|
|
Joined: Sep 2000
Posts: 3,223
"Absolut Rara."
|
"Absolut Rara."
Joined: Sep 2000
Posts: 3,223 |
Couldn't have said it any better myself Todd.
Though, I disagree with Jon in some ways.
First, heat capacity is always important, whether its your first stop or your fifth stop. How much heat your rotor can absorb is key because heat is generated extremely quickly while stopping, and you tend to have much much more time to dissapate the heat to the air after the stop.
Also, while the items you list will reduce the cross-drilled rotors tendancy to crack, it does not eliminate the source of the cracks, the stress concentration created by the drilled hole. Cracks are still likely to happen far more regularly than on a solid faced rotor (ie not drilled).
The best solution I have seen for rotors with holes, is some of the very high dollar Porsche rotors, where the holes are cast into the rotor rather than drilled. Though, if you notice, you very rarely see even these rotors on the serious Porsche race cars.
Honestly, the best application for drilled rotors I know of, is motorcycles. Bikes typically use a solid rotor(s) in the front of very large diameter (relative to the wheel size, when compared to car packages). Now on a bike, the rotational inertia of the wheel/tire/rotor plays a HUGE part in the steering feel and responsiveness, so the rotors are drilled to save weight and improve steering feel on the bike. And the heat capacity of the very large rotor(s) is such that the loss due to drilling has little to no effect on performance.
|
|
|
|
|
Joined: Dec 2001
Posts: 49
New CEG\'er
|
|
New CEG\'er
Joined: Dec 2001
Posts: 49 |
How important is heat capacity? How much mass do you remove when drilling and machining a rotor? I would estimate about 2% or a bit less based on rotors I have machined and drilled. You also remove a similar amount of material when resurfacing rotors
dQ = Cp*m*dT where:
dQ = heat input,
Cp = heat capacity of a material
m = mass of material being heated
dT = temp rise
The equation can be rewritten:
dT = dQ /(Cp*m)
Assume dQ and Cp are constant and dQ results in a temp rise of 600F from 100F to 700F for an undrilled rotor. If you reduce the rotor mass by 2% you get a 612F rise.
I agree that rotors have to be corectly sized to the mass of the vehicle. However, the reduction in heat capacity by drilling a correctly sized rotor in insignificant.
Do you really think an additional temp rise of 12F is going to make a significant difference? If you removed 25-30% or more metal from the rotor, you might see some effect from lost heat capacity, but this will involve more than just drilling the rotors.
Assuming that everyone here is using brake pads with an epoxy or polyimide binderm I used 700F as my example for a continuous use temp. If you wanted to be optimistic 900F could have been taken as an upper limit. If you see rotor temps much above this you will need to go to sintered metal or carbon-carbon pads or you will overhat the pads.
Don't limit yourself to what happens after one or two hard stops. Consider the vehicle dynamics at a NASCAR short track or a road course such as Lime Rock or Watkins Glen. The brakes must disappate heat for hundereds of barking cycles over a 100-500 mile race. Clearly heat disappation not heat capacity (as long as the brakes are properly sized) is the most critical factor.
The better race shops use sufficient cutting fluid to cool the rotors during machining and heat treat the rotors afterward to remove stress. Usually a very light resurfacing at low speed and low pressure is used to true the rotor surface after heat treat. I've had this done on about 6 pairs of rotors and never had one crack.
One thing you must watch out for on the street is stress induced corrosion. Machined rotors do have greater opportunity for corrosion problems over undrilled rotors.
|
|
|
|
|
Joined: Sep 2000
Posts: 3,223
"Absolut Rara."
|
|
"Absolut Rara."
Joined: Sep 2000
Posts: 3,223 |
Jon,
Heat capacity is important because if dQ exceeds Cp at any point during the very short period of time of a brake application, all of that excess heat has to go somewhere, and it obviously isn't into the rotor. Also, while total heat capacity may be constant, available heat capacity is ever changing, so for a dynamic system, it is not safe to assume Cp is the same at any given moment. And as to your mass assumption of 2%, I will leave it be, simply because I do not wish to calculate it at the moment. Perhaps if I get bored at work tomorrow, I will run through it . . .
The real point is the stress concentration induced by the holes themselves. You can apply any process you wish to minimize the stress induced from machining, etc., but the fact remains that simply by the presence of the hole (or absence of presence?) you have introduced a geometric stress concentration into the rotor that you cannot eliminate by any method you can name, aside from not making the holes in the first place. Further, if they work as well as you claim in aiding heat dissapation, then you are also inducing significant thermal stresses in the rotor as well.
