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Originally posted by SleeperZ:
Originally posted by Rara:
Originally posted by SleeperZ:
and really high thermal expansion.
Nope, that alone would make the cracking problem worse.
I think not.
One should fully understand thermodynamics before teaching it.
With the proper alloy, a C/D rotor WITH high thermal expansion (and with the other properties I described above as well as a few I didn't describe) would expand into the holes, not outward.
This would only be possible with an alloy that would transfer heat into inner layers of the rotor, where a heat sink is mounted between the 2 sides of the rotor.
(To better understand the statement above; Think of a brake rotor as a stack of 3 pancakes, the top pancake transfers heat to the middle pancake, which inturn transfers heat to the bottom pancake. Finally the bottom pancake transfer heat to the plate. And the plate disipates the heat.)
Sounds cool. Can you make 'em without the holes? I'll take four.
Function before fashion.
'96 Contour SE
"Toss the Contour into a corner, and it's as easy to catch as a softball thrown by a preschooler." -Edmunds, 1998
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So your telling Rara how brakes work? 
I think he pretty much has that figured out by now.
"Cobb - It's not just for corn anymore!"
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SleeperZ, you're only addressing (weakly) the issue of cracking. You are conveniently ignoring the primary issue at hand, for what must now be the thousand-and-third time: Brakes work by converting kinetic energy to heat energy through the use of friction.
The more heat energy the rotors can accept, the more braking power you have.
Cross-drilling reduces mass more than they add cooling capacity thanks to the holes.
Reduced mass means reduced heat sink.
Reduced heat sink means the rotors can't accept as much heat energy. Therefore reduced braking power is the result.
Period. End of story. Now come all the idiots saying "but PORSCHE does it" and blah blah blah...well, they have rotors that are big enough to accept the required thermal energy and STILL be filled with holes (they're cast that way, not drilled) so they're big enough to generate the rotor torque they're looking for. Practically nobody on the street and most assuredly single-digit numbers of people with Contours have the capacity to use this effect.
Pacific Green '96 Contour LX V6
â??98 GTP, light mods, 14.66/94
Calypso Green '92 Mustang LX coupe, 13.56/101
Crown Autocross Club 1999 Street Tire Champion, 2000/2001/2002 Street Modified Champion
KCR SCCA 2002 Solo II Street Modified Champion
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"Absolut Rara."
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"Absolut Rara."
Joined: Sep 2000
Posts: 3,223 |
Originally posted by SleeperZ:
Originally posted by Rara:
Originally posted by SleeperZ:
and really high thermal expansion.
Nope, that alone would make the cracking problem worse.
I think not.
One should fully understand thermodynamics before teaching it.
With the proper alloy, a C/D rotor WITH high thermal expansion (and with the other properties I described above as well as a few I didn't describe) would expand into the holes, not outward.
This would only be possible with an alloy that would transfer heat into inner layers of the rotor, where a heat sink is mounted between the 2 sides of the rotor.
(To better understand the statement above; Think of a brake rotor as a stack of 3 pancakes, the top pancake transfers heat to the middle pancake, which inturn transfers heat to the bottom pancake. Finally the bottom pancake transfer heat to the plate. And the plate disipates the heat.)
I must ask you to follow your own advice. Keep in mind that a material with a high thermal expansion rate would be more prone to cracking at points where the thermal gradient is high, especially if a high stress concentration was at the same point, oh, something like a drilled hole would do the trick.
Also, in order to run a rotor with a higher than typical thermal expansion rate, you would have to increase your caliper seal rollback to limit serious DTV problems over time, and this would toss pedal feel and initial response right on out the window.
Further, lowering the thermal conductivity of the outer face of the rotor would limit the ability of the rotor to absorb the heat, generated while braking, fast enough, which means the temperature of the pad, caliper and fluid would increase significantly over a short period of time. This is bad. Also, if you can figure out way to vary the thermal conductivity across the depth of the rotor, fantastic, except that you have just decreased the thermal stability of your rotor, especially if you have a high thermal expansion rate, because as heat is transferred into the rotor during a braking event, you create tremendous internal stresses into the rotor itself because they will all be expanding at different rates (because of the temperature gradient during the transient event). If they existed, I wouldn't be surprised at all to see all 3 "pancakes" seperate under anything other than mild use.
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How am I weakly addressing the issue of cracking?
How am I ignoring the issue of how brakes work?
I interjected a suggestion that solves the problem of cracking and allows for maximum heat absorbtion.
