brake temperature calculations and effect of uprated brakes
10-26-2012, 11:00 PM
#31
Nordschleife Master
You can add fluid recirculation / auto bubble bleed ?
http://books.google.com/books?id=sjd...lation&f=false
http://books.google.com/books?id=sjd...lation&f=false
Last edited by Indycam; 10-26-2012 at 11:20 PM.
10-27-2012, 07:20 AM
#32
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So I spent some time thinking...
Another pointfor bigger rotors: more torque due to the larger lever, or same torque using less force (=less heat). But also higher rotor speeds due to higher distance from the hub. My gut and conservation of energy tells me it evens out...
I Dont feel to good doing a FEA. I will get some nice pictures with a rotor turning from blue to red, no question. But I have no means of controlling if its anywhere near realistic. There's too much going on...Also cars intertia will be hard to implement.
My new plan is write a bit of code solving three coupled equations for Trotor,Tpad, and cars velocity (coupled over the HTC, which is in case of ducting connected to the cars velocity. I will check that influence first. If its small, Ill leave it. Its a major PITA. But i feel its important....) I think thats more general and maybe even more informations in this
Another pointfor bigger rotors: more torque due to the larger lever, or same torque using less force (=less heat). But also higher rotor speeds due to higher distance from the hub. My gut and conservation of energy tells me it evens out...
I Dont feel to good doing a FEA. I will get some nice pictures with a rotor turning from blue to red, no question. But I have no means of controlling if its anywhere near realistic. There's too much going on...Also cars intertia will be hard to implement.
My new plan is write a bit of code solving three coupled equations for Trotor,Tpad, and cars velocity (coupled over the HTC, which is in case of ducting connected to the cars velocity. I will check that influence first. If its small, Ill leave it. Its a major PITA. But i feel its important....) I think thats more general and maybe even more informations in this
10-27-2012, 09:38 AM
#33
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Did some math. Those are the simplified T-equations fairly ready to solve in a math software. Just did this, no proof thats right.
Implementing car velocity and step-function-braking will make it a bit more complex, but still solvable numerical. The system here looks closed and I think its solvable analytically. Have no time right now, sorry. And the use is not too big. So I will try to implement step-braking and car velocity and then put it into code...
Implementing car velocity and step-function-braking will make it a bit more complex, but still solvable numerical. The system here looks closed and I think its solvable analytically. Have no time right now, sorry. And the use is not too big. So I will try to implement step-braking and car velocity and then put it into code...
10-29-2012, 09:42 AM
#35
Three Wheelin'
Thread Starter
That is fascinating!
Slowly getting my head around the various elements.
The duct size looks a good estimate of the kinds of ducts we'd see on a 964. I can see with that design the air velocity at the duct outlet is ~3 x the inlet speed. I googled incompressible flow a bit, and it looks like that assumption works upto Mach ~0.3 ~=228mph so from a car speed of ~70 mph the duct outlet velocity would start to deviate from incompressible flow. I understand that makes calculations somewhat more difficult but what's effect of that? Would the air start to heat as it compresses?Does the intake start to "choke"? would a NACA duct on the side on the bumper be better?
It's funny how as you understand a topic more and learn some of the terminology you get better results in google!
I've now found some cracking papers:
NUMERICAL AND EXPERIMENTAL ANALYSIS OF A PEGS-WING VENTILATED DISK BRAKE ROTOR, WITH PADS AND CYLINDERS
http://www.lista.it/atnet/EAEC05YU-AS04.pdf
This uses fea approach and compares w experiment. I think there are enough details given to plug in the parameters into my simplistic calculation and adjust the calc to put all the heat into the rotor, then I could see just how bad an approximation my "instantaneous" model is. Hopefully its not too far out, and even in the simpler model the scale and direction of responses to proportional changes in rotor dimensions and materials are fairly realistic.
