warped my brembo rotors
01-24-2007, 01:35 PM
#22
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The material is the most determining factor on the rate at which heat is aborbed. What Kolia is talking about is that with a specific material, the more material you have the larger heat sink you have. Thus Q=mC(p)detaT, so as m gets larger your detaT is less (since Cp is a constant for a given material), thus your temperature gain is less for the same energy input.
So Kolia is correct.
The reason for two piece rotors is to allow more cooling, yet these are much more expensive to build than a single. Thus, why we still have a single piece.
And for us that track - your rotors are never at a steady state temperature, you are always heating them up and cooling them down, most of the time 5+ times per minute!
So Kolia is correct.
The reason for two piece rotors is to allow more cooling, yet these are much more expensive to build than a single. Thus, why we still have a single piece.
And for us that track - your rotors are never at a steady state temperature, you are always heating them up and cooling them down, most of the time 5+ times per minute!
Originally Posted by spacemn_spiff
I agree with theory , but dont agree with the application and we have been on this road before. 
Its not desirable the rotors to be heavy, if that was the case no one would have 2 piece rotors or composite rotors and definitely would not have vanes if mass of the rotor was enough to cool it down or absorb & dissipate the heat. Vanes are not very effective in dissipating heat at very low speeds, other than that they do most of the cooling even when the temperature has not reached the peak or steady state, the mass allows it absorb heat at a certain rate and raise the temperature which is governed by specific heat capacity of a substance.
This should help.
Its not desirable the rotors to be heavy, if that was the case no one would have 2 piece rotors or composite rotors and definitely would not have vanes if mass of the rotor was enough to cool it down or absorb & dissipate the heat. Vanes are not very effective in dissipating heat at very low speeds, other than that they do most of the cooling even when the temperature has not reached the peak or steady state, the mass allows it absorb heat at a certain rate and raise the temperature which is governed by specific heat capacity of a substance.
This should help.
01-24-2007, 02:00 PM
#23
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Originally Posted by 350Zenophile
If rotors and pads are fine, can re-bedding solve a squeaking issue I'm having under light braking?
Odds are your rotors are heat glazed, if so, they will continue to squeak until replaced.... Which sucks, I know, b/c mine did the same until I replaced the rotors...
If rotors and pads are fine, can re-bedding solve a squeaking issue I'm having under light braking?
Odds are your rotors are heat glazed, if so, they will continue to squeak until replaced.... Which sucks, I know, b/c mine did the same until I replaced the rotors...
01-24-2007, 02:11 PM
#24
I've got technical allies now... 
All I meant to say is, if i have to sets of identical rotor and I shave .025 inch off on set, the thinner set will feel the heat more.
This is why I said that surfacing a rotor to fix an overheating issue is a no go.
Nice to see you again Spiff
All I meant to say is, if i have to sets of identical rotor and I shave .025 inch off on set, the thinner set will feel the heat more.
This is why I said that surfacing a rotor to fix an overheating issue is a no go.
Nice to see you again Spiff
Last edited by Kolia; 01-24-2007 at 02:22 PM.
01-24-2007, 03:46 PM
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Originally Posted by MoodDude
The material is the most determining factor on the rate at which heat is aborbed. What Kolia is talking about is that with a specific material, the more material you have the larger heat sink you have. Thus Q=mC(p)detaT, so as m gets larger your detaT is less (since Cp is a constant for a given material), thus your temperature gain is less for the same energy input.
So Kolia is correct.
The reason for two piece rotors is to allow more cooling, yet these are much more expensive to build than a single. Thus, why we still have a single piece.
And for us that track - your rotors are never at a steady state temperature, you are always heating them up and cooling them down, most of the time 5+ times per minute!
So Kolia is correct.
The reason for two piece rotors is to allow more cooling, yet these are much more expensive to build than a single. Thus, why we still have a single piece.
And for us that track - your rotors are never at a steady state temperature, you are always heating them up and cooling them down, most of the time 5+ times per minute!
If one wants to calculate the temperature rise -delta, you have to equate the heat transfer equation to m.C.dT. Thus in general terms dq=U.dT.A
dq/dt=m.C.dT/dt(rotor) ~ d/dt(dm.C.dT)(air) ~ U.dT/dt.A - assuming energy transfer is efficient. (ignoring sign convention)
Also two piece rotors are lighter, so less of a heat sink, so larger theoretical delta T. But if one takes the dissipation/heat transfer into factor, then dT for both rotor types might not be too different or far apart.
I never thought two piece rotors would be cheaper to fabricate than single piece ones.
About the steady state temperature part - depends on what one uses as boundaries for the integral, is it between a smaller brake on and off event or the entire track event or any combination of those or other events. I think the steady state effects can be ignored for now and used if needed in the detail scenarios.
