Hot brakes
05-19-2004, 09:19 PM
#1
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Hot brakes
Last Friday Danno and I went out racing with Speed Ventures at Streets of Willow . As you can see on another tread Danno’s car was acting up, but my car did very well. I finally got the boost under control and had set it to 25psi max. The crowd was essentially Miata, EVO, WRX S2000 in various state of tune and a couple of "Radical Racecars" . It was my first experience on this track and I was beginning to learn it at the end. Unfortunately one thing held me back, namely the brakes, which after only a few turns were fading and the pedal was getting soft (boiling fluid). I know for a fact that my OG Racing brake ducts help a lot, but also know that the Design-90 rims I use are terrible in the cooling department.
So therefore this question: What brake upgrade(s) would you recommend? Low cost, max effect.
I am leaning toward drilled rotors and would also like to hear if the ones for our cars crack around the vent holes as some do.
Oh, lap-times? Quite a few under 1:40 (as measured on my GPS system ), not impressive, but OK under the circumstances, especially under the stinging sun and since we had to get up at 4:30AM to drive 130 miles to the track. Here is something to compare to.
The amount of water consumed (by my car) in five 15 min sessions was about a quart. Since then I increased the water pressure (flow) and adjusted the wastegate to give less roll off with rpm.
Laust
So therefore this question: What brake upgrade(s) would you recommend? Low cost, max effect.
I am leaning toward drilled rotors and would also like to hear if the ones for our cars crack around the vent holes as some do.
Oh, lap-times? Quite a few under 1:40 (as measured on my GPS system ), not impressive, but OK under the circumstances, especially under the stinging sun and since we had to get up at 4:30AM to drive 130 miles to the track. Here is something to compare to.
The amount of water consumed (by my car) in five 15 min sessions was about a quart. Since then I increased the water pressure (flow) and adjusted the wastegate to give less roll off with rpm.
Laust
05-19-2004, 10:00 PM
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Drilled rotors won't do a thing for you (or anyone else for that matter, they're solely an appearance mod) - what fluid were you running, and how old was it? Was the ducting on the rotor end channeled into a nice backing plate that would force the air to go through the rotor and not be wasted around it? Also, how worn were your pads?
Sam
Sam
05-19-2004, 10:05 PM
#3
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Laust,
I went with Zimmerman Cross Drilled rotors. They were a massive improvement over the stock ones. In part maybe due to newer rotors, but still very noticable. Porsche cross drill their rotors on higher performance models, so I figure its not for show.
I went with Zimmerman Cross Drilled rotors. They were a massive improvement over the stock ones. In part maybe due to newer rotors, but still very noticable. Porsche cross drill their rotors on higher performance models, so I figure its not for show.
05-20-2004, 12:29 AM
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Drilled rotors, even on porsche's new cars, are for show. Also, they all crack (cast or drilled) under heavy usage. Drilled rotors just crack sooner and at lower temps. I've changed my share of Big Red rotors with cracks that have reached from hole to hole, and from hole to the edge of the rotor.
Good brake upgrades for the track address the top issue affecting braking performance. Heat.
So the first line of upgrades should always address heat in the best manner possible:
- Brake fluid with a higher boiling temp (wet and dry). I like Ate Blue. Not the best but perhaps one of the better combinations of cost, performance on the track, and ease of maintenance on both street and track.
- Pads effective at the temperatures you see at the track. Street pads simply crumble under heavy track usage and fade quickly. Track pads develop their most effective friction at high temps and also wear best at those temps. I am currently using SBS pads but have also liked Pagid Oranges and Blacks (for really heavy use). Lots of people like Hawk, as well, but I didn't feel their performance was better and I didn't like the corrosive dust they produce. I particularly dislike EBC pads. Others' experiences may vary of course.
- Cooling. Vents, ducts, whatever it may be. These can make a nice difference.
If that's still not enough, there are two possibilities.
1. The most common is that the driver needs to change their braking technique. There's so much more speed to be gained on a track by effective use of the brakes and that includes the ability to conserve them. Lots of drivers are on their brakes far too long, far too much, and/or far too harshly.
2. You really do need better brakes. This is common with most cars out there but not with porsches. Porsche isn't perfect but they've been remarkably good about equipping their cars with just a bit more braking ability than power. Caveat: As they come from the factory. With 944T's, it's my opinion that at over 280rwhp, it becomes a whole lot of work to drive at the limit and still conserve the brakes enough to put down good laps.
Merely my opinion.