Further, we are not talking about competition use here anyway, I have conceded that there are some specialized situations where cross-drilling is more beneficial than detrimental, and all situations where that applies, excluding street-driven motorcycles, is in competitive motorsport.
I do not dismiss the importance of dissipating the heat in the rotor, but how much can the holes actually improve heat dissapation? Sure you increase the surface area, but where? Not in the prime spots of cooling airflow, and perpendicular to the direction of the normal flow. I believe at best they would be turbulence generators for existing cooling air flow. Heat dissipation depends on airflow, and lots of it, over the rotors, and through the open center section of a vented rotor, the holes don't do anything to increase the amount of airflow, and the additioanl srface area they do provide, isn't in the best place to make use of the existing airflow anyway.
If you are really having that much of a heat dissipation issue in a given application, rather than make your rotors weaker, I would recommend improving the brake ducting.
|
|
|
|
|
Joined: Mar 2002
Posts: 337
CEG\'er
|
|
CEG\'er
Joined: Mar 2002
Posts: 337 |
Now that this has gotten back to some reasonable conversation....
My personal opinion is that it's less the loss of mass that adds to the problems but rather the huge difference in temps that the rotors see when drilled rotors are exposed to high temps and ambient air. Consider the 900 degree rotor that has 70 degree air passing over the holes. This makes for some really difficult tasks on the part of the iron to expand and contract. And if anyone thinks that they don't move that much consider the value of floating rotors in extreme applications for much the same reason.
|
|
|
|
|
Joined: Dec 2001
Posts: 49
New CEG\'er
|
|
New CEG\'er
Joined: Dec 2001
Posts: 49 |
Originally posted by Rara:
Jon,
Heat capacity is important because if dQ exceeds Cp at any point during the very short period of time of a brake application, all of that excess heat has to go somewhere, and it obviously isn't into the rotor. Also, while total heat capacity may be constant, available heat capacity is ever changing, so for a dynamic system, it is not safe to assume Cp is the same at any given moment. And as to your mass assumption of 2%, I will leave it be, simply because I do not wish to calculate it at the moment. Perhaps if I get bored at work tomorrow, I will run through it . . .
I'll get back in more detail later, but it sounds like there is a limited understanding of thermodynamics or materials in this discussion.
"Heat capacity is important because if dQ exceeds Cp at any point during the very short period of time of a brake application, all of that excess heat has to go somewhere"
dQ is in units of cal/sec or joules/sec depending on whether you are using English or scientific units. Cp is in cal/(deg-F - lb) or joule/(deg-K - g). The total heat capacity of of an object is Cp*m. I don't know how you compare dQ to Cp to determine if one exceeds the other
"all of that excess heat has to go somewhere, and it obviously isn't into the rotor"
Some of it is into the brake pad and calipers. for obvious reasons you have to limit this to avoid overheating the pads.
Besides transferring the braking energy as heat, there are a limited number of ways you can extratct heat from the brakes. Heat can conduct through the brake pad and caliper and eventually heat the surrounding air and structure, it could be transformed into acoustical energy, or you could spray water on the rotor. The resulting evaporation of water will carry heat away very effectively. If you want to get complicated you could magnetically couple the rotor to another structure, however the hardware to do so would be heavier than the brakes. Do you know of another way that heat could be disappated from the rotors?? In any case the mechanism should be measurable given the large amount of heat genereated by braking.
So where do you think all the excess heat is going? Perhaps it is being dissapated from the rotor to the air via infrared radiation. Hot objects have very distinct infrared signatures.
"Also, while total heat capacity may be constant, available heat capacity is ever changing, so for a dynamic system, it is not safe to assume Cp is the same at any given moment. "
Over a finite temperature range Cp changes very little (less than 1-2% over a 100F range) unless a phase change occurs. One example of a phase change is melting... I'm sure no one here wants to even get close to the melting pont of the rotors. (2500-2700 F)
I've studied the thermal characteristics of materials for over 25 years. There are no surprises with ferrous materials such as steel, cast steel and cast iron in the room temp to 1200F range. Tempering is usually done in the 400-1550F range with melting in the 2500-2700F range. Internal phase changes (going from martensitic to austenic phases in steel, as an example) do slightly alter the Cp, but once a part is at equilibrium the Cp is continuous with a very flat slope over the entire RT to 1200+ F range.
