Not trying to stir the pot

[yody]

qship how are you arguing? you haven't put any information out there?? or really said much at all on this thread. I feel the same way you do, that you won't ever change your mind. I think the crossdrilling being bad thing is an internet trend, and most people are just going along with it.(I do appreciate your input on my rear brakes though Well see ya guys on other posts

[Zefhix]

Best thread ever
38 to go; I wanted in somehow...

[Fuelie Fan]

[QUOTE=dennis68]Pad area very much plays into braking performance, the same way pad composition does. Nice try though, throw the 3rd year engineering major-math out there to confuse the point-almost got me.QUOTE]


Okay.....care to elaborate as to how? Again, not saying wrong or right, but you offered really nothing to substantiate other than "it just does". I'm totally not a braking expert, so I'd like to hear the explanation.

Pad composition most certainly affects mu_k, and also how mu_k will change with temperature. I agree 100% on that point, but you can't just make an arbitrary connection between pad material and pad area.

A quote from another site that I came across purely by accident today (I swear I made my post before I read it):

"Coefficient of friction does not depend on surface area, but rather the friction value, multiplied by the force applied (by the caliper pistons to the pad). Surface area has a lot to do with pad wear, or think of a rotor that has a ¼" swept area. It will still stop the car the same amount, but the pads won't last as long."

Seems to support that derned engineering crap that nobody wants to believe

[dennis68]

In a stone cold braking contest I would agree that brake pad size has little to do with stopping distance, however I am not aware of anybody who operates their brakes under those conditions. Since a larger surface area is more capable of disipating heat and heat is the leader in brake fade, it stands to reason that under normal operating conditions brake pad size doea have a substantial impact on stopping distance. Sorry, I don't have a formula to show how this works and I actually don't use them at all for the most part, practical application wins over text book everytime. :icon996:

[Fuelie Fan]

That seems reasonable.

BUT, does this argument directly apply to drilled rotors (which was supposed to be the focus, I think we got slightly off-track)? A thought experiment says that the holes provide no area to create and conduct heat (which is what braking is, Kinetic to thermal conversion), or in other words the contact area has been reduced. At first this seems to suggest that it would act in the same way as a reduced pad area. BUT, I have to wonder what is going on at the unused portion of the pad at any instantaneous second. Also, it is not like the holes are circumferential; they pass through the pad, and are followed by solid material. So how does this affect things? It does not seem entirely unplausible for well placed holes to allow the brake pads to perform as effectively (or at least as far as the a moderately skilled driver can tell) as if they were against a solid rotor.

I tend to lean toward the holes not reducing braking performance nearly as much as physically removing material from the pad. Either that, or I don't have a good mental image of how much "equivalent area" each hole removes. You could do a ratio of areas (solid vs drilled) perhaps, then decrease the pad on the solid rotor by the corresponding percentage? Would that be equivalent?

[harshman]

yup! now i got a nose blead.

[Fuelie Fan]

Oh, and it bugs me when people say things like "practical over theory". Just about everything can be accounted for with an accurate model. I guarantee the pros HAVE theories and equations that precisely explain what we're talking about, it's just we don't know about it. So really all that statement says is we don't know enough yet to account for what we've seen.

the best exception: tires. They're incredibly complicated. But the science is getting better and the mathematical models more accurate every day.

[dennis68]

Yeah Firestone and Goodyear know all about building better tires these days. Yes, I have worked for both and have done my share of tire recalls. My point was that theroy is good to get the mind thinking but you can never plan for practicality on paper. I deal with DCX engineering on this all the time, they design , and it's good until you try and put it to use in a real world enviornment. That when I get to re-design to work correctly.

[yody]

So Dennis, since you are such a real world type of guy, what kind of real world testing have you done? I know I have been stepping on your feet this whole post, but it seems as if we are representing to differnet sides of an argument or should i say discussion. The only reason why I ask you for testing data is that you are making a point that holes do not work, I am merely questioning that theory. If I was to say They definetly do work, I would have to back up my presumption with hard facts and data which i don't really see here, I mostly see people just getting irritated because I am not going along with their ideas. All I see is bland statements with no real facts to back it up. The only real problem with these engineering discussions is that it seems so easy to argue back and forth and find loop holes in peoples ideas, kinda like Kerry v Bush. One person is always going to think the other way and there is no way to convince them otherwise. Basically this is a good discussion. THere isn't going to be any for sure answer. I just think that people should not state things that they overheard somewhere else, and state them as a fact here, because that is the new trend to go along with. I am not pretending to know much at all here, just exploring a different side to these theorys. Well maybe we can agree that there is no for sure answer. I guess it would be pretty hard to do a real world test to see how they helped in brake fade. I don't think a test like Baers is sufficient, i believe all they did was put the brakes on a fixed machine spin them up fast, apply the brakes and thest the temperature output, I don't think that replicates real life standards, where you have a weight of a vehicle and brakes inside of a wheel next to a hot engine, and the acceleration of a car to stop

[jannes_z-28]

Haven't read all posts in this thread yet but I will throw in some thoughts anyway.