Numerical Prediction of Brake Fluid Temperature Rise During Braking and Heat Soaking
http://am.delphi.com/pdf/techpapers/1999-01-0483.PDF
Some interesting take away points in this one:
Aluminium calipers increase brake fluid temperatures/stationary heat soak (leaving reduced unsprung mass as key benefit)
The vast majority of the energy is transfered to the disc and pad (pad is very bad conductor)
Analysis of heat conduction in a disk brake system
http://www.fem.unicamp.br/~phoenics/...referencia.pdf
Finally the one perelet sent me, which walks you through it nicely.
http://www.engineeringinspiration.co.uk/brakecalcs.html
There are some more interesting looking paid for academic journals too, but few of us will have access to these:
Experimental heat transfer and flow analysis of a vented brake rotor
http://www.sciencedirect.com/science...90072907000853
Fun one about the improvements in heat transfer (20%) in vented discs that use a tear drop vane instead of a straight one.
http://www.ingentaconnect.com/conten...0592f3f3b2cac3
In terms of approaches it generally it seems right to start with the kinetic energy, but you need to account for the heat flux, convection and time - rather than assume the energy is intantaneously converted to heat and diffused equally throught the various components. This way you balance the build up of heat during the brake time and the dissipation of heat via convection during this time. If you want extract accuracy you can look at the changing specific heat capacity and conductivity vs temperature, and even "microscopic" surface issues and pad wear.
Other common assumptions (more informed and evidenced and allround better than my assumptions):
Ignore air resistance
Ignore engine braking
Pad is a good enough insulator that in short time frames you only need to consider the heat in the rotor/pad surface
Slowly getting my head around the various elements.
The duct size looks a good estimate of the kinds of ducts we'd see on a 964. I can see with that design the air velocity at the duct outlet is ~3 x the inlet speed. I googled incompressible flow a bit, and it looks like that assumption works upto Mach ~0.3 ~=228mph so from a car speed of ~70 mph the duct outlet velocity would start to deviate from incompressible flow. I understand that makes calculations somewhat more difficult but what's effect of that? Would the air start to heat as it compresses?Does the intake start to "choke"? would a NACA duct on the side on the bumper be better?
It's funny how as you understand a topic more and learn some of the terminology you get better results in google!
I've now found some cracking papers:
NUMERICAL AND EXPERIMENTAL ANALYSIS OF A PEGS-WING VENTILATED DISK BRAKE ROTOR, WITH PADS AND CYLINDERS
http://www.lista.it/atnet/EAEC05YU-AS04.pdf
This uses fea approach and compares w experiment. I think there are enough details given to plug in the parameters into my simplistic calculation and adjust the calc to put all the heat into the rotor, then I could see just how bad an approximation my "instantaneous" model is. Hopefully its not too far out, and even in the simpler model the scale and direction of responses to proportional changes in rotor dimensions and materials are fairly realistic.
Numerical Prediction of Brake Fluid Temperature Rise During Braking and Heat Soaking
http://am.delphi.com/pdf/techpapers/1999-01-0483.PDF
Some interesting take away points in this one:
Aluminium calipers increase brake fluid temperatures/stationary heat soak (leaving reduced unsprung mass as key benefit)
The vast majority of the energy is transfered to the disc and pad (pad is very bad conductor)
Analysis of heat conduction in a disk brake system
http://www.fem.unicamp.br/~phoenics/...referencia.pdf
Finally the one perelet sent me, which walks you through it nicely.
http://www.engineeringinspiration.co.uk/brakecalcs.html
There are some more interesting looking paid for academic journals too, but few of us will have access to these:
Experimental heat transfer and flow analysis of a vented brake rotor
http://www.sciencedirect.com/science...90072907000853
Fun one about the improvements in heat transfer (20%) in vented discs that use a tear drop vane instead of a straight one.
http://www.ingentaconnect.com/conten...0592f3f3b2cac3
In terms of approaches it generally it seems right to start with the kinetic energy, but you need to account for the heat flux, convection and time - rather than assume the energy is intantaneously converted to heat and diffused equally throught the various components. This way you balance the build up of heat during the brake time and the dissipation of heat via convection during this time. If you want extract accuracy you can look at the changing specific heat capacity and conductivity vs temperature, and even "microscopic" surface issues and pad wear.
Other common assumptions (more informed and evidenced and allround better than my assumptions):
Ignore air resistance
Ignore engine braking
Pad is a good enough insulator that in short time frames you only need to consider the heat in the rotor/pad surface
Last edited by alexjc4; 10-29-2012 at 10:12 AM.
