Not so Hot Brakes
06-25-2004, 04:46 AM
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Not so Hot Brakes
Here is a follow up on my prior thread on “Hot Brakes”.
In that post I ended up with 9 suggestions for cooling the brakes and inspired by contributions from you guys and some independent creativity, I have come up with 8 more cooling options.
I ended up implementing 1), 5), 6), 10) and 15) and plan to add 11) and 14) and have 13) and 17) as a backup.
Here are my top 5 cooling solutions:
1. Cross drilled rotors (1)
2. High temperature brake fluid (5)
3. Better venting rims (10)
4. Add a heat sink/conductor to the front rotors (7)
5. Add a thin coat of high temperature black paint (15)
Feel free to comment with your favorites and any other comments you may have.
Below is a slightly edited version of one my prior posts with more items added and some further comments.
Thanks a lot for the input. Here a some of my views and analysis on the subject:
The brakes do nothing more than convert kinetic energy to thermal energy (heat) and the amount of heat converted is independent of the hardware installed. As a matter of fact it is exactly equal to ½ m (V2^2 - V1^2) or in mph being careful with the units the heat generation is: 0.0346 * (V2^2 - V1^2) kCal assuming the weight of the car with driver is 3200 lbs, that’s 412 kcal from 120 to 50 mph. The trick however is to transfer the heat as fast as possible to the surrounding air before the heat source builds up too high a temperature and there are many means of doing that.
1) Cross drilled rotors. The heat source is the interface between the pad and the rotor and whatever can be done to ventilate that area has great benefit since the temperature is highest here. Sam, in spite of your statement to the contrary, I strongly believe in the cooling effect of cross drilling, since this brings venting to the very hottest place. As a matter of fact the very cracks around the holes may be evidence of their cooling effect. I don’t think that those cracks are not from external forces, but are a result of the thermal gradients within the disc (big temperature difference between periphery of hole and the surrounding friction surface), large enough that the difference in thermal expansion will induce stress beyond the materials tensile strength.
2) Slotted rotors. Same argument as above except that the venting is probably less, but without the cracking. Anybody with experience in slotted rotors who want to comment?
3) Ducting/baffles. Essentially throwing air on the rotors, which is always a good idea, but it requires speed to build airflow. The ducts I use can be found here (http://www.ogracing.com/eshop/itemd...amp;showbrake=1). The fit to the inside of the disc is far from perfect and there may be a ¼” gap all around.
4) Brake pads. Using different pads does not change the heat generated, but some pads generate more friction-reducing gas/dust than others, increasing the pedal pressure (fading). The ones I used (don’t know the brand, 30% left when finished) faded more in the beginning, probably being de-gassed with use. They don’t grind the rotors down as some pads do. I have tried Mintex and found them to need too much pressure for street use.
5) ATE Blue brake fluid is what I now have ordered to get the boiling temperature up. The old fluid was from the PO, therefore unknown and more that 3 years old.
In addition to the above I am generating some alternative solutions. They may seem unsafe at first, but if properly implemented they will work to various degrees.
6) Add an aluminum heat sink to the outer cylindrical section of the disc. I may post a picture when I get the parts already ordered.
7) Add an aluminum heat sink to the back of the pads (between the pads and the caliper). This will reduce the heat transfer to the calipers (and brake fluid) and in general dissipate dome heat.
8) Utilize my water injection system to spray water on the inside of the disc. The nozzles create a fine enough mist that the cooling will be uniform (no warping). The spray will happen simultaneous with the water injection, i.e. under boost so the chance of getting water on the friction surface while braking is eliminated.
9) Weld some fins on the inside of the wheels to better circulate the air. I believe that the rim temperature got up to 80 deg C.
10) Buy a set of better venting rims.
11) Remove dust shields on rear brakes as proposed by xxx
12) Active brake cooling as proposed by Danno (oddly enough I thought about that too, but forgot to list it).
13) Use a venting spacer.
14) Improve existing ducting with smoother ducting hose and back plate with better tolerance.
15) A thin coat of high temperature black paint on all but wear surfaces for better heat radiation. Too thick and the isolation effect will take over the benefit.