Here is a link with some facts:
http://www.apracing.com/car/brakedisc/face.htm

THis site has a lot of brake information, AP Racing is one of Europes leading high performance brake supliers. they have a few Formula One teams as their customers.

You will find a lot to read here.

So what do they say about drilled rotors?

"Mainly used for aesthetic reasons on road applications."


I guess the last word on this will never be said :-)

Jan

[Steve Chryssos]

I can't take this **** anymore!! This isn't one guys opinion vs. another. This is BASIC Physics. What's the problem? Need some proof? You don't need brake manufacturer or sports car maker. Go to any high school physics textbook and look up the chapter on The First Law of Thermodynamics. Search the entire internet.

If you take two identical solid rotors--no make that solid OBJECTS (same, material, dimensions, features) and drill holes in one of them, the drilled rotor--I mean object's-- ability to ABSORB HEAT will be lower than it's solid counterpart. The little metal shavings on your work bench resulting from the drilling process represent LOST MASS. Mass is a fundamental component of the heat absorption equation.

THAT'S ALL. THAT'S IT. ASTA LUIGI!

So, given identical circumstances, the solid rotor will always work better because it can absorb more heat.

Does that mean that drilled rotors are junk? No. As long as that drilled rotor is used in an application that does not EXCEED IT'S ABILITY TO ABSORB HEAT, it's fine. Any rotor--drilled or not--will warp or crack when it's heat capacity is exceeded. But the otherwise equivalent UNdrilled rotor has a greater capacity to absorb and is therefore less prone to failure under identical circumstances. WHY IS THIS SO HARD TO UNDERSTAND?

My hot rod has Baer drilled and slotted rotors because they they reduce unsprung weight and look cool. I can take advantage of the looks and weight reduction every day on the street.

So in case I have lost you, solid rotors are better than drilled rotors. A drilled rotor is entirely effective and acceptable when the system and circumstances in which that rotor is used does not exceed the rotor's capacity to absorb (and dissipate) heat. At any given time, you can plug a solid rotor in place of that drilled rotor and extend that otherwise identical system's braking capacity. See? All that and we never even had to touch the Second Law of Thermodynamics!!


Yody,
One of life's best lessons is:
Disagreeing for the sake of argument does not make you any more intelligent or wise--nor does it make you appear any more intelligent or wise to others.

Search:
-Specific Heat Capacity
-Heat Absorption
-First Law of Thermodynamics

[dennis68]

It may take a while to recover.

[Steve Chryssos]

Now now Dennis. I'm not trying to flame him. I'm 37 and I just learned that lesson like, 3 weeks ago. (Not the thermo's--the other one.) I'm just sharing the wealth and spreading the love.

[Norm Peterson]

If you take two identical solid rotors--no make that solid OBJECTS (same, material, dimensions, features) and drill holes in one of them, the drilled rotor--I mean object's-- ability to ABSORB HEAT will be lower than it's solid counterpart. The little metal shavings on your work bench resulting from the drilling process represent LOST MASS. Mass is a fundamental component of the heat absorption equation.

Scanning through this thread, this seems to be the first real mention of heat capacity. It's not simply that the shavings on the bench represent heat sink mass that's been intentionally thrown away - it's mass that's been discarded precisely from where temperatures and rates of temperature change are going to be the highest. So you must absolutely expect rotor swept area temperatures to increase by more than the mass of shavings relative to total rotor mass would suggest.

Now let's talk heat flow. This is a far more complex issue than static snapshots of temperature over the rotor area (volume?). The time element is involved, and the rates of temperature change matter. In addition to being very sensitive to the rate of temperature change, these thermal transient terms are highly sensitive to local geometric discontinuities. And increasing the magnitudes of these goes to reducing fatigue life. Need I mention the connection between fatigue and cracking? Or only that pad coefficient of friction is not constant even over the range of temperatures encountered in street driving?

By way of explanation of thermal transient effects, the total picture can be reasonably well distilled down to the evaluation of three separate effects that are functions of the material properties (heat conductivity, specific heat, expansion, etc.). One is a linear temperature distribution through the thickness of a given part. Another is a nonlinear component of temperature distribution through the same thickness of the same part (this is a peak stress effect, and is the first of these three to reach its max value). The third is the effect of the difference in average temperature between adjacent sections of different thickness or between similar thicknesses subjected to different rates of temperature change (this one peaks last). I spent a fair part of my 20+ years in the nuclear power industry making evaluations that included precisely this sort of stuff.

At least slots don't interrupt the radial heat flow path even though they do increase the thermal effects (but with a professional race team budget I'd be discarding rotors far more frequently than I would care to on my daily driver or auto-X car).

I'll quote something I posted over two years ago (partly because I'm too lazy to retype it and partly to illustrate that there's more than one brake tech thread over at corner-carvers.

Quote:
Originally posted by jp99gt
. . . It would be interesting to make a finite element model of a rotor and dump the heat energy in at different rates, like 2.5, 3.0, 3.5, ... 5.0 seconds and see the differences in thermal stresses. Maybe I'll run a solid rotor problem for simplified geometry and see how it works out.

