A little analysis on brake wear...completely useless

so here we go. This is something I always think about when I brake, especially when driving my little scooter instead of the wr as I have to brake a lot more energetically. The basic question, do brakes wear faster if I brake with a lot of pressure for a short period of time, or with less pressure for a longer period of time. (say for example when stopping at a traffic light)

Well my physics tells me that the brakes basically convert kinetic energy into heat. And kinetic energy is a function of speed and mass. Therefore for a kinetic energy of X, (in a perfect world ), X Joules of Heat should be produced.


So at first it would seem, that applying different pressure really doesn't make a difference as it's all about the initial KE.

However, although the THERMAL ENERGY is going to be the same in both scenarios (that is the total heat produced) the peak temperature COULD be different.

So the real question is, do the discs heat up more under hard braking for short periods or softer braking for longer periods? Although I am not 100% sure on this, I would say they heat up more under heavy braking even if it is for a shorter time period, as less of the heat can be dissipated in say 0.5 sec braking compared to 2 sec braking.

So if the peak temperature really is achieved under hard, short braking, this would use up the pads faster and they deteriorate at greater rate the higher the temperature.

Thus the conclusion I arrive to, is that if you stop very quickly (aka apply a lot of pressure for a short amount of time), you will use up your brakes faster than if you apply less pressure but hold it for a longer period, even if the Kinetic Energy was the same to begin with.

So this is the really, quite pointless, conclusion to my thoughts. Any comments, thoughts, improvements (even calling my whole argument bs ) welcome. After all I am in my summer break and my physics is a bit rusty!

I see your point. I think it makes sense. I usually try to avoid hard braking, but it's not due to concern of the brake pads but more for the rotors. I have friends who brake hard a lot and they usually have to replace their rotors pretty frequently since they are so warped. The cars I drive the rotors last forever. A little shaving after each brake pad change and I'm good to go.

My normal stopping procedure for a motorcycle is let the transmission do all of the braking. By the time I get about 30 feet from my stopping point I start applying the brakes, but lightly.

By this doing I've never had to change a brake pad on any of the motorcycles I've owned since I sell them before I can actually see the life span on them. I usually keep my motorcycles for about 15,000 miles of riding.

BoredAzHell20 wrote:

My normal stopping procedure for a motorcycle is let the transmission do all of the braking.

is it really all tranny? gravity and rolling resistance would play a part too.

I concur with your logical theory. Hard braking will cause a faster build up of heat and higher temps than a slower stop using lite pressures.
+1 on the disc warping issue as well.

From what I know about our buddies in Japan we have about 50% redundancy there, meaning we have about twice the durability needed for the expected life Span
Our brakes are just off the shelf stuff at the factory, they have been used on supersport bikes and are in retirement on our WR's
I have been doing this since the early 80's and I have only seen a few cases of warped rotors and that was in the bad metallurgy years in the mid 80's
Lately the stuff is pretty good, usually stuff like this that goes bad has a story behind it, people do stooopid shit sometimes and wonder why their stuff breaks

bigg wrote:

Thus the conclusion I arrive to, is that if you stop very quickly (aka apply a lot of pressure for a short amount of time), you will use up your brakes faster than if you apply less pressure but hold it for a longer period, even if the Kinetic Energy was the same to begin with.

So... cartridge propellant powders burn at various temperatures, but a value in the middle would be about 3000 K. The bullet and case are only exposed to the flame for a very short time.

With that in mind, why can I fire a lead bullet made from wheelweights (melting temp about 490 F. or thereabouts), and they show no signs of melting from either friction with the steel barrel or from being exposed to the propellant flame at the base?

And why can I touch that rifle case without burning myself just a second or so after all that powder burned at 3000 K inside of it?

Jäger wrote:

bigg wrote:

Thus the conclusion I arrive to, is that if you stop very quickly (aka apply a lot of pressure for a short amount of time), you will use up your brakes faster than if you apply less pressure but hold it for a longer period, even if the Kinetic Energy was the same to begin with.

So... cartridge propellant powders burn at various temperatures, but a value in the middle would be about 3000 K. The bullet and case are only exposed to the flame for a very short time.

With that in mind, why can I fire a lead bullet made from wheelweights (melting temp about 490 F. or thereabouts), and they show no signs of melting from either friction with the steel barrel or from being exposed to the propellant flame at the base?

And why can I touch that rifle case without burning myself just a second or so after all that powder burned at 3000 K inside of it?

Very simple. Because the gunpowder only release a very small amount of thermal energy. Do not confuse thermal energy with temperature. Since you are blasting only a few grams of powder, thermal energy is minimal.