10-29-2012, 10:29 AM
#36
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The speed increase builds on incompressibility. If you look at mass flow rate m. you will easily obtain that its for internal flow:
m. = rho * A * c
due to continuity (no mass lost..). Dividing inlet by outlet:
Ain/ Aout = cout/ cin
for CONSTANT desity.
so if pressure / temperature (or better enthalpy) rises with higher velocity and desity changes, you will have to account for that. A real easy approach is ideal gas law
rho = p/ RT , R=spec. gas constant
I did a CFD for an impulse turbine blade running around 100m/s recently. The total enthalpy change was low, almost neglectably, but mach was lower due to higher temp. Also, being an impulse turbine, flow was redirected rather than accelerated.
Still I think regading the flow as imcompressible will not be the main concern here. Too many assumptions for an exact solution anyways.
Lots of papers out there. Got to take a look.
As already stated, not really my field of work.
m. = rho * A * c
due to continuity (no mass lost..). Dividing inlet by outlet:
Ain/ Aout = cout/ cin
for CONSTANT desity.
so if pressure / temperature (or better enthalpy) rises with higher velocity and desity changes, you will have to account for that. A real easy approach is ideal gas law
rho = p/ RT , R=spec. gas constant
I did a CFD for an impulse turbine blade running around 100m/s recently. The total enthalpy change was low, almost neglectably, but mach was lower due to higher temp. Also, being an impulse turbine, flow was redirected rather than accelerated.
Still I think regading the flow as imcompressible will not be the main concern here. Too many assumptions for an exact solution anyways.
Lots of papers out there. Got to take a look.
As already stated, not really my field of work.
10-29-2012, 11:04 AM
#37
Burning Brakes
Dr,
Whilst reading up on brake disc options (as part of future upgrade), came across this comment
"Other than material composition, another major factor that affects the thermal performance of brake rotors is the surface area within the ventilation design. The more surface area, the more heat that can be removed from the brake rotor."
from... http://www.dba.com.au/kangaroo-paw-ventilation/
Whilst reading up on brake disc options (as part of future upgrade), came across this comment
"Other than material composition, another major factor that affects the thermal performance of brake rotors is the surface area within the ventilation design. The more surface area, the more heat that can be removed from the brake rotor."
from... http://www.dba.com.au/kangaroo-paw-ventilation/
10-29-2012, 11:07 AM
#38
Three Wheelin'
Thread Starter
Funny that, when I read the abstract of one of the pay per view papers above first thing I though of was DBA's "kangaroo paw":
http://www.ingentaconnect.com/conten...0592f3f3b2cac3
Looks like a seriously good effect, the paper says 20% better heat transfer with good vane design. Of course that may be old hat now and most manufacturers may now use an optimized vane design.
http://www.ingentaconnect.com/conten...0592f3f3b2cac3
Looks like a seriously good effect, the paper says 20% better heat transfer with good vane design. Of course that may be old hat now and most manufacturers may now use an optimized vane design.
Last edited by alexjc4; 10-29-2012 at 11:42 AM.
10-29-2012, 12:43 PM
#39
Three Wheelin'
Thread Starter
Lol well that's torn it 
I've adjusted the assumptions so all the energy goes into the rotor and plugged the same numbers for weights, speeds and rotor dimensions etc as used this paper http://www.lista.it/atnet/EAEC05YU-AS04.pdf into my simplistic model. The simple model which assumes the heat will be evenly spread through the disc comes up with much lower peak temperatures that the experimental and FEA derived figures given in the paper.
Mass of car 1445 kg
Speed 44.4444 in m/s
Kinetic energy of moving car 1427157.64 Joules (1/2 * mv^2)
Specific heat of Iron 450 joules per kg per deg C
Weight of one 304mm brembo front Fe rotor 8.2 kg
% of energy transferred to Front vs Rear brakes 70 %
Energy transferred to front brakes for full stop 999010.3477 joules
% Energy into Rotors (vs other components) 100 %
Energy into one Rotor for full stop 499505.1738 joules
Temp rise for Rotor for full stop 135.3672558 deg C
Plus ambient (25 deg C) 160.3672558 deg C
Compared to the temps seen in the paper after a single stop, this figure is roughly inline with the cool part of the disc, as shown in the heat map in page 9 http://www.lista.it/atnet/EAEC05YU-AS04.pdf BUT the hottest part of the disc (under the pads) is 100 deg C hotter! The paper adds an "inertial coefficient" (not sure what that's for?) of 1.1 to the calculation for the energy dissipated in a stop but that's not going to makeup 100 deg C.