Assuming a phase change doesn't occur 'all that excess heat' just results in more temperature rise, dT. It doesn't magically do anything else. It could melt the brake pads or boil the brake fluid to disappate excess heat. However, I think we are trying to avoid those scenarios.
I stand by my original statement, if you reduce the mass of a rotor by 2%, expect dT to increase by 2% assuming the same heat input, dQ.
|
|
|
|
|
Joined: Sep 2000
Posts: 3,223
"Absolut Rara."
|
|
"Absolut Rara."
Joined: Sep 2000
Posts: 3,223 |
Quote:
I'll get back in more detail later, but it sounds like there is a limited understanding of thermodynamics or materials in this discussion.
No, the difference is I'm looking at it from a practical system level.
Quote:
dQ is in units of cal/sec or joules/sec depending on whether you are using English or scientific units. Cp is in cal/(deg-F - lb) or joule/(deg-K - g). The total heat capacity of of an object is Cp*m. I don't know how you compare dQ to Cp to determine if one exceeds the other
Again, I am looking at it in a practical system level, I didn't think I had to be overly explicit here, I thought you were intelligent enough to figure it out. You have situation where a stop has a rate of heat input over time of dQ, and a time of S; using those two parameters, you can calculate a total amount of heat into the system. Then, since your rotor will not appreciably change mass during a stop (unless your cross-drilling causes a catastrophic failure) m will be constant, so Cp*m doesn't change either. Now, the system will have a temperature at which it no longer functions adequately, typically limited by the pad material, or by the brake fluid boiling in the lines, but there is some temperature for a given system at which "fade" sets in.
This means, that for a given system, there is only a certain amount of Q that the rotor can absorb before the system goes over that temperature, typically in the neighborhood of 700 deg F for a street pad.
With regard to Cp changing, again, I was oversimplistic. Cp itself does not change, but the amount of available heat capacity of the rotor at any given time depends on the temperature of the rotor, and the difference between that temperature and the maximum temperature at which the system can adequately operate. Again, my problem for using the term Cp incorrectly, but my intent should have been obvious. In my defense, I posted that late at night, and I've been sick. 
Quote:
I stand by my original statement, if you reduce the mass of a rotor by 2%, expect dT to increase by 2% assuming the same heat input, dQ.
I don't argue your calculations on this, I even verified the 2% on a typical rotor. Just remember, the heat is also going into the pads and the caliper at the same time, and at likely a very different rate (Cp will be different, as will mass). Its just that the temperature of the rotor isn't the real problem, because as you state, there are no surprises even in to the 1200 deg F range, its in the pads and fluid where you have to be very careful about the heat. The rotor is a heat sink for them, and as such, should have as much heat capacity as possible to avoid excess heat into the pads, calipers and fluid. Once also must consider that OEM rotors are designed pretty much right on the edge of required thermal capacity for meeting the Federal FMVSS135 stopping requirements, so 2% could in fact make a big difference on an OEM rotor design.
And once again, I must point out, that you are still missing the main importance of why cross-drilling is bad; the geometric stress concentrations introduced into the face of the rotor, and then compounded by the thermal issues associated with dissimilar cooling at the holes. These rotors tend to crack, that's all there is to it. Any percieved improvements in cooling performance cannot offset the added danger associated with having rotors that tend to crack and fail. Granted, light use on the street will not likely induce cracking except over a long period of time, but honestly, how many of us are really easy on our brakes? Cross-drilling was originally intended to allow an escape path for gases generated by older type pad materials under heavy use. "Out-gassing" of pads is not a problem with modern pad materials, so the original function of the holes is no longer existant, and yet they remain for their "bling factor". Which is fine, as long as people understand the ramifications of enjoying the "bling factor".
|
|
|
|
|
Joined: May 2001
Posts: 682
Veteran CEG\'er
|
|
Veteran CEG\'er
Joined: May 2001
Posts: 682 |
Originally posted by Rara:
I do not dismiss the importance of dissipating the heat in the rotor, but how much can the holes actually improve heat dissapation? Sure you increase the surface area, but where? Not in the prime spots of cooling airflow, and perpendicular to the direction of the normal flow. I believe at best they would be turbulence generators for existing cooling air flow. Heat dissipation depends on airflow, and lots of it, over the rotors, and through the open center section of a vented rotor, the holes don't do anything to increase the amount of airflow, and the additioanl srface area they do provide, isn't in the best place to make use of the existing airflow anyway.