Take this example. What would you prefer, a drop of 100C water on your hand, or a bucket of 70C on your heard? Without doubt, the 1 drop at 100C. Even though it has a higher temperature, you will hardly feel it because it has very small thermal energy. Thermal energy is a function of average random kinetic energy plus the potential energy of the molecules. Thus the more molecules you have, the more thermal energy. This means, thermal energy is mass related. And this is why a few grams of gun powder, even if at 3000K will not heat the barrel by much.

Jäger wrote:

bigg wrote:

Thus the conclusion I arrive to, is that if you stop very quickly (aka apply a lot of pressure for a short amount of time), you will use up your brakes faster than if you apply less pressure but hold it for a longer period, even if the Kinetic Energy was the same to begin with.

So... cartridge propellant powders burn at various temperatures, but a value in the middle would be about 3000 K. The bullet and case are only exposed to the flame for a very short time.

With that in mind, why can I fire a lead bullet made from wheelweights (melting temp about 490 F. or thereabouts), and they show no signs of melting from either friction with the steel barrel or from being exposed to the propellant flame at the base?

And why can I touch that rifle case without burning myself just a second or so after all that powder burned at 3000 K inside of it?

Lead bullets are most frequently loaded into low velocity target rounds. At some point just under 1000fps the burning powder/friction factor catches up with the lead and will start to leave heavy deposits of said lead in the barrel. Thus the introduction of copper clad bullets. You must remember the powder burning behind the bullet is in an ever expanding chamber as the bullet is pushed forward. Copper will do the same as lead only at a much higher velocity.

IndigoWolf wrote:

Lead bullets are most frequently loaded into low velocity target rounds. At some point just under 1000fps the burning powder/friction factor catches up with the lead and will start to leave heavy deposits of said lead in the barrel. Thus the introduction of copper clad bullets. You must remember the powder burning behind the bullet is in an ever expanding chamber as the bullet is pushed forward. Copper will do the same as lead only at a much higher velocity.

Copper has a much, much higher thermal conductivity value than lead (although that value varies with the alloy being used) so you run into a problem right there.

The rest is mostly "everybody knows" fairy tales in the shooting world. My .358 Win loads chronograph at about 2400 fps; .303 British to about 2150 fps. For cast bullet benchrest, I load those same bullets, heat treated to about 30 Bhn, to about 2500 fps. With no leading. As do other competitors to various degrees. Tom Gray who held various small group records for years had .308 Win loads at just under 2900 fps, if I remember correctly, until he decided that things just got too finickity up there for no additional benefit.

And I can still fire one of those lead alloy bullets at those velocities and immediately pick it up, incidentally, just as you can do with an all-copper alloy bullet fired at high velocity. Frank Marshall published extensive work on lead bullets at high velocity, going all the way back to the sixties and forward.

Lead is used for inexpensive and/or target loads for various reasons, chief among them being cost. Target loads are kept at just under 1000 fps because that is the velocity at which the least amount of wind drift is achieved - you don't get less wind drift until velocities are way, way up there (go ahead, use a ballistics calculator and check it out - wind drift is a sine wave with its first low point being around 980 fps). Lead bullets do depend on alloying and heat treating along with fitting to the individual firearm to get those kinds of results - something that wasn't available back in the days of lead bullets in older firearms. Hence first the paper patch, and then the copper jacket.

Leading has nothing to do with burning powder, nor friction. It has everything to do with proper fit to obtain obduration of the chamber lede (which is why guys like Mountain Moulds can charge so much for a mould), and matching the heat treating (strength) of the cast bullet to the working pressure of the load. When the bullet fails under pressure of firing and you get gas blow-by, that's when leading starts to occur - and why you usually will find evidence of gas cutting on recovered bullets.

bigg wrote:

Very simple. Because the gunpowder only release a very small amount of thermal energy. Do not confuse thermal energy with temperature. Since you are blasting only a few grams of powder, thermal energy is minimal.

So would you consider the thermal energy in a mortar round minimal? The drill for the mortar to prevent blowing your hands off by forgetting to get your hands out of the way is to slide your hand to the base of the tube when the round is dropped. I've fired dozens of rounds at one time, successively on a fire mission, where I slid my right hand to the base of the tube as each round was dropped. That's with charge six, which has got to be getting close to a pound of powder (never weighed it). Bare hands all the way to be able to fine tune the C79 sight knobs while chasing the bubble. You can do a shoot and scoot mission and touch every part of the mortar with your bare hands after firing to load and go.

Lots of thermal energy going on in those examples, burning powder by the pound, and yet no burns.