Clearly I put more emphasis is on heat dissipation, while Kolia is putting more emphasis on heat sink capacity. And I know from my limited experience that brake rotors are not getting any heavier, in fact they are becoming lighter and are using better dissipation techniques.
Last edited by spacemn_spiff; 01-24-2007 at 03:52 PM.
01-24-2007, 06:20 PM
#26
Originally Posted by will350z
Originally Posted by 350Zenophile
If rotors and pads are fine, can re-bedding solve a squeaking issue I'm having under light braking?
Odds are your rotors are heat glazed, if so, they will continue to squeak until replaced.... Which sucks, I know, b/c mine did the same until I replaced the rotors...
If rotors and pads are fine, can re-bedding solve a squeaking issue I'm having under light braking?
Odds are your rotors are heat glazed, if so, they will continue to squeak until replaced.... Which sucks, I know, b/c mine did the same until I replaced the rotors...
01-25-2007, 04:51 AM
#27
Originally Posted by spacemn_spiff
Clearly I put more emphasis is on heat dissipation, while Kolia is putting more emphasis on heat sink capacity. And I know from my limited experience that brake rotors are not getting any heavier, in fact they are becoming lighter and are using better dissipation techniques.
If I'm right, doesn't it make sense to say that, as the rotor wears down and get thinner, it looses of its heat sink properties. Since cooling will remain fairly constant, the rotor will experience higher temperature. Leading to all sorts of nasty problems.
01-25-2007, 05:36 AM
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Originally Posted by Kolia
I've been under the impression that the heat dissipation capability of a given rotor would be fairly constant during its service. Putting rust asside, the vanes aren't getting more efficient and surface area barely increases on the pad side of the rotors as it wears down. Am I wrong in that assumption?
If I'm right, doesn't it make sense to say that, as the rotor wears down and get thinner, it looses of its heat sink properties. Since cooling will remain fairly constant, the rotor will experience higher temperature. Leading to all sorts of nasty problems.
If I'm right, doesn't it make sense to say that, as the rotor wears down and get thinner, it looses of its heat sink properties. Since cooling will remain fairly constant, the rotor will experience higher temperature. Leading to all sorts of nasty problems.
But that again is not taking into account where the temperature differential will settle if heat dissipation is taken into account. In effect, the real life effect might not too different before and after mass reduction, but I cant say for sure, but my gut feeling says that effect should be minimal unless the rotor doesnt have vanes. And since manufacturers recommend turning of the rotors, I dont think the effect should be very significant.
A lot of times the effect is non-linear, as the temperature increases heat transfer effect doesnt change linearly. But lets not go there.
01-25-2007, 06:42 AM
#29
I see.
So thermodynamically speaking, resurfacing would have a minimal impact, leaving the rotor performing pretty much the same way.
I guess resurfacing a rotor can help fix a brake problem (shudder, squeal…) if that that problem came to be following an extraordinary event. Like an emergency braking.
If the problem arises from daily driving, resurfacing will just be a temporary fix since the conditions will still be unchanged.
I still believe that resurfacing de facto every time we change pads or encounter some brake shudder is not a good thing. It only shortens the longevity of the rotors, making you spend more money, more often.
So thermodynamically speaking, resurfacing would have a minimal impact, leaving the rotor performing pretty much the same way.
I guess resurfacing a rotor can help fix a brake problem (shudder, squeal…) if that that problem came to be following an extraordinary event. Like an emergency braking.
If the problem arises from daily driving, resurfacing will just be a temporary fix since the conditions will still be unchanged.
I still believe that resurfacing de facto every time we change pads or encounter some brake shudder is not a good thing. It only shortens the longevity of the rotors, making you spend more money, more often.
01-25-2007, 09:15 AM
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^^ . Nice to see a civil discussion. Thanks for being open minded.
Now lets get on with what OP wanted to discuss, did his rotors get warped or not? According to Stoptech they didnt get warped.
. Nice to see a civil discussion. Thanks for being open minded.Now lets get on with what OP wanted to discuss, did his rotors get warped or not? According to Stoptech they didnt get warped.
01-25-2007, 09:51 AM
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I will attempt the rebedding process and post the results. I have 2 bald tires so I'm actually trying to steer away from driving the car until they are replaced. Having problems with that too..
01-25-2007, 10:01 AM
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Originally Posted by davidv
So did you resurface the rotors or not?
01-25-2007, 11:25 AM
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Gosh - good stuff. It has been 15 years since I had heat transfer in Grad school. Brings back good memories!
One thing - I am not so sure the two piece rotors are lighter than one piece.
