quickly, but that force would be applied in less time.

Since work = force x distance

The numbers could play out in an interesting way. The real thing would be how the kinetic friction changes (linear,grow faster, logarithmic/etc) as the amount of force is applied...one could argue that the coefficient of friction decreases as the brakes increase in temperature -- causing the more force being needed and more wear from them when they are applied longer.

While I can make up some more stuff, nobody likely wants to hear Ben's wild ... well guess on that matter

.

Thx,
Ben

**4by4** · #4 10-04-2009, 09:05 PM

Feeding the brakes slowly and smoothly will increase the lifetime a lot. I always get exceptional brake life compared to other drivers of the same make/model car, whatever it is.

**K-Man S** · #5 10-04-2009, 09:32 PM

I find that not using the brakes at all gives the best brake life.

Since most of the time you drive your car you are not doing panic/hard stops, I doubt in the real world it matters very much, but the answer should be a gradul on and off the brakes should provide the longest life because you are using "just enough" of the brake materials to get you slowed to where you want to be, vs. a hammering on of the brakes which in all likelihood will end up being more force than needed to stop or slow to the desired speed.

tc_sting · #6 10-04-2009, 10:42 PM

Due to the conservation of energy, it takes the same amount of energy to stop a car going a particular speed regardless if the stopping distance was long or short.

However, reality paints a complicated picture of summation of forces. There are more forces retarding the forward motion of the car than just the brakes.

Other than brakes, two other major retarding forces would be friction (from the road surface) and wind resistance.

The long, slow braking motion causes less wear on the brakes, because it allows the other two retarding forces to help out considerably.

Hard and fast brake appliacation requires the brakes to do all the work by converting all the kinetic energy into potential, therefore causes excessive wear.

008 · #7 10-05-2009, 10:01 AM

Type of pad material and its state will play a role in this as well. A cold race pad wears quicker than a properly bedded and warmed up pad as the friction surface is between the transfer material and the pad not just the rotor face. Maybe one of our resident brake experts can chime in but my vote goes for more pressure causing less wear on a track pad that is bedded and up to temp.

**Gator Bite** · #8 10-05-2009, 10:35 AM

Heat changes the equation. This is a complex question.

**DaveN007** · #9 10-05-2009, 10:37 AM

"Heat changes the equation. This is a complex question."

-Gator Bite, in his gutless "it depends" engineering answer.....LOL

People who rest a foot on the brake pedal and therefore have the brakes lightly engaged all of the time wear out their pads more quickly, therefore maniacally hard threshold braking is what I do. Always. To save wear and discourage tailgaters.

This logic is unimpeachable. Before you even try to match wits with me you should know that I took a basic physics class in college so I am practically a scientist.

Bricks and feathers fall at the same speed inside a vacuum cleaner. You probably didn't know this.

+

**PistolPete** · #10 10-05-2009, 10:45 AM

Thank you for the laugh Dave!

Originally Posted by DaveN007

People who rest a foot on the brake pedal and therefore have the brakes lightly engaged all of the time wear out their pads more quickly, therefore maniacally hard threshold braking is what I do. Always. To save wear and discourage tailgaters.

This logic is unimpeachable. Before you even try to match wits with me you should know that I took a basic physics class in college so I am practically a scientist.

Bricks and feathers fall at the same speed inside a vacuum cleaner. You probably didn't know this.

hawc · #11 10-05-2009, 07:13 PM

Originally Posted by tc_sting

Other than brakes, two other major retarding forces would be friction (from the road surface) and wind resistance.

At just 70 km/h do either road friction or wind really cause that much resistance?

tc_sting · #12 10-05-2009, 07:43 PM

Originally Posted by hawc

At just 70 km/h do either road friction or wind really cause that much resistance?

Yes.

The force of friction is easiest to determine.

Dry roads have a coefficient of kinetic friction value of approximately 0.70, which is high, hence the reason they use asphalt for paving roads.