It's hard to visualize but each point on the disc heats up significatly as it passes through the pads and then cools quickly through the rest of its travel before being reheated as its passes through the pads again.
The uneven distribution of the heat illustrates the value of Aluminium rotor bells/hats as they allow you to replace a region of the iron rotor where the properties of iron are of less use to us, with a smaller volume of less dense material, reducing unsprung mass.
I've adjusted the assumptions so all the energy goes into the rotor and plugged the same numbers for weights, speeds and rotor dimensions etc as used this paper http://www.lista.it/atnet/EAEC05YU-AS04.pdf into my simplistic model. The simple model which assumes the heat will be evenly spread through the disc comes up with much lower peak temperatures that the experimental and FEA derived figures given in the paper.
Mass of car 1445 kg
Speed 44.4444 in m/s
Kinetic energy of moving car 1427157.64 Joules (1/2 * mv^2)
Specific heat of Iron 450 joules per kg per deg C
Weight of one 304mm brembo front Fe rotor 8.2 kg
% of energy transferred to Front vs Rear brakes 70 %
Energy transferred to front brakes for full stop 999010.3477 joules
% Energy into Rotors (vs other components) 100 %
Energy into one Rotor for full stop 499505.1738 joules
Temp rise for Rotor for full stop 135.3672558 deg C
Plus ambient (25 deg C) 160.3672558 deg C
Compared to the temps seen in the paper after a single stop, this figure is roughly inline with the cool part of the disc, as shown in the heat map in page 9 http://www.lista.it/atnet/EAEC05YU-AS04.pdf BUT the hottest part of the disc (under the pads) is 100 deg C hotter! The paper adds an "inertial coefficient" (not sure what that's for?) of 1.1 to the calculation for the energy dissipated in a stop but that's not going to makeup 100 deg C.
It's hard to visualize but each point on the disc heats up significatly as it passes through the pads and then cools quickly through the rest of its travel before being reheated as its passes through the pads again.
The uneven distribution of the heat illustrates the value of Aluminium rotor bells/hats as they allow you to replace a region of the iron rotor where the properties of iron are of less use to us, with a smaller volume of less dense material, reducing unsprung mass.
10-29-2012, 12:57 PM
#40
Burning Brakes
Funny that, when I read the abstract of one of the pay per view papers above first thing I though of was DBA's "kangaroo paw":
http://www.ingentaconnect.com/conten...0592f3f3b2cac3
Looks like a seriously good effect, the paper says 20% better heat transfer with good vane design. Of course that may be old hat now and most manufacturers may now use an optimized vane design.
http://www.ingentaconnect.com/conten...0592f3f3b2cac3
Looks like a seriously good effect, the paper says 20% better heat transfer with good vane design. Of course that may be old hat now and most manufacturers may now use an optimized vane design.
It's hard to visualize but each point on the disc heats up significatly as it passes through the pads and then cools quickly through the rest of its travel before being reheated as its passes through the pads again.
The uneven distribution of the heat illustrates the value of Aluminium rotor bells/hats as they allow you to replace a region of the iron rotor where the properties of iron are of less use to us, with a smaller volume of less dense material, reducing unsprung mass.
The uneven distribution of the heat illustrates the value of Aluminium rotor bells/hats as they allow you to replace a region of the iron rotor where the properties of iron are of less use to us, with a smaller volume of less dense material, reducing unsprung mass.
10-29-2012, 01:07 PM
#41
Three Wheelin'
Thread Starter
http://www.rallydesign.co.uk/pdf/des...pot_brakes.pdf
I don't think wilwood have a a great rep in UK and they're in 'orrible imperial sizes and fittings but they are very cheap. I spose if you take a wheelbarrow full of cash to AP they'll sort you out nicely
10-29-2012, 01:11 PM
#42
Burning Brakes
lol well I was googling that over the weekend, and wilwood do generic kits, you buy the rotor and caliper you want along with the bell closest to your needs and a blank caliper bracket, then get them machined by your friendly local machine shop to the correct dimensions. They even do bells that can take drum handbrakes.