One thought is- Since during the heat up cycle (braking) the energy input from the pads will be so much greater than the energy disapation of convection, that at this point we can assume that the rotor will need to absorb all the energy into the rotor material. If we assume that our two piece and one piece are made out of the same material, than in order not to boil the fluid or the brake pads than there is a maximum temperature that we want our rotor to not go past. Therefore, the heat sink of our rotor must be the same for a single cycle of the brake.
Another thought is that if we assume that we have numorous brake cycles in a very short period of time, and each cycle the energy taken in has a short time to disapate. Than, you would need a much larger heat sink for a single rotor rather than a two piece since you have more convective cooling.
Therefore, my conclusion is that you need the same weight rotor for a street application if it is a single or two piece. Whereas if you are racing, than you need a two piece or a larger single rotor.
One thing - I am not so sure the two piece rotors are lighter than one piece.
One thought is- Since during the heat up cycle (braking) the energy input from the pads will be so much greater than the energy disapation of convection, that at this point we can assume that the rotor will need to absorb all the energy into the rotor material. If we assume that our two piece and one piece are made out of the same material, than in order not to boil the fluid or the brake pads than there is a maximum temperature that we want our rotor to not go past. Therefore, the heat sink of our rotor must be the same for a single cycle of the brake.
Another thought is that if we assume that we have numorous brake cycles in a very short period of time, and each cycle the energy taken in has a short time to disapate. Than, you would need a much larger heat sink for a single rotor rather than a two piece since you have more convective cooling.
Therefore, my conclusion is that you need the same weight rotor for a street application if it is a single or two piece. Whereas if you are racing, than you need a two piece or a larger single rotor.
Originally Posted by spacemn_spiff
Good point. I agree that m-mass affects the rate of temperature increase, its desirable to have a heat sink, but at some point there is a need to dissipate that heat and that dissipation comes from vanes and to some extent radiation. What matters in that case is the surface area and flow rate (vel) & density of the fluid- in this case for cooling the rotor down.
If one wants to calculate the temperature rise -delta, you have to equate the heat transfer equation to m.C.dT. Thus in general terms dq=U.dT.A
dq/dt=m.C.dT/dt(rotor) ~ d/dt(dm.C.dT)(air) ~ U.dT/dt.A - assuming energy transfer is efficient. (ignoring sign convention)
Also two piece rotors are lighter, so less of a heat sink, so larger theoretical delta T. But if one takes the dissipation/heat transfer into factor, then dT for both rotor types might not be too different or far apart.
I never thought two piece rotors would be cheaper to fabricate than single piece ones.
About the steady state temperature part - depends on what one uses as boundaries for the integral, is it between a smaller brake on and off event or the entire track event or any combination of those or other events. I think the steady state effects can be ignored for now and used if needed in the detail scenarios.
Clearly I put more emphasis is on heat dissipation, while Kolia is putting more emphasis on heat sink capacity. And I know from my limited experience that brake rotors are not getting any heavier, in fact they are becoming lighter and are using better dissipation techniques.
If one wants to calculate the temperature rise -delta, you have to equate the heat transfer equation to m.C.dT. Thus in general terms dq=U.dT.A
dq/dt=m.C.dT/dt(rotor) ~ d/dt(dm.C.dT)(air) ~ U.dT/dt.A - assuming energy transfer is efficient. (ignoring sign convention)
Also two piece rotors are lighter, so less of a heat sink, so larger theoretical delta T. But if one takes the dissipation/heat transfer into factor, then dT for both rotor types might not be too different or far apart.
I never thought two piece rotors would be cheaper to fabricate than single piece ones.
About the steady state temperature part - depends on what one uses as boundaries for the integral, is it between a smaller brake on and off event or the entire track event or any combination of those or other events. I think the steady state effects can be ignored for now and used if needed in the detail scenarios.
Clearly I put more emphasis is on heat dissipation, while Kolia is putting more emphasis on heat sink capacity. And I know from my limited experience that brake rotors are not getting any heavier, in fact they are becoming lighter and are using better dissipation techniques.
01-25-2007, 11:40 AM
#35
Originally Posted by shanecrosby
No, I planned on trying the rebedding process first. Waiting for info on where I can perform this feat w/o getting busted for reckless driving.
01-26-2007, 05:01 AM
#36
Originally Posted by MoodDude
One thing - I am not so sure the two piece rotors are lighter than one piece.
One other advantage of running a 2 pieces rotor is it allows for different material to be used in the hub and disc area. Aluminium for the hub and steel for the disc.
Over time, the saving in replacement part also help (the discs are cheaper than a full rotor).
I can't find my notes here so I'm not sure of the temperature the disc section of my rotors would go up to (1,200F?). But the hub went to 800F (Thermal paint) while the outer caliper was at 450F.
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