On the other hand, fire a belt of .50 cal through an M2. If you pulled all those bullets, you might have about as much powder as two or three mortar rounds. The barrel of the .50 weighs about as much as the barrel of the 81mm mortar, although its ID is tremendously smaller. Walls are much, much thicker. Touch a .50 cal barrel with your bare hands after firing a belt - you'll only do it once. That's why the Number Two always has those thick, thick gloves for handling barrels. And if you give a barrel a chance to cool by swapping it out before reinstalling it again (no more headspacing and timing with quick change barrels - yahoo), you'll get normal barrel life. Don't change it, and you will very quickly ruin a barrel once you get it too hot.

The big difference as I see it is the .50 has a long, long bearing surface for it's projectiles, while mortar rounds for example usually have a very small surface with most contact provided by a plastic obdurating ring. There is far, far more thermal energy being released in that thin little mortar tube to throw that eight pound bomb than in that .50 to fire a 750 grain bullet. On the other hand, friction with the .50 is tremendously greater than you have with the 81mm. How that would apply to the brake question, if at all, I don't know. But I do know much more thermal energy - by far - is being released in those thin mortar barrels than those thick, heavy .50 barrels. But the .50 barrel will brand you, while you can pick up the 81mm tube in your bare hands without burning yourself. And a mortar tube has a much, much longer service life than a .50 cal barrel, probably having a lot to do with the fact they never get anywhere as hot as a .50 does every time it is used.

Quote :

Take this example. What would you prefer, a drop of 100C water on your hand, or a bucket of 70C on your heard? Without doubt, the 1 drop at 100C.

I'll take the 70C bucket for a tenth of a second over the 100C drop for five seconds, please Alex. How about you?

Time for thermal transfer to take place is probably somewhat important, no? Which is what the gunpowder example was trying to get at.

I'm not a physics or engineer guy. But related to your brakes question (which I don't have an answer to), I don't think it is as simple as "shorter duration/harder pressure is worse than longer duration/less pressure". I might be right out to lunch, but dealing with different projectiles, casings, etc over the last thirty years or so seems to suggest it just isn't that simple. I gather you're an engineer type guy, and I would expect somebody in your field has already used various remote temperature sensing setups to determine best-case braking application for various industrial requirements. And the higher you get temperatures of a material, generally speaking the faster wear occurs as the material gets closer to its failure/melting point, no?

So how about the suggestion that short, hard braking provides a lower time frame for thermal conductivity between the pads and rotors to take place, thereby minimizing the heating of the brake pads, giving them more time between applications to cool down, keeping their temperature down, and therefore reducing their wear rate?

Jäger wrote:

So how about the suggestion that short, hard braking provides a lower time frame for thermal conductivity between the pads and rotors to take place, thereby minimizing the heating of the brake pads, giving them more time between applications to cool down, keeping their temperature down, and therefore reducing their wear rate?

Respect for all your knowledge on weaponry I have none


Anyway returning to the quoted statement, the heat produced is because of the friction between the pads and the rotor. Thus the "amount" of friction is the same in both scenarios. If the bike has 100 J of kinetic energy to begin with, and ends with 0, then (removing rolling resistance and wind resistance) 100 J of heat energy HAS to be produced. That is one thing we can't get around of (conservation of energy). So, 100 J is converted from friction to heat in the rotor. If it takes 5 seconds to produce this heat of 100 J, then a fairly significant amount can be lost to the surroundings, as this is what rotors are designed to do (thus the varying styles which are supposed to dissipate heat better when turning).

If however, 100 J is transferred in 0.5 seconds, a lot less of the 100J can be lost to the surroundings during our braking time. Thus higher temperature is achieved.


I'm gonna try a gun powder example. Lets assume you fire a gun with a gun powder quantity which contains 100J. Because gun powder has (in this example) a very fast combustion time, very little of the heat is transferred to the gun. On the other hand, using 100J of coal, with really get your gun hot, but probably won't propel your bullet at all because it combusts too slowly.

So in a way your comment is right on. Except you are missing the point (I think) that X amount of heat HAS to be produced in the rotor. Its all about how quickly it is able to lose this heat to the air. so in 0.5 seconds you are able to loose very little (thus high temp) but in 5 seconds you can loose a lot more (thus lower temp)

I think

Jäger wrote:

Quote :

Take this example. What would you prefer, a drop of 100C water on your hand, or a bucket of 70C on your heard? Without doubt, the 1 drop at 100C.

I'll take the 70C bucket for a tenth of a second over the 100C drop for five seconds, please Alex. How about you?