16) Change brake bias. This is a suggestion from Jason at Paragon and can make the rear brake do more of the work.
17) Drill radial holes (maybe four ¾” holes) in the cylindrical portion of the discs. This also gets air to the space between the disc and the wheel.
Reg. 1) I bought cross drilled Zimmerman rotors and interestingly enough the holes are smaller in the front rotors (3/16” vs. ¼” in the back), both 40 holes (one per internal fin). The disc is really a radial fan with lowest pressure at the inner radius so it stands to reason that the airflow through the holes will be from the outside in. There is no doubt in my mind that the larger surface area will generate more cooling (calorie removal). It appears that high end sports car manufacturers agree (Porsche, Lamborghini, Ferrari, etc.).
Reg. 5) I flushed the system with ATE Gold brake fluid.
Reg. 6) See one of the pictures posted. These are made from extruded aluminum (cut in half) used for add on oil filter cooler and can be found here http://www.jcwhitney.com/webapp/wcs/...&storeId=10101
The small tube shown is heat sink conducting paste. Apart from having a large surface area the fins are of (heat conducting) aluminum, also transporting heat to the wheels, which therefore will remove more calories (if not already removed by the fins themselves).
Reg. 8) The interesting question is how much water is needed. Let’s say that a racetrack needs 10 speed reductions from 90 to 60 mph. Using the formulas above, that amounts to about 1560 kcal, assuming 25% of the cooling is done by the water that amounts to a water consumption of about ¾ liters per lap, which is on the limit of being tolerable.
Reg. 10) I bought a set of 8”/10”x 17” rims expected to have better venting, see picture.
Reg. 12) Active brake cooling. Let’s look at a 70 mph race track. With brake ducts at 2.5” that is a flow of about 210 cfm, so a fan has to flow more than that to make a relevant contribution.
Reg. 13) Venting spacer. See one of the pictures posted. This is/was a patentable idea (until this post) and consists of layers of perforated (1/32”) sheet aluminum spaced by washers of same thickness and material. I personally like the idea, which would work well with 6), but the wheel/hub geometry does not allow for air to the center section without grinding slots in either hub of wheel center. Something I prefer not to do unless absolutely necessary.
Reg. 14) A can be seen in the picture I like the idea. This hopefully also postpones the ugly rust. The paint accidentally sprayed on the disc itself will be scraped off in 10 feet of braking.
To test whether the changes (new wheels, drilled, painted discs with heat sink) worked Danno and I went out for a brief test drive with the standard brakes and Design-90 wheels diagonally on RF and LR and the changes on the LF and RR. The average of 4 sets of measurements gave the following temperatures (in Centigrade): LF: 212, RF: 231, LR: 185, RR: 173 or a 19 degree difference on the fronts and 12 degree difference on the backs. This was not as much as I had hoped, but the difference should be larger with the higher temperatures on the track.
Laust
Front disc with heat sink:
Perforated aluminum cooling spacer:
In that post I ended up with 9 suggestions for cooling the brakes and inspired by contributions from you guys and some independent creativity, I have come up with 8 more cooling options.
I ended up implementing 1), 5), 6), 10) and 15) and plan to add 11) and 14) and have 13) and 17) as a backup.
Here are my top 5 cooling solutions:
1. Cross drilled rotors (1)
2. High temperature brake fluid (5)
3. Better venting rims (10)
4. Add a heat sink/conductor to the front rotors (7)
5. Add a thin coat of high temperature black paint (15)
Feel free to comment with your favorites and any other comments you may have.
Below is a slightly edited version of one my prior posts with more items added and some further comments.
Thanks a lot for the input. Here a some of my views and analysis on the subject:
The brakes do nothing more than convert kinetic energy to thermal energy (heat) and the amount of heat converted is independent of the hardware installed. As a matter of fact it is exactly equal to ½ m (V2^2 - V1^2) or in mph being careful with the units the heat generation is: 0.0346 * (V2^2 - V1^2) kCal assuming the weight of the car with driver is 3200 lbs, that’s 412 kcal from 120 to 50 mph. The trick however is to transfer the heat as fast as possible to the surrounding air before the heat source builds up too high a temperature and there are many means of doing that.