Good brake upgrades for the track address the top issue affecting braking performance. Heat.
So the first line of upgrades should always address heat in the best manner possible:
- Brake fluid with a higher boiling temp (wet and dry). I like Ate Blue. Not the best but perhaps one of the better combinations of cost, performance on the track, and ease of maintenance on both street and track.
- Pads effective at the temperatures you see at the track. Street pads simply crumble under heavy track usage and fade quickly. Track pads develop their most effective friction at high temps and also wear best at those temps. I am currently using SBS pads but have also liked Pagid Oranges and Blacks (for really heavy use). Lots of people like Hawk, as well, but I didn't feel their performance was better and I didn't like the corrosive dust they produce. I particularly dislike EBC pads. Others' experiences may vary of course.
- Cooling. Vents, ducts, whatever it may be. These can make a nice difference.
If that's still not enough, there are two possibilities.
1. The most common is that the driver needs to change their braking technique. There's so much more speed to be gained on a track by effective use of the brakes and that includes the ability to conserve them. Lots of drivers are on their brakes far too long, far too much, and/or far too harshly.
2. You really do need better brakes. This is common with most cars out there but not with porsches. Porsche isn't perfect but they've been remarkably good about equipping their cars with just a bit more braking ability than power. Caveat: As they come from the factory. With 944T's, it's my opinion that at over 280rwhp, it becomes a whole lot of work to drive at the limit and still conserve the brakes enough to put down good laps.
Merely my opinion.
05-20-2004, 11:17 AM
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I would do the following; better brake fluid, better pads and colling ducts. I run ATE blue (& amber) in my track car, Pagid 14 compound pads and have installed the OG cooling ducts - I have yet to have any brake fade or fluid boiling with this setup.
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05-20-2004, 12:54 PM
#8
Three Wheelin'
Drilled rotors are a step backward. The thread that Danno mentions is a long way to this short answer.
Laust,
If you can provide any info on your hp or tq, that might indicate the issue. You could very well be overpowering your existing brakes. If that's the case, it's time to pony up the cash for Big Reds.
-Jon
Laust,
If you can provide any info on your hp or tq, that might indicate the issue. You could very well be overpowering your existing brakes. If that's the case, it's time to pony up the cash for Big Reds.
-Jon
05-20-2004, 02:05 PM
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Originally posted by Danno
Don't make me bring up that 30-page Altima thread now....
BTW, could a metallurgist clarify the process by which holes can be casted into the rotors?
Don't make me bring up that 30-page Altima thread now....
BTW, could a metallurgist clarify the process by which holes can be casted into the rotors?
Chris
05-20-2004, 02:27 PM
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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/itemde...13&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.
NZ, what do you mean by “improvement”. I need to remove heat, have you push your car enough to detect an improvement in fade?
Danno, OK maybe I should read the dreaded 30 page Altima thread. When you get time, could you please email it to me. As you can see I start out with a belief in the cooling effect of drilled rotors, but try to keep an open mind.
Jon, I am pretty sure, that I have 350 rw ft-lb of torque, but it is falling off with rpm. I’ll be on the dyno “soon”.
Laust
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/itemde...13&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.
NZ, what do you mean by “improvement”. I need to remove heat, have you push your car enough to detect an improvement in fade?
Danno, OK maybe I should read the dreaded 30 page Altima thread. When you get time, could you please email it to me. As you can see I start out with a belief in the cooling effect of drilled rotors, but try to keep an open mind.
Jon, I am pretty sure, that I have 350 rw ft-lb of torque, but it is falling off with rpm. I’ll be on the dyno “soon”.
Laust
05-20-2004, 03:00 PM
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drilled or slotted are an improvement over stock, I don't care what anyone says test #'s are easily manipulated. I would change rotors, pads flush the system add new fluid and remove the dust shields in the rear. Works for me.
05-20-2004, 03:19 PM
#12
Three Wheelin'
Responses:
1) You are reducing the mass of the heat sink, thus allowing higher peak temperatures. The rotor may cool slightly more, due to the cross drilling, but you'll still have higher peak temps, which means you're more likely to boil fluid.
2)Slotting seems to be accepted by the racing community.
3)Better cooling is always good. Improvements in the cooling setup are always worthwhile.
5) Big mistake in running old fluid. That's probably most of your problem.
6) I'd guess that you'll have problems balancing, and adhering the heat sinks to the disc. Plus, you'll have to do this every time you get new rotors. At this point, I'd just start looking at larger rotors, ala Big Reds.