A dense enough alloy with a suffecient enough thermal pass-through rate will absorb the the heat created by the friction and transfer it (at a rate greater than or equal to the rate at which it accepts it) to a heat sink in the center of the rotor that dissipates heat (at a rate greater than or equal to the rate at which it accepts it).
Most likely an alloy that absorbs heat quick enough will expand.
An alloy that only expands in a single dimension (planar expansion), not an alloy that swells (expands in all dimensions), needs to be chosen.
The expansion of the alloy can be directed inwards by adding a ring of a diffent alloy at a density and tencil strenth greater than the alloy of the rotor.
Then the rotor is given an area to expand by crossdrilling in holes.
The ring would also need to be very thermally reflective.
All I was saying, is that it's possible to make a rotor that is better when C/D than when it is not.
The cost of doing so would be great, exotic alloys would need to be used and the process directly thermal bonding two alloys even in small amounts would cost more (1" x 1" square surface area is about $2000) than most brake packages currently offered.
Knowing how brakes work, and understanding thermodynamics is two different things.
I am not telling anybody how brakes work, I am stating principles of thermodynamics (and a little bit of material science) that can be applied to braking.
Frank McCoy aka Mod-deth aka Mid Life Crisis aka SVT Doood aka mcgainer is a SCAM ARTIST
Pre98 Zetec - Some Mods
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Quote:
The cost of doing so would be great, exotic alloys would need to be used and the process directly thermal bonding two alloys even in small amounts would cost more (1" x 1" square surface area is about $2000) than most brake packages currently offered.
I love iron.
Function before fashion.
'96 Contour SE
"Toss the Contour into a corner, and it's as easy to catch as a softball thrown by a preschooler." -Edmunds, 1998
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"Absolut Rara."
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"Absolut Rara."
Joined: Sep 2000
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SleeperZ, I think you are missing any semblance of common sense on this one. Quote:
How am I weakly addressing the issue of cracking?
Because you haven't addressed the source of the cracking; the series of added stress concentrations through the rotor, ie the holes. At least Porsche has reasonably addressed the issue when they really want to use them, by casting the holes into the rotor.
Quote:
How am I ignoring the issue of how brakes work?
You stated that the outer layer, ie next to the pad, should have low thermal conductivity to force the heat to "flow" to the center of the rotor where it could be dissapated by the "heat sink" portion of the rotor (your use of heat sink here isn't really correct, but I'll let it go). If the outer layer has a lower thermal conductivity, then the heat isn't readily transferred into the rotor, which means it won't transfer as rapidly, which means a couple of things.
1) The heat generated at the pad rotor interface won't be absorbed as quickly by the rotor, which means it has to go somewhere during the transient event; that would be right into the pad, then the caliper, then the fluid; none of which you want absorbing the heat if you can help it.
2) If the conductivity is low, then you can pretty much kiss the outer surface of the rotor, as well as the outer third of the cross drilled holes goodbye for radiating the stored heat to the air. Especially considering you can't leave any coatings or anything on the rotor surface, becasue it would get worn away.
3) I'm sure there's more, but that's enough for now.
Quote:
I interjected a suggestion that solves the problem of cracking and allows for maximum heat absorbtion.
No, your solution would lead to additional cracking, and in additional areas and planes than before. Differing thermal properties across a rotor's thickness will lead to much higher stressed zones across the boundary layers between each region.
Quote:
A dense enough alloy with a suffecient enough thermal pass-through rate will absorb the the heat created by the friction and transfer it (at a rate greater than or equal to the rate at which it accepts it) to a heat sink in the center of the rotor that dissipates heat (at a rate greater than or equal to the rate at which it accepts it).
Now, see this is slightly different from your original proposal, now you simply refer to a "suffecient" (sic) thermal conductivity, implying a single rate, where before you proposed three different rates across the thickness of the rotor. The part that is still screwed up here though, is you make the statement that it should transfer the heat at an equal or greater rate than it received it. If you could do this, your rotors would be more or less ambient temperature regardless of the braking event. If you can figure this one out practically, more power to you. I'll tell you this, in a practical situation, you want the rotor to absorb the heat from the braking event as fast as possible, keeping as much heat out of the rest of the foundation components as possible. Then, after the braking event is done, you worry about getting the heat out of the rotor. And unfortunately, you are practically limited to ambient air as the only cooling medium (yes, liquid cooled brakes have been tried, and there is a reason they are extremely uncommon still)
Quote:
Most likely an alloy that absorbs heat quick enough will expand.