1) Cross drilled rotors. The heat source is the interface between the pad and the rotor and whatever can be done to ventilate that area has great benefit since the temperature is highest here. Sam, in spite of your statement to the contrary, I strongly believe in the cooling effect of cross drilling, since this brings venting to the very hottest place. As a matter of fact the very cracks around the holes may be evidence of their cooling effect. I don’t think that those cracks are not from external forces, but are a result of the thermal gradients within the disc (big temperature difference between periphery of hole and the surrounding friction surface), large enough that the difference in thermal expansion will induce stress beyond the materials tensile strength.
2) Slotted rotors. Same argument as above except that the venting is probably less, but without the cracking. Anybody with experience in slotted rotors who want to comment?
3) Ducting/baffles. Essentially throwing air on the rotors, which is always a good idea, but it requires speed to build airflow. The ducts I use can be found here (http://www.ogracing.com/eshop/itemd...amp;showbrake=1). The fit to the inside of the disc is far from perfect and there may be a ¼” gap all around.
4) Brake pads. Using different pads does not change the heat generated, but some pads generate more friction-reducing gas/dust than others, increasing the pedal pressure (fading). The ones I used (don’t know the brand, 30% left when finished) faded more in the beginning, probably being de-gassed with use. They don’t grind the rotors down as some pads do. I have tried Mintex and found them to need too much pressure for street use.
5) ATE Blue brake fluid is what I now have ordered to get the boiling temperature up. The old fluid was from the PO, therefore unknown and more that 3 years old.
In addition to the above I am generating some alternative solutions. They may seem unsafe at first, but if properly implemented they will work to various degrees.
6) Add an aluminum heat sink to the outer cylindrical section of the disc. I may post a picture when I get the parts already ordered.
7) Add an aluminum heat sink to the back of the pads (between the pads and the caliper). This will reduce the heat transfer to the calipers (and brake fluid) and in general dissipate dome heat.
8) Utilize my water injection system to spray water on the inside of the disc. The nozzles create a fine enough mist that the cooling will be uniform (no warping). The spray will happen simultaneous with the water injection, i.e. under boost so the chance of getting water on the friction surface while braking is eliminated.
9) Weld some fins on the inside of the wheels to better circulate the air. I believe that the rim temperature got up to 80 deg C.
10) Buy a set of better venting rims.
11) Remove dust shields on rear brakes as proposed by xxx
12) Active brake cooling as proposed by Danno (oddly enough I thought about that too, but forgot to list it).
13) Use a venting spacer.
14) Improve existing ducting with smoother ducting hose and back plate with better tolerance.
15) A thin coat of high temperature black paint on all but wear surfaces for better heat radiation. Too thick and the isolation effect will take over the benefit.
16) Change brake bias. This is a suggestion from Jason at Paragon and can make the rear brake do more of the work.
17) Drill radial holes (maybe four ¾” holes) in the cylindrical portion of the discs. This also gets air to the space between the disc and the wheel.
Reg. 1) I bought cross drilled Zimmerman rotors and interestingly enough the holes are smaller in the front rotors (3/16” vs. ¼” in the back), both 40 holes (one per internal fin). The disc is really a radial fan with lowest pressure at the inner radius so it stands to reason that the airflow through the holes will be from the outside in. There is no doubt in my mind that the larger surface area will generate more cooling (calorie removal). It appears that high end sports car manufacturers agree (Porsche, Lamborghini, Ferrari, etc.).
Reg. 5) I flushed the system with ATE Gold brake fluid.
Reg. 6) See one of the pictures posted. These are made from extruded aluminum (cut in half) used for add on oil filter cooler and can be found here http://www.jcwhitney.com/webapp/wcs/...&storeId=10101
The small tube shown is heat sink conducting paste. Apart from having a large surface area the fins are of (heat conducting) aluminum, also transporting heat to the wheels, which therefore will remove more calories (if not already removed by the fins themselves).