--Jim



I think you'd eventually end up modeling the vanes and the holes. The thermal stress terms are quite sensitive to geometric discontinuities (local thickness changes as well as the presence of the holes) and the rate of heat input. Remember also that there's more energy to be dissipated from the first 10 mph of decel than from the last 10, so with constant deceleration assumed the thermal effects will be more severe during the early portion of the event.

With that in mind, perhaps the more aggressive pads are putting yet more heat into the rotor early in the event even if the total duration of braking remains about the same. IOW, the early deceleration with them is higher than before, indicating that the assumption of constant deceleration isn't entirely accurate. This makes the thermal transient event more closely approximate a step temperature change.

Re upscale Euro cars w/drilled rotors - I suspect that their owners tend to fall into two categories. Those who will never tax their brakes sufficiently to cause problems to surface, and those who can/will replace rotors with no more thought than that given to replacing spark plugs.

Norm

[dennis68]

Ah, c'mon Norm-I had the heat capacity affect nailed back in post 52 and again in 64. Albeit not as elequently put as Steve, but the same principle.

I promise to encourage Yody anymore, I promise to encourage Yody anymore, I promise to encourage Yody anymore, I promise to encourage Yody anymore, I promise to encourage Yody anymore, I promise to encourage Yody anymore......
Sorry my wife is a teacher, says I have to write on the screen a hunderd times not to encite a riot anymore, I figure that was close enough.

[yody]

SO after all that mumbo jumbo, do holes have any benefits to brake fad/performance?
BTW steevo, i am not arguing just for the sake of argument nor to look smart, maybe that is a trait of yours? I just don't see sufficient info, just self proclaimed braking genious's stating things with no backup. Obviously Norm is a very educated person, and knows his physics, but maybe i didn't rear or undersatnd correctly, but I still don't see an answer to if holes work or not. and it seems even the genious's can't come up with a solid reason why These expensive cars use them. I guess all of their R+D in racing and 50+ years in teh exotic high performance market and racing has resulted in them putting brakes on cars that give up performance for looks? I will admit that lately in some of these posts there has been some better info, enough to make me even more vague on what the real answer is. It one respect I do agree with Dennis, people tend to over anaylize things in their head than putting it to the road.

[immrtransam]

WHEW I never thought I would finish reading this post. As far as I know as far as motorcycles are concerned the holes on brake rotors are for cooling(greater surface area) and to help shed water. On my 1975 Suzuki GT750 the rotors were solid, and on the front fork leg was a warning sticker saying that "in wet conditions braking would be effected ". the next year they had drilled rotors and no warning stickers.

Plus they look cool

[yody]

by the way, don't beat me up at sema

[jannes_z-28]

Without knowing for facts I think that the European "hot cars" that have drilled rotors do have that more as a marketing feature than anything else. It makes the cars look racier and faster. People think it looks cool. I think that almost everyone driving those cars never will brake so much that it will hurt the heat-transfer or warp the rotors.

I'll bet that the engineers lost the battle to the marketing guys on this one. Drilled rotors sell better.

Everyone follows and think it is a good thing because Mercedes have it. In the auto industry there are a lot of "followers" that follow without knowing why.

I heard this story once, it is supposed to be true.

In the sixties one English factory motorcycle racing team constructe a fuelpump that got its movement from the rearsuspension. It reduced weight over the conventional setup. However to make the engine start the mecanics had to run behind the bike and jacking the rearsupension so that the engine could start. Of course they won the race.

On the next race all the other teams did the same thing, jacking their rearsuspensions in the believe that the first team had done this to warm up the shocks and therefore had an advantage.

This is an example of how easy it is to do things that others do without knowing why.


Jan

[Norm Peterson]

SO after all that mumbo jumbo, do holes have any benefits to brake fad/performance?

Not unless you have pads that significantly outgas and then drive such that you make them do so.

. . . but I still don't see an answer to if holes work or not.

In that respect, the answer is "it depends". With cheapie pads (and relatively small OE-size brakes) it's quite possible that you could improve fade resistance somewhat, though that would come at the expense of increased likelihood of cracking. First stop performance would remain about the same, plus or minus whatever the temperature effect over the time duration of the stop (temperature time history of the pads & rotors) has on the coefficient of friction and whether or not the driver can accurately correct his pedal pressure continuously to suit.

Fatigue life is a complex thing to estimate, as it's a function of how severe each stress cycle is and how many there have been. It's an extremely nonlinear thing, BTW. The quick and dirty estimate that I'm most familiar with is that fatigue life is approximately an inverse 5th power function of alternating stress intensity. The simplest example that I can think of is to assume that you have a rotor that's good for exactly 100,000 cycles at some thermal stress intensity. Then double that stress intensity. It's now only good for around 3125 cycles, or around 3% of what it was good for at the lower stress level. Fortunately for us all, most street usage is at some small percentage of the 100,000 cycle stress level, so the cycles in actual service go into the millions.

Norm