Tell you what, you find me any alloy that won't expand when temperature is increased. In all of my engineering classes, Thermo, Materials Science, etc. I don't recall a single material that had a coefficient of thermal expansion that was a dead flat zero.
Quote:
An alloy that only expands in a single dimension (planar expansion), not an alloy that swells (expands in all dimensions), needs to be chosen.
Got any guesses on any alloys that really only expand in one direction? Any that would be terribly useful here?
Quote:
The expansion of the alloy can be directed inwards by adding a ring of a diffent (sic) alloy at a density and tencil (sic) strenth greater than the alloy of the rotor. Then the rotor is given an area to expand by crossdrilling in holes. The ring would also need to be very thermally reflective.
Yup and at the same time it can dramatically increase the internal stresses of the rotor, woohoo! this is exactly what we want! MORE STRESS AT THE CONCENTRATIONS THAT ALREADY EXIST!!
btw, this rotor is getting pretty damn complicated, as I believe we are up to 4 different very fancy unobtianium alloys. On a side note, I'm not sure i would trust the design of someone that can't spell "Tensile" or "different" or "sufficient". . . (and no way can you say you fat fingered the typing on those, as none of the wrong letters are even close to each other)
And why would the ring need to be thermally reflective? So it would be colder than the rest of the rotor? So it could induce MORE thermal stresses into the mix? I'm not gonn even touch the part about the rotor expanding into the holes, since it can only locally expand into the holes, what about all the areas of the rotor that aren't right next to a hole? You would need a lot of holes; which further reduces the thermal capacity of the rotor, raising rotor temperature, which creates more stresses. It's a vicious cycle.
Quote:
All I was saying, is that it's possible to make a rotor that is better when C/D than when it is not.
I think its pretty clear that this rotor isn't better.
Hey, here's a better idea, rather than work SO hard to save your precious cross-drilling, why wouldn't you work to come up with a rotor material that lives at a much higher temperature, and then work to improve cooling airflow to that rotor? You know, kinda like Formula One does with carbon composite rotors. . .
Quote:
The cost of doing so would be great, exotic alloys would need to be used and the process directly thermal bonding two alloys even in small amounts would cost more (1" x 1" square surface area is about $2000) than most brake packages currently offered.
Hmm, wait, I have an idea, how about we just don't drill holes in the rotor in the first place? Then we can just stick with a good old cast iron plain face rotor that is dirt cheap and will have similar performance anyway.
Quote:
Knowing how brakes work, and understanding thermodynamics is two different things.
I dunno, maybe they aren't, I mean, you don't seem to have quite the grasp you need on either one. Then again, maybe they are, I didn't know nearly as much about brakes back when I was taking my thermo classes.
Quote:
I am stating principles of thermodynamics (and a little bit of material science) that can be applied to braking.
And not doing a terribly good job at it unfortunately. But keep working at it, you'll get there.
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damn, this horse died a long time ago.  everyone has seen both sides so let them take from this what they want but end it already. peace. 
PS: rara i asked about your twin turbo set-up in the pics and vids section. if you could go take a look. thanks.
Oo (xxx)oO
o xxxxxxxx o
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Originally posted by mikey boy:
damn, this horse died a long time ago. everyone has seen both sides so let them take from this what they want but end it already. peace.
PS: rara i asked about your twin turbo set-up in the pics and vids section. if you could go take a look. thanks.
Can cross-drill rotors be machined?
Actually, I'm rather enjoying the technical discussion, so if you can't handle it...
Function before fashion.
'96 Contour SE
"Toss the Contour into a corner, and it's as easy to catch as a softball thrown by a preschooler." -Edmunds, 1998
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CEG\'er
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LOL! Now we get right into big, fat bootylicious J-lo meaty TECH and people want to abandon the discussion to let others "take from it what they will". EXACTLY why it should go on as long as the people providing actual tech can stand banging their head against the wall. The more people that come away from this knowing more about how brakes REALLY work, the better. If you're done, then...well...buh-bye then. I'm still listening, I hope others are too.
Pacific Green '96 Contour LX V6
â??98 GTP, light mods, 14.66/94
Calypso Green '92 Mustang LX coupe, 13.56/101
Crown Autocross Club 1999 Street Tire Champion, 2000/2001/2002 Street Modified Champion
KCR SCCA 2002 Solo II Street Modified Champion
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