Reg. 8) The interesting question is how much water is needed. Let’s say that a racetrack needs 10 speed reductions from 90 to 60 mph. Using the formulas above, that amounts to about 1560 kcal, assuming 25% of the cooling is done by the water that amounts to a water consumption of about ¾ liters per lap, which is on the limit of being tolerable.
Reg. 10) I bought a set of 8”/10”x 17” rims expected to have better venting, see picture.
Reg. 12) Active brake cooling. Let’s look at a 70 mph race track. With brake ducts at 2.5” that is a flow of about 210 cfm, so a fan has to flow more than that to make a relevant contribution.
Reg. 13) Venting spacer. See one of the pictures posted. This is/was a patentable idea (until this post) and consists of layers of perforated (1/32”) sheet aluminum spaced by washers of same thickness and material. I personally like the idea, which would work well with 6), but the wheel/hub geometry does not allow for air to the center section without grinding slots in either hub of wheel center. Something I prefer not to do unless absolutely necessary.
Reg. 14) A can be seen in the picture I like the idea. This hopefully also postpones the ugly rust. The paint accidentally sprayed on the disc itself will be scraped off in 10 feet of braking.
To test whether the changes (new wheels, drilled, painted discs with heat sink) worked Danno and I went out for a brief test drive with the standard brakes and Design-90 wheels diagonally on RF and LR and the changes on the LF and RR. The average of 4 sets of measurements gave the following temperatures (in Centigrade): LF: 212, RF: 231, LR: 185, RR: 173 or a 19 degree difference on the fronts and 12 degree difference on the backs. This was not as much as I had hoped, but the difference should be larger with the higher temperatures on the track.
Laust
Front disc with heat sink:
Perforated aluminum cooling spacer:
Last edited by Laust Pedersen; 01-14-2013 at 03:32 PM.
06-25-2004, 04:50 AM
#2
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and here is a sample of the "Cooling Spacer":
(I wish I knew how to add more than one picture per post)
(I wish I knew how to add more than one picture per post)
Last edited by Laust Pedersen; 01-14-2013 at 03:32 PM.
06-25-2004, 01:53 PM
#6
Nordschleife Master
JB Weld strikes again!
I doubt an tech inspection would pass that. Interesting thought, but the execution needs some work imo. Looks like that heat conducting paste is going to migrate onto the pads when things heat up, no?
I doubt an tech inspection would pass that. Interesting thought, but the execution needs some work imo. Looks like that heat conducting paste is going to migrate onto the pads when things heat up, no?
06-25-2004, 02:19 PM
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I'm with Rich, that rotor appears to be on the wrong side of the car. Your hypothesis on airflow being from outside in is wrong. Install them in the correct orientation and watch brake temps drop by 50F in the same panic stop test. Replace the drilled rotors with solid and watch temps drop by another 30-50F. Most sports car makers provide drilled rotors because they look far better on the street - the vast majority of customers do not drive their cars hard enough to find the issues with drilled. You'd be best served looking at hard-core track cars.
Mike, JB Weld wouldn't work in this app because it won't hold up to brake heat. The fins are in a location of zero airflow and are worthless anyway. I share you and Dave's concerns, that's not something I would want to be on track with, and even the damage to your own wheel/brake caliper if it comes off is something to worry about if you don't hurt others.
Cooling hub is another bad idea, early 911s have open hubs, and the first step in cooling those brakes is to block those openings. The hub is the coolest part of the brake, it's a waste of air to try and cool it when that same air can go through the vanes and cool the hottest part. The hub gets all the cooling it can ever use in the wheel itself.
If your pads faded more in the beginning as you describe, you need to start following a methodical break-in on all new pads, as they were not. After break-in, pads do not out-gas, and will fade more with more use/heat, not less.
The high-temp black paint does more to hurt than to help, radiant heat is a tiny percentage of actual heat transfer out of the rotor, the direct conductive heat is the only effective source, and the paint is only an insulator.
Active brake cooling often is effective because in no case is flow the amount you cite - the ducting, inlet, outlet, all add restrictions. Add in the fact that inlet locations and outlet locations do not provide favorable pressures, and flow is far lower. Again, look at racing, there is much you can pick up free there.