The retarding force due to friction = (coefficient of fricition) * (mass of Porsche) * (gravity constant). This force is measurable, cars are heavy.

Simple to test on a level road I expect.

**ValveTubeHead** · #13 10-24-2009, 04:33 AM

of course, it depends

resistence: wind, tire friction, inertia, drivetrain... all sources are components of a complex equation which vary; some of which can be affected by the driver...eg. scrubbing off speed (swirving) to increase tire friction, compression braking by downshifting, which dissapates more energy (heat) through clutch and engine, a headwind, rain even.... aerodynamic drag. Energy is conserved (one way or many others) in our 3D world.

wpmjr · #14 10-24-2009, 06:36 AM

OK, here's the answer:

The big difference is weight transfer in hard braking. This wears the front pads faster for sure. And who cares about the rears-they last forever. So save your front brakes! Or not!

Jeff G in NE GA · #15 10-25-2009, 12:37 PM

Hi Gents, I didn't weigh in yet, because I sensed the answer was complex, like Gator Bite said. Knowing my limitations, and also knowing some true experts - I asked their opinions. Below are the answers from the two most qualified brake engineers I know. (Thank You Nic and Darrick):

Hi Jeff

Interesting question, but as some of the forum members have pointed out its very complex and depends on many factors. As the forum members have pointed out the energy dissipated in a stop of the same speed will be the same, it depends on where the energy is dissipated on how much it will wear the pads.

For a road type pad which works well at lower temperatures I’m with the slow soft braking guys, as the rolling resistance and drag will slow the car a reasonable amount. The higher the speed the greater the drag co-efficient will be. I agreed with the guy who also mentions usually front pads wear out first, due to the higher loads on them, due to the weight transfer. On a typical road car with a relatively high centre of gravity the dynamic brake bias, even with a car that has static rear weight bias, will be about 70/30% to the front.

With certain race type pad and particularly carbon/carbon the wear can be higher at low temperatures (about 2x normal wear), then it reaches it optimum, and then once it exceeds its higher temperature limit it starts to wear faster again (about 20x normal wear!). Often with race type compounds the wear against temperature curve is a “U” shape. Therefore with race pads hard and short is often better, but again it depends on so many factors, including how big a stop you are looking at, as this will give different rises in temperature, some of which may exceed the pads optimum temperature/friction range.

With road pads they due to the fact they are designed to work well from cold, they often don’t have a particularly high maximum optimum temperature so again a very hard high energy short stop will cause a higher temperature rise than a longer slower one, as the heat is lost, due to disc/caliper cooling over a shorter period of time. Once the pad exceeds its optimum working temperature quite often the wear rate will rise dramatically. Also if the pad to disc interface temperature is higher than the maximum operating temperature of the pad it will “fade”. This fade can be caused by excessive gasses being given off by the volatile carbon compounds (resins etc) in the pad oxidising and the pad being forced away from the disc face thus lowering the friction. The normal reaction is to press harder, this creates more heat and more fade. The unsupported pad structure left once all the resins burn out easily crumbles and thus wears faster. Again pads are made by a variety of methods these days so this may not be true of all pads.

The downside of a long slow brake, particularly say on a long downhill, is that the heat from the disc/pad interface will have a longer time to be transferred through the pad material into the caliper piston and thus into the brake fluid. This eventually, with generally lower specification road type brake fluids ( and the fact its not changed that often, thus it has absorbed more water vapour than would be found in a race car brake system) will cause the fluid to “boil” and due to the compressibility of these gases created, cause the brake pedal to travel further, maybe to the floor.

I know this doesn’t give a short yes/no answer but I’m afraid as with many things in life there isn’t one.

Hope this helps and feel free to use as much or little of this information on your forum.

Best Regards

Nic Olson
Race Engineer
AP Racing
Wheler Road
Coventry
CV3 4LB
United Kingdom
Website: AP Racing Competition Car Products

Hi Jeff,
The answer is yes to both questions. Friction materials are a very complex matrix of various elements and “phasing” linkages through different processes.

