still confused as to why a thicker CD'ed rottor will crack before a thinner one...

and why this principal doens't hold true for blank rotors

 

What he brings up is differential in temperature, this is why solids have the cooling problem. If you brake duct a solid you need to do it on both sides about as equally as you can get it, on a vented rotor you can be a little more careless about placement of the duct setup, if the vent gets air then it will do what it can to cool it.

If one side is cold and one side is hot, remember that cold things "shrink" and warm things expand. I might even guess that you could see an inch of difference between a cold day/warm day skyscraper. This is similar to how toaster oven switches work, a bimetalic strip that when heated flexes at different rates on either side, making it "throw its own switch" instead of being electronically actuated.

 

buy since the holes dont cool much at all.. i dont get it ...


also

i was thinking about the rotor thing...

I still think a thicker rotor would be cooler than a thinner rotor. The smaller rotor would be "maxed out" whereas the larger rotor could still have more "braking" left in it.

the larger one will absorb and disipate engergy faster.

the smaller one will absorb it slower and disipate it slower.

however in the end, both absorbed close to the same amount of evergy, the larger one disipating a bit more. However, temp is the AVG Kenetic engergy of the molecules

if the total Energy is the same, the one with more "molecules" will be coolder right?

 

I already posted at least twice, a rotor with more mass will be more effective with heat dissipation, this is obvious.

The samller rotor will absorb heat no slower, it just wont be able to get rid of it as well because it has less surface area to dissipate the heat and less mass to hold it, ergo it will be overpowered quicker. A smaller object will not absorb things slower, think about that for a second... If someone punches a smaller person they aren't going to take any less impact than a larger person, however they most certainly wont be able to absorb it as well mass vs mass the larger object wins every time.

Drilled rotors lose thier structural integrity, which is why an as-cast rotor is a better choice, however since the holes offer no real cooling, there is no real reason to spend the money on such an item other than the looks - since people think that they are a performance item.

 

ok, nevermind-
i'm a dumb**** and misread a post. its been a long day


as far as this

quote:
--------------------------------------------------------------------------------
Originally posted by Eric Bryant
Adding thickness to a drilled rotor only makes the cracking problem worse, as you typically increase the temperature difference (thermal gradient) between the inner and outer surface of the rotor "cheek".

Of course, this isn't a problem with unvented rotors, which is why bikes get away with properly-manufactured drilled rotors.
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I understad why temp differeances cause cracking, but why would a thicker (bigger) rotor icrease the temp differance?

 

Pretty close: If the material is the same and the input power is the same, the rotor with the least mass will have a higher delta T (temp change), because it will absorb the same amount of heat in less mass but in the same amount of time: it heats faster.
I'm just saying what Dr.Mudge said.


The rotor with the most surface area exposed to the air should dissipate heat faster. Here is where a well vented smaller dia rotor of less mass than a larger solid or poorly vented rotor could perform better over the course of a shorttrack or road race. When sanctioning organizations limit wheel diameter (NASCAR 15 in. F1 13 inch), they force brake designers to make do with smaller braking systems, which they do well.

Obviously brake rotors heat cycle every time they are used. Iron grows and shrinks about .007 inches per foot per 100 deg F temp change. In other words, a 12 dia rotor heating to 1300F (starting to glow) grows 3/32 if a inch (.093). If the heating is uneven side to side, there is a lot of stress put into the metal. Even on the street, during hard driving you might see over 500 F delta T.

Any stress risers, which can be disruptions in the cast surface like cross holes, and/or sharp edges are the favorite place for cracks to start during this thermal cycling. Think of a sharp groove in a transmission shaft which gets torsional load cycling. The shaft often fails at the groove. IMO, the situation is similar.

FWIW, vented rotors pump air from the middle to the outer edge. Air really doesn't like to make sharp 90 degree turns into little holes, so it just kind of ignores the holes, except perhaps to draw air into them from the hot friction surface.

I'm a 'no cross drill' team member, obviously.

 

that explains why every blue print i've been staring at says "remove all burrs and sharp edges" on it at the top

 

Since heat is energy, and it takes more energy to penetrate deeper into a mass (as well as time), this is why a thicker rotor may have more temperature differential from the center to the outside versus a thiner one. Just as someone with more subcutaneous fat will stay warmer for longer when out in the cold, than someone with more visceral fat.

Would you like a more obnoxious example?

 

just for fun

 

Ok, your name is Arch E. Bunker and you work for the Air Force, your position is classified so I can't get into that here. You spotted a crash landing at the north face of Area 51, so you take off with your team after the alien, but its nightfall already and as you and I both know, nights are cold in the desert because the ground doesn't retain heat well out in these parts.

What is the first thing to get cold? You will notice that your head, hands, and feet will get cold faster than the core of your body. Think of cold and hot as polar opposites, and again it takes time for either to penetrate deep within the core of a mass. So, next time your out stalking aliens dont forget a good set of boots and some military issue wool socks, along with some kind of hat, maybe a ski mask because they will scare the alien and it will be submissive. 80% of your body heat escapes through the top of your head.

 

so another question-

why does this temp penetration deal only hold true to CD's rotors and not blank rotors? (re: eric bryant)


If what your saying causes further cracking and weakening, why are the heaviest duty rotors the tickest?

Why do the thicker ones last longer in the 155mph to 0 (repeat 100x) testing?

why do fools fall in love?

 

You usually give something up when you gain something, right? A more massive rotor will add unsprung weight, and perhaps have more temperature differential if it is a very thick one, yet it still has the mass to take the most energy from the brakes without heating to a point that a smaller rotor would see.

As for bikes, look how thin the rotor is. Now a solid rotor on a car is going to be much more massive, and brake ducting on a solid rotor car is a little finicky, and they definately have a tendency to warp, vented rotors are king when talking cars, bikes I dont know.

 

i meant in regards to his "thicker rotors on a dross drilled will crack easier"

ahhh i've had it. i'm done till monday.

I'm upto the vette rotors. I think i'll stop now so I have something to look foward to monday

 

Dude, two principles that were mentioned:

* Thicker rotor will have more temperature differential causing different expansion rates across the rotor.

* Cross drilling destroys the structural integrity of the rotor, cryoing them can help "realign" molecules to help bring some structural strength back.

Combine the two of those and you have a double whammy.

 

... but thicker ones have LESS tendancy to crack...

the way eric said it (or i read it) it seemed like this only held true for CD'ed rotors.


The thicker rotors last MUCH longer than thinner ones udner high speed tests and all other tests for strength