Brake water injection is not anything new, many racers (sound familiar?) run it - you do not need to "waste" your Aquamist setup to drive this though, there are far cheaper setups available. The droplet size is far less critical in this app, even if solid drops hit the rotor, they flash into steam immediately, and unless it's a solid stream, will not remove enough heat to cause warpage. One thing to look at, race water spray systems operate proportionally to brake pressure, so exactly when the brakes are generating the most heat - again, there is no chance of getting water onto the braking interface unless the brakes are very cool. This has happened on caution/pace laps, where there is not enough heat to vaporize the water, yet the brakes are being used, and the car loses most of its braking ability. Something to watch out for, but not a serious issue as it only happens in very special circumstances. Some cars have now put in a manual switch for the driver to shut off brake water when in cases like that. Real fancy would be a thermal switch, but that's simply overkill.
I still believe your original issue was simply your old brake fluid, racers bleed if not flush before every event at the least - your old fluid was certainly operating at its "Wet" temp, which in all cases is around 300F - hardly worth mentioning over water's 212F!
Heat sinking the pad back - again, nothing new, but your use of aluminum is a terrible choice, short of copper it's the fastest heat transfer readily available (why it's used on nearly all heat sinks!). Many racers replace aluminum pistons with Stainless Steel or Titanium, to slow the heat transfer in, with very good results. Others use a ceramic insulator (Porsche does this themselves on the yellow calipers) between pad and piston. Both are readily available and will provide you great results.
Welding stuff onto the rim to circulate air? You show that it's unneccessary - wheel temps are 80C - whoopeee, who cares? brake temps get up to 600C and above, the wheel temp is nothing to be concerned about. I'd be far more concerned about localized stress and embrittlement from the welding on my wheel, as well as rotational inertia, and unsprung mass.
You're trying to solve issues that have been very heavily researched by many groups, all with more resources than any of us. It'd be wise to learn from them instead of reinventing the wheel (or brake).
Sam
Mike, JB Weld wouldn't work in this app because it won't hold up to brake heat. The fins are in a location of zero airflow and are worthless anyway. I share you and Dave's concerns, that's not something I would want to be on track with, and even the damage to your own wheel/brake caliper if it comes off is something to worry about if you don't hurt others.
Cooling hub is another bad idea, early 911s have open hubs, and the first step in cooling those brakes is to block those openings. The hub is the coolest part of the brake, it's a waste of air to try and cool it when that same air can go through the vanes and cool the hottest part. The hub gets all the cooling it can ever use in the wheel itself.
If your pads faded more in the beginning as you describe, you need to start following a methodical break-in on all new pads, as they were not. After break-in, pads do not out-gas, and will fade more with more use/heat, not less.
The high-temp black paint does more to hurt than to help, radiant heat is a tiny percentage of actual heat transfer out of the rotor, the direct conductive heat is the only effective source, and the paint is only an insulator.
Active brake cooling often is effective because in no case is flow the amount you cite - the ducting, inlet, outlet, all add restrictions. Add in the fact that inlet locations and outlet locations do not provide favorable pressures, and flow is far lower. Again, look at racing, there is much you can pick up free there.
Brake water injection is not anything new, many racers (sound familiar?) run it - you do not need to "waste" your Aquamist setup to drive this though, there are far cheaper setups available. The droplet size is far less critical in this app, even if solid drops hit the rotor, they flash into steam immediately, and unless it's a solid stream, will not remove enough heat to cause warpage. One thing to look at, race water spray systems operate proportionally to brake pressure, so exactly when the brakes are generating the most heat - again, there is no chance of getting water onto the braking interface unless the brakes are very cool. This has happened on caution/pace laps, where there is not enough heat to vaporize the water, yet the brakes are being used, and the car loses most of its braking ability. Something to watch out for, but not a serious issue as it only happens in very special circumstances. Some cars have now put in a manual switch for the driver to shut off brake water when in cases like that. Real fancy would be a thermal switch, but that's simply overkill.
I still believe your original issue was simply your old brake fluid, racers bleed if not flush before every event at the least - your old fluid was certainly operating at its "Wet" temp, which in all cases is around 300F - hardly worth mentioning over water's 212F!