There are;

Cold wear properties
Hot wear properties
Oxidation wear rates
Pad material density
Noise damping
Abraded wear from disc conditioning
Abraded wear from disc rubbing speed
Dynamic disc symmetry
Non Uniform force distribution effects

To consider from the short list of what affects pad wear. I could write papers on each of the points listed but there is no simple answer but “Brake wear is mostly influenced by driver input.”

It’s the driver who can most influence dynamic wear as he’s the one who adjusts to the car’s performance capabilities. So the friction companies work at various areas for improvement and often hit a combination that improves performance from a wear, bite, torque or pedal firmness standpoint but tend to be tougher to be quiet under all conditions. The noise sensitivity issue is #1 on the OE’s list of priorities so compromises in density, added noise damping tends to have higher wear and dusting problems, so there is no free lunch wither it’s for road cars or racecars.

Darrick Dong
Director of Motorsports
Performance Friction Brakes
Performance Friction

Reynard20 · #16 10-25-2009, 08:45 PM

I really like the brick and feather analogy. My wife says I have a vacuum in between my ears and the last time she threw a brick (which has more mass) at my vacuum it hurt much more than the feather. Therefore, using this analogy, we can deduce that the heavy braking will do more damage.

hawc · #17 10-26-2009, 06:04 AM

Originally Posted by Jeff G in NE GA

Best Regards

Nic Olson
Race Engineer
AP Racing
Wheler Road
Coventry
CV3 4LB
United Kingdom
Website: AP Racing Competition Car Products

Darrick Dong
Director of Motorsports
Performance Friction Brakes
Performance Friction

Awesome!

sixisenuff · #18 10-26-2009, 03:21 PM

Originally Posted by Jeff G in NE GA

Hi Gents, I didn't weigh in yet, because I sensed the answer was complex, like Gator Bite said. Knowing my limitations, and also knowing some true experts - I asked their opinions. Below are the answers from the two most qualified brake engineers I know. (Thank You Nic and Darrick):

Hi Jeff

Interesting question, but as some of the forum members have pointed out its very complex and depends on many factors. As the forum members have pointed out the energy dissipated in a stop of the same speed will be the same, it depends on where the energy is dissipated on how much it will wear the pads.

For a road type pad which works well at lower temperatures I’m with the slow soft braking guys, as the rolling resistance and drag will slow the car a reasonable amount. The higher the speed the greater the drag co-efficient will be. I agreed with the guy who also mentions usually front pads wear out first, due to the higher loads on them, due to the weight transfer. On a typical road car with a relatively high centre of gravity the dynamic brake bias, even with a car that has static rear weight bias, will be about 70/30% to the front.

With certain race type pad and particularly carbon/carbon the wear can be higher at low temperatures (about 2x normal wear), then it reaches it optimum, and then once it exceeds its higher temperature limit it starts to wear faster again (about 20x normal wear!). Often with race type compounds the wear against temperature curve is a “U” shape. Therefore with race pads hard and short is often better, but again it depends on so many factors, including how big a stop you are looking at, as this will give different rises in temperature, some of which may exceed the pads optimum temperature/friction range.

With road pads they due to the fact they are designed to work well from cold, they often don’t have a particularly high maximum optimum temperature so again a very hard high energy short stop will cause a higher temperature rise than a longer slower one, as the heat is lost, due to disc/caliper cooling over a shorter period of time. Once the pad exceeds its optimum working temperature quite often the wear rate will rise dramatically. Also if the pad to disc interface temperature is higher than the maximum operating temperature of the pad it will “fade”. This fade can be caused by excessive gasses being given off by the volatile carbon compounds (resins etc) in the pad oxidising and the pad being forced away from the disc face thus lowering the friction. The normal reaction is to press harder, this creates more heat and more fade. The unsupported pad structure left once all the resins burn out easily crumbles and thus wears faster. Again pads are made by a variety of methods these days so this may not be true of all pads.