Mmm this is tricky because you have added time to it. However in this case the drop would still be better. Ever get some boiling water drops on you while you cook? they sting for a moment but then stop. In 5 seconds a drop of water would be back to 30C.

Although water has a high energy density (thus great for cooling the engine), a drop simply contains very little energy (small thermal energy) thus it won't burn, because burning of skin is mostly due to thermal energy transfer to the skin. Since the drop doesn't really have any to begin with, no burn!

Would 70C for a tenth of a second burn you? I don't know I guess we would have to know the mass of water and the surface area of your hand submerged to find out, but am too lazy for that right now

bigg wrote:

Mmm this is tricky because you have added time to it.

How can you NOT have time involved when discussing thermal transfer - particularly when your question involves duration of brake application?

Quote :

However in this case the drop would still be better.

You and I will have to disagree on that. Exposing your skin to the option you prefer for that amount of time would result in a pretty good burn. Even if the surface area exposed were exactly the same.

Quote :

Ever get some boiling water drops on you while you cook? they sting for a moment but then stop. In 5 seconds a drop of water would be back to 30C.

I inadvertently dip the tips of my fingers in lightly boiling water fairly frequently when adding food to water and don't get burned. I HAVE been burned when not being cautious enough, so I'm pretty skippy when working with my fingers that close to the surface of the water and so generally exposure is usually for a fraction of a second. No burns.

This is the same mechanism that allows you to pinch a candle flame out with your finger. Orsweep your hand through a flame considerably hotter than boiling water - of consistent temperature because its temperature is not decreasing - and not burn yourself if your hand is only exposed momentarily. You can do that with a wood flame, and a candle flame. A flame from propane or a welding torch... ah, not so much, or at least I'm not going to try it.

Your point that the fixed amount of energy has to be converted is correct and where my head starts to hurt. To convert the same amount of energy over a shorter period of time has to result in higher temperatures, doesn't it? So why were we all told to not ride the brakes when slowing down when we first got our driver's license - or was that just another "everybody knows" legend?

I guess the point is we're assuming the vast majority of the bike energy is converted/tranferred to the braking system, while flames, weapons components, etc have many other avenues for conversion of that energy.

mm ok so we are not agreeing on the water thing. (although I think we are just misunderstanding each other). just very quickly. How much energy did it take to heat a pot of water? how much did it take to heat a drop of water? that energy is then given to your skin. heating a drop of water is ridiculous and takes place almost instantly on the stove, while heating a whole pot takes several minutes.

few energy = little/no pain
a lot of energy = burn
(btw time with the water example does make it more complitaced because the drop will cool off almost instantly (just like it heated almost instantly) but the pot will not)

Anyway lets get to the important bit! The brakes

I didn't quite catch if we agreed or not! But actually you brought up an interesting point. On say a long downhill run, you are not supposed to hold the brakes the whole time to prevent over heating. but in this case we also have gravitational potential energy in the equation, so basically the rotors have to keep converting and converting energy to heat and this adds up. causing overheating.

big question: so assuming no rolling resistance, wind resistance and engine brake, would it be better for the rotor/pads to wait till the end of the slope and brake all at once even tho by that time we have reached like 250 mph? I don't know...

ah this thing is become more confusing than I initially thought

On the subject of guns, anyone got a way to haul a long rifle on a bike? lol. My shooting place is my riding place too, and 30 miles away. Also, my S&W .500 happily lobs 440 grains of lead at 2800fps. The brisnell hardness is pretty far outside of what the standard mathematical formula I'm supposed to be using but it still isn't leading too bad.


Back to the bikes brakes, rather than just the heat, what is the affect of the heat cycling? Rapid baking puts heat in to them quickly, meanwhile braking gently more gradually heats them. I would think the faster heat cycle would be more harsh on the materials.

Not sure about the pad part...but maintaining the integrity of the rotor seems straight forward.
Like in welding different materials, with different conductivities. Welding steel is pretty easy, because of the rate it conducts heat...compared to stainless steel, which conducts heat slower, and is more prone to warping, from the hot spots. To prevent the material from expanding unevenly, preheating will lessen the tendency of these hot spots.
Now if applied to the rotor...you heat the surface rapidly, it could expand faster than the core material...probably causing stress fractures or microfractures in the surface, which would allow surface to break down faster, and no doubt the pads as well. As compared to a slower more evenly heated rotor which wasn't subjected to that uneven expansion and stress.
Anywho, that's my best guess.

My front brakes are screwed. I've recently taken up stoppies, and to get the pad temp up for best grip I drag 'em a little before getting on 'em hard for the stoppie.