7) You mean on the sides of the pads where it doesn't contact the piston? Not much surface area or air flow there.
8)Hello, cracked rotor!
9)Doable. Before adding to the unsprung weight, and potentially damaging the structural integrity of your wheels, you may want to just think about cleaning the mating surface between the wheels and the rotors. Alloy wheels are often under-utilized in their ability to aid brake cooling. There are some cool gels that aid in heat transfer between rotor and wheel. I've used these before, and think that they're worth looking into.
-Jon
1) You are reducing the mass of the heat sink, thus allowing higher peak temperatures. The rotor may cool slightly more, due to the cross drilling, but you'll still have higher peak temps, which means you're more likely to boil fluid.
2)Slotting seems to be accepted by the racing community.
3)Better cooling is always good. Improvements in the cooling setup are always worthwhile.
5) Big mistake in running old fluid. That's probably most of your problem.
6) I'd guess that you'll have problems balancing, and adhering the heat sinks to the disc. Plus, you'll have to do this every time you get new rotors. At this point, I'd just start looking at larger rotors, ala Big Reds.
7) You mean on the sides of the pads where it doesn't contact the piston? Not much surface area or air flow there.
8)Hello, cracked rotor!
9)Doable. Before adding to the unsprung weight, and potentially damaging the structural integrity of your wheels, you may want to just think about cleaning the mating surface between the wheels and the rotors. Alloy wheels are often under-utilized in their ability to aid brake cooling. There are some cool gels that aid in heat transfer between rotor and wheel. I've used these before, and think that they're worth looking into.
-Jon
05-20-2004, 03:22 PM
#13
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Laust - You have some good thoughts but you have to carefully analyse the conditions brakes are under. I don't want to recreate the altima thread here so I'll take one shot at this and the leave it alone.
You're targeting ways to decrease the heat in the braking system but as you've already stated, the amount of heat being introduced is always a fixed amount. It doesn't matter how fast you remove heat from the system if the initial introduction of heat exceeds tha capability of the system to begin with.
Analogy: Water boils at 100C. It doesn't matter if you can find a way to cool water quickly if you are flashing the temperature of the water to 130C every 40 seconds. You need to find a way to handle the 130C first. Brake systems are a balance between how much total heat they can withstand and how fast they can be cooled.
Cross drilled holes in a rotor. Consider exactly how air is going to flow through those holes. Inside a rotor are vanes which direct air from the hub out to the radius of the disk. For each vane, there are maybe four holes between the hub and radius of the disk. This is overly simplistic but air will flow in the direction of least restriction and I promise you, it's not the holes. At least not in any significant amount. Cooling isn't happening at all while the brake pad is in contact with the disc. Far more heat is being introduced than cooling elements can remove them.
Slotted rotors: The venting effect isn't there. Don't forget that most of the cooling of a vented rotor is happening from the air being pushed through the inside of the rotor, not the outside surfaces. Slotted rotors may benefit from ensuring even pad wear (and hence even application to the rotor surface) but I don't see much other opportunity there.
Brake pads: The issue isn't gasses or dust. It's the pad materials friction coefficient at temperature. Your pads faded more in the beginning because there were bedding in better. Different materials can maintain or develop friction at different temperatures. Track pads are effective not because they out gas less or create less dust. They are effective because they can maintain a high coefficient of friction at high temperatures. Which segues nicely to the next topice which is...
Brake fluid. You've got the exact right idea here. Increase the temperature range in which your braking system is effective.
Aluminum heat sink. You'll probably have packaging issues with that idea but further more, the heat sink will do nothing to alleviate the massive amount of heat being introduced when you're on the brakes. If the brakes become uneffective over X degrees, it doesn't matter if you can cool them quickly if you put in more than X degrees of heat during usage. And I promise there is no way to create a reasonable cooling system that can pull heat out of a rotor or any component as fast as it's being put in under braking.
Water injection and fins inside the rim: We're talking about temps way higher than I think you're thinking are in the brakes. Track pads are effective up to 550C and some up to 700C which is about the range of temps that the brakes will see under very heavy usage. An 80C wheel is going to seem very cool to components that are at that temperature.
The techniques you're discussing might be effective at cooling something that is having problems performing at 100C. But cooling something that is being heated to 500C+ in a matter of seconds?
Again, I think you have great ideas but there is a better end of the equation which will get you exactly the benefits you're looking for at a realistic cost. You'll note that brake technology improvements often center around increasing their ability to perform at high temperatures, not reducing the temperature at which they perform.