The downside of a long slow brake, particularly say on a long downhill, is that the heat from the disc/pad interface will have a longer time to be transferred through the pad material into the caliper piston and thus into the brake fluid. This eventually, with generally lower specification road type brake fluids ( and the fact its not changed that often, thus it has absorbed more water vapour than would be found in a race car brake system) will cause the fluid to “boil” and due to the compressibility of these gases created, cause the brake pedal to travel further, maybe to the floor.

I know this doesn’t give a short yes/no answer but I’m afraid as with many things in life there isn’t one.

Hope this helps and feel free to use as much or little of this information on your forum.

Best Regards

Nic Olson
Race Engineer
AP Racing
Wheler Road
Coventry
CV3 4LB
United Kingdom
Website: AP Racing Competition Car Products

Hi Jeff,
The answer is yes to both questions. Friction materials are a very complex matrix of various elements and “phasing” linkages through different processes.

There are;

Cold wear properties
Hot wear properties
Oxidation wear rates
Pad material density
Noise damping
Abraded wear from disc conditioning
Abraded wear from disc rubbing speed
Dynamic disc symmetry
Non Uniform force distribution effects

To consider from the short list of what affects pad wear. I could write papers on each of the points listed but there is no simple answer but “Brake wear is mostly influenced by driver input.”

It’s the driver who can most influence dynamic wear as he’s the one who adjusts to the car’s performance capabilities. So the friction companies work at various areas for improvement and often hit a combination that improves performance from a wear, bite, torque or pedal firmness standpoint but tend to be tougher to be quiet under all conditions. The noise sensitivity issue is #1 on the OE’s list of priorities so compromises in density, added noise damping tends to have higher wear and dusting problems, so there is no free lunch wither it’s for road cars or racecars.

Darrick Dong
Director of Motorsports
Performance Friction Brakes
Performance Friction

Darrick:

You sound like an economist.

Take glazing of pad material out of the equation and assume that brakes are street brakes at operating temp and the hard stop will wear them faster.

Now dragging the brakes to stop from 100mph in 2 or 3 minutes, would cause some bad effects to the brakes...They wonldn't be worn, but they'd be very glazed and wouldn't work very well.

Agree this is another complicated question with 3 simple and not the whole truth multiple choice answers.

Fun, aren't they?

**Caymancouver** · #19 10-26-2009, 04:11 PM

Let's say I do two stops from 50 mph on a flat asphalt surface:

The first stop I do by taking the car out of gear about a quarter mile back and coasting to a stop without even touching the brake pedal. Wear on brakes = zero. Other frictional forces are what end up bringing me to a stop.
The second stop I do by waiting til the last minute and then standing on the brakes hard. Brakes do practically all the stopping, with little contribution from other frictional forces. [Okay this isn't quite true, the tires get a lot more wear too]. Lots of heat is given off and the brakes take a lot of wear. If I stopped like this all the time I'd need pads every 5000 miles.

With those as the two extremes, I would say that if your normal braking is closer to the former, you'll have less brake wear. If they're closer to the latter, more wear.

Or am I oversimplifying?

Tonson · #20 11-03-2009, 04:26 AM

A very interesting question and some equally interesting replies.

Because most of the vehicle's weight shifts to the front during braking, that means that the front brakes have to handle most of the work. That also means the front brakes wear faster. I think everyone accepts this as fact.

According to
car-reviews.suite101.com/article.cfm/the_basics_of_brake_maintenance

"Brake wear will also vary on different types of driving. Aggressive drivers who slam hard on the brakes will obviously wear their front brakes out faster than those with a more sanguine style. Also, drivers who spend a lot of time in stop and go urban traffic will wear their brakes out faster."

I would contest the use of the word "obviously" but am otherwise in agreement with the statement.

In any event the choice of braking method is not always down to the driver thanks to other idiots unpredictability which, sadly, means you are hitting the brakes harder and more often than you might choose.... Just be thankful the Cayman's brakes are among the very best!

Read more: The Basics of Brake Maintenance: The Importance of Good Stopping Power | Suite101.com