Heat sinking the pad back - again, nothing new, but your use of aluminum is a terrible choice, short of copper it's the fastest heat transfer readily available (why it's used on nearly all heat sinks!). Many racers replace aluminum pistons with Stainless Steel or Titanium, to slow the heat transfer in, with very good results. Others use a ceramic insulator (Porsche does this themselves on the yellow calipers) between pad and piston. Both are readily available and will provide you great results.
Welding stuff onto the rim to circulate air? You show that it's unneccessary - wheel temps are 80C - whoopeee, who cares? brake temps get up to 600C and above, the wheel temp is nothing to be concerned about. I'd be far more concerned about localized stress and embrittlement from the welding on my wheel, as well as rotational inertia, and unsprung mass.
You're trying to solve issues that have been very heavily researched by many groups, all with more resources than any of us. It'd be wise to learn from them instead of reinventing the wheel (or brake).
Sam
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06-25-2004, 02:27 PM
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I hate to be so negative to all of your efforts, but Sam has it nailed down for the most part, I cannot add anything productive.
In my honest opinion, you ought to reconsider what you're doing to the brakes of that car. Cross drilling came up last time, if you do research, you will see that it's not helpful for what you're trying to do.
Ahmet
In my honest opinion, you ought to reconsider what you're doing to the brakes of that car. Cross drilling came up last time, if you do research, you will see that it's not helpful for what you're trying to do.
Ahmet
06-25-2004, 04:15 PM
#10
Nordschleife Master
Um. why would you want to insulate the wheel from the brakes, when you can use it as a giant heat sink??? If anything I would sand down the paint on the back of the wheel and put conductive grease on it.
I was being cautious last night when I wrote that I "thought" the rotors are on backwards. but after a good night's sleep I'm actually sure they are on backwards and will definately be detrimental to performance in that configuration.
The wheel should rotate counterclockwise in that photo. brake gases go into the little holes and escape out the vanes inside the rotor. When the rotors are on backwards, the rotor vanes are scooping up air instead of throwing air out.
If the internal vanes are sweeping opposite direction to the design of the vents, than you should ignore me.
I was being cautious last night when I wrote that I "thought" the rotors are on backwards. but after a good night's sleep I'm actually sure they are on backwards and will definately be detrimental to performance in that configuration.
The wheel should rotate counterclockwise in that photo. brake gases go into the little holes and escape out the vanes inside the rotor. When the rotors are on backwards, the rotor vanes are scooping up air instead of throwing air out.
If the internal vanes are sweeping opposite direction to the design of the vents, than you should ignore me.
06-25-2004, 09:28 PM
#12
Originally posted by Laust Pedersen
(I wish I knew how to add more than one picture per post)
(I wish I knew how to add more than one picture per post)
As you upload each picture, copy and paste the url you just uploaded and use the IMG button in the thread to link to it.
Does this make sense?
06-25-2004, 09:31 PM
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Your hypothesis on airflow being from outside in is wrong.
Most sports car makers provide drilled rotors because they look far better on the street - the vast majority of customers do not drive their cars hard enough to find the issues with drilled.
I also agree that paint may reduce the heat transfer, it will act as a barrier and it will reduce surface area.
One problem with a street driven car is that rust forms inside the disc vanes and drastically reduce heat transfer. I've sand blasted the vanes in the disc with decent result.
If the vanes are curved in the same direction as the arc of the holes then the discs are on the wrong side of the car.
Tomas
06-25-2004, 11:10 PM
#15
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Hi Laust
I have hear that the only benefit of cross drilled is in the Wet.
As by drilling you would lose what would otherwise be braking surface area ...
Also there are a few companies that offer Heat Emitting coatings like the ones
you had mentioned in the thread on coatings (that would be for the calipers
regards
Ed
I have hear that the only benefit of cross drilled is in the Wet.
As by drilling you would lose what would otherwise be braking surface area ...
Also there are a few companies that offer Heat Emitting coatings like the ones
you had mentioned in the thread on coatings (that would be for the calipers
regards
Ed