http://www.rallydesign.co.uk/pdf/des...pot_brakes.pdf
I don't think wilwood have a a great rep in UK and they're in 'orrible imperial sizes and fittings but they are very cheap. I spose if you take a wheelbarrow full of cash to AP they'll sort you out nicely
http://www.rallydesign.co.uk/pdf/des...pot_brakes.pdf
I don't think wilwood have a a great rep in UK and they're in 'orrible imperial sizes and fittings but they are very cheap. I spose if you take a wheelbarrow full of cash to AP they'll sort you out nicely
10-29-2012, 01:21 PM
#43
Nordschleife Master
http://www.performancefriction.com/news-and-events.aspx
"PFC Preffered Over Pagid by Endurance Racers
by Lesley on October 22, 2012
Forum Page: https://rennlist.com/forums/997-gt2-...ml#post9928476
(Clover, SC) October 22, 2012- Performance Friction brake pads become the topic of choice
for the Porsche and Endurance Racing World on rennlist.com . A debate over PFC and Pagid pads had an obvious winner—PFC. "
http://www.performancefriction.com/m...ake-discs.aspx
"PFC Preffered Over Pagid by Endurance Racers
by Lesley on October 22, 2012
Forum Page: https://rennlist.com/forums/997-gt2-...ml#post9928476
(Clover, SC) October 22, 2012- Performance Friction brake pads become the topic of choice
for the Porsche and Endurance Racing World on rennlist.com . A debate over PFC and Pagid pads had an obvious winner—PFC. "
http://www.performancefriction.com/m...ake-discs.aspx
Last edited by Indycam; 10-29-2012 at 01:37 PM.
10-29-2012, 01:52 PM
#44
Nordschleife Master
Join Date: Feb 2011
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I haven't read this entire thread, but appreciate the math. Anyway...
...regarding the reference to AP Racing rotors and $$, I can tell you that the buy in might be high, but the value for $$ is enormous.
I'm on the same front rotors we installed two seasons ago. These rotors have seen at least 10 brake pad changes, some 40 very hard track days last year, some 30 this year on the new (very fast) build, race training and full race weekends.
They're still way good to go and I suspect they'll last well into next season. I've worn about 0.5mm on each face, see no cracking whatsoever and consider them almost 75% solid.
Up front, yes, they cost a bit....but given the conditions under which they get used in my car, they've paid for themselves several times over.
And they're pretty!!! Claudia likes that....
...regarding the reference to AP Racing rotors and $$, I can tell you that the buy in might be high, but the value for $$ is enormous.
I'm on the same front rotors we installed two seasons ago. These rotors have seen at least 10 brake pad changes, some 40 very hard track days last year, some 30 this year on the new (very fast) build, race training and full race weekends.
They're still way good to go and I suspect they'll last well into next season. I've worn about 0.5mm on each face, see no cracking whatsoever and consider them almost 75% solid.
Up front, yes, they cost a bit....but given the conditions under which they get used in my car, they've paid for themselves several times over.
And they're pretty!!! Claudia likes that....
10-29-2012, 03:15 PM
#45
Burning Brakes
I haven't read this entire thread, but appreciate the math. Anyway...
...regarding the reference to AP Racing rotors and $$, I can tell you that the buy in might be high, but the value for $$ is enormous.
I'm on the same front rotors we installed two seasons ago. These rotors have seen at least 10 brake pad changes, some 40 very hard track days last year, some 30 this year on the new (very fast) build, race training and full race weekends.
They're still way good to go and I suspect they'll last well into next season. I've worn about 0.5mm on each face, see no cracking whatsoever and consider them almost 75% solid.
Up front, yes, they cost a bit....but given the conditions under which they get used in my car, they've paid for themselves several times over.
And they're pretty!!! Claudia likes that....
...regarding the reference to AP Racing rotors and $$, I can tell you that the buy in might be high, but the value for $$ is enormous.
I'm on the same front rotors we installed two seasons ago. These rotors have seen at least 10 brake pad changes, some 40 very hard track days last year, some 30 this year on the new (very fast) build, race training and full race weekends.
They're still way good to go and I suspect they'll last well into next season. I've worn about 0.5mm on each face, see no cracking whatsoever and consider them almost 75% solid.
Up front, yes, they cost a bit....but given the conditions under which they get used in my car, they've paid for themselves several times over.
And they're pretty!!! Claudia likes that....
I'm considering giro disc (run these on the 996t) or AP.... You only get what you pay for!
What pads do you run, the same front and rear?