You're targeting ways to decrease the heat in the braking system but as you've already stated, the amount of heat being introduced is always a fixed amount. It doesn't matter how fast you remove heat from the system if the initial introduction of heat exceeds tha capability of the system to begin with.
Analogy: Water boils at 100C. It doesn't matter if you can find a way to cool water quickly if you are flashing the temperature of the water to 130C every 40 seconds. You need to find a way to handle the 130C first. Brake systems are a balance between how much total heat they can withstand and how fast they can be cooled.
Cross drilled holes in a rotor. Consider exactly how air is going to flow through those holes. Inside a rotor are vanes which direct air from the hub out to the radius of the disk. For each vane, there are maybe four holes between the hub and radius of the disk. This is overly simplistic but air will flow in the direction of least restriction and I promise you, it's not the holes. At least not in any significant amount. Cooling isn't happening at all while the brake pad is in contact with the disc. Far more heat is being introduced than cooling elements can remove them.
Slotted rotors: The venting effect isn't there. Don't forget that most of the cooling of a vented rotor is happening from the air being pushed through the inside of the rotor, not the outside surfaces. Slotted rotors may benefit from ensuring even pad wear (and hence even application to the rotor surface) but I don't see much other opportunity there.
Brake pads: The issue isn't gasses or dust. It's the pad materials friction coefficient at temperature. Your pads faded more in the beginning because there were bedding in better. Different materials can maintain or develop friction at different temperatures. Track pads are effective not because they out gas less or create less dust. They are effective because they can maintain a high coefficient of friction at high temperatures. Which segues nicely to the next topice which is...
Brake fluid. You've got the exact right idea here. Increase the temperature range in which your braking system is effective.
Aluminum heat sink. You'll probably have packaging issues with that idea but further more, the heat sink will do nothing to alleviate the massive amount of heat being introduced when you're on the brakes. If the brakes become uneffective over X degrees, it doesn't matter if you can cool them quickly if you put in more than X degrees of heat during usage. And I promise there is no way to create a reasonable cooling system that can pull heat out of a rotor or any component as fast as it's being put in under braking.
Water injection and fins inside the rim: We're talking about temps way higher than I think you're thinking are in the brakes. Track pads are effective up to 550C and some up to 700C which is about the range of temps that the brakes will see under very heavy usage. An 80C wheel is going to seem very cool to components that are at that temperature.
The techniques you're discussing might be effective at cooling something that is having problems performing at 100C. But cooling something that is being heated to 500C+ in a matter of seconds?
Again, I think you have great ideas but there is a better end of the equation which will get you exactly the benefits you're looking for at a realistic cost. You'll note that brake technology improvements often center around increasing their ability to perform at high temperatures, not reducing the temperature at which they perform.
05-20-2004, 03:32 PM
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like what Jon said; you may have just been boiling the water in your brake system! It's just a bad idea to go to the track w/ questionable brake fluid.
New high quality fluid and a good set of pads, along w/ your cooling system you already have would be worth trying at another track day, you may find that to be adequate until you really start braking late.
A 951S front brake upgrade would probably be pretty cost effective (wait until your current rotors are gone!).
I've have very good luck with Motul-600 brake fluid, and porterfield R4 pads (just for another option).
New high quality fluid and a good set of pads, along w/ your cooling system you already have would be worth trying at another track day, you may find that to be adequate until you really start braking late.
A 951S front brake upgrade would probably be pretty cost effective (wait until your current rotors are gone!).
I've have very good luck with Motul-600 brake fluid, and porterfield R4 pads (just for another option).
05-20-2004, 04:47 PM
#15
Race Director
I think we need active brake cooling....
One thing I don't like about the brake-ducting hoses is that they are corrugated. This limits how fast they can move air to the brakes, especially with a long run to the front of the car. I like the 968/996 brake-scoops much more.
How about this idea, cut short the brake-ducting hose and attach each one to one of those e-Ram electric superchargers! Supposed they flow quite a few CFM and they'd help conduct heat away from the rotors.
One thing I don't like about the brake-ducting hoses is that they are corrugated. This limits how fast they can move air to the brakes, especially with a long run to the front of the car. I like the 968/996 brake-scoops much more.
How about this idea, cut short the brake-ducting hose and attach each one to one of those e-Ram electric superchargers! Supposed they flow quite a few CFM and they'd help conduct heat away from the rotors.