Intuitively it seems to me that the holes would tend to act as an inlet to the central venting, and that if the rotor is spinning at a decent speed the center vents will produce a bit of a vacuum that would pull air in through the holes. Wouldn't this contribute some cooling?
(Remember that the reason I tried cross-drilled rotors was for mountain descents, in which heat capacity is of minimal importance but dissipation is all-important. Under these conditions there are no individual hard stops, but heat may accumulate steadily over as long as an hour. My last two imports had no trouble with this but the tour overheats badly.)
Originally posted by Rara:
If you are really having that much of a heat dissipation issue in a given application, rather than make your rotors weaker, I would recommend improving the brake ducting.
What sort of ducting might help if the speed at which one is doing this braking is only like 35 MPH? Is ducting useful then?
|
|
|
|
|
Joined: Sep 2000
Posts: 3,223
"Absolut Rara."
|
|
"Absolut Rara."
Joined: Sep 2000
Posts: 3,223 |
Quote:
Intuitively it seems to me that the holes would tend to act as an inlet to the central venting, and that if the rotor is spinning at a decent speed the center vents will produce a bit of a vacuum that would pull air in through the holes. Wouldn't this contribute some cooling?
Not nearly as much as you think. A vented brake rotor works much like a centrifugal air pump, as the rotor spins, it creates low pressure areas behind the vanes, and air is drawn in from the center of the rotor to fill that low pressure area. So basically air is pumped from the center of the rotor radially outward (this is why you direct the ducting to the center of the rotor, for it to pick up the colder air, though, because the contour is vented on the outer face at the center, its a bit tough). When you drill holes in the face, it takes a lot away from the centrifugal pump effect by not allowing the low pressure area to build up nearly as effectively.
And proper ducting is always useful, well, as long as you are moving anyway.
|
|
|
|
|
Joined: Dec 2001
Posts: 49
New CEG\'er
|
|
New CEG\'er
Joined: Dec 2001
Posts: 49 |
Originally posted by Rara:
And once again, I must point out, that you are still missing the main importance of why cross-drilling is bad; the geometric stress concentrations introduced into the face of the rotor, and then compounded by the thermal issues associated with dissimilar cooling at the holes. These rotors tend to crack, that's all there is to it.
I'm not missing your point. I fully understand the issues with stress in materials and the effect of geometry. I've worked as a materials scientist for over 30 years and with materials & structural engineering for the last 25 years. Aircraft disc brakes and very large elevator disc brakes are just two of the many components I've had fun working with.
The problem with drilled brakes is not primarily due to the geometry, but what happens during the machining process. The geometric problems are no more severe than the geometric effects and subsequent stress distribution during heating that result from th webbing.
The problem with cross drilling is what happens from the microscopic to local levels, if the drilling is not done correctly. If the machinist uses a less than sharp drill bit, works too fast and does not use cooling lubricant damage can be done to the metal. The two major problems are that significant stress is left in the metal surrounding the holes and that a lot of microcracks can form in the metal.
However, if the machinist uses a sharp tool with lubricant, uses a slow feed rate and if the rotor is heat treated after machining the risk of cracking is very small.
This is an example of a layman looking at a cracked brake rotor and concluding that the drilled holes were the cause of cracking. An experienced engineer looks at a cracked rotor, analyzes the problem two levels deeper and determines the machining process was at fault. The layman concludes that drilled rotors are bad, while the engineer figures out how to properly manufacture a machined brake rotor.
Originally posted by Paul Kienitz:
(Remember that the reason I tried cross-drilled rotors was for mountain descents, in which heat capacity is of minimal importance but dissipation is all-important. Under these conditions there are no individual hard stops, but heat may accumulate steadily over as long as an hour. My last two imports had no trouble with this but the tour overheats badly.)
Paul -- if you are experiencing severe fade during mountain descents, it is likely that overheating of the pads is the root cause of fade. Using stock drilled rotors will only help a small amount with stock pads. I would suggest that you look for pads that start to fade 100-200F higher in temp. If that doesn't work, unfortunately the next step would be larger diameter rotors and pads with a larger surface area, also selecting premium pad materials. You need to spread the heat generating surface over a larger area and increase the ability of the rotor to dissapate heat. A larger caliper structure will also help increase heat dissapation though that path.
|
|
|
|
|
Joined: Aug 2002
Posts: 1,228
Hard-core CEG\'er
|
|
Hard-core CEG\'er
Joined: Aug 2002
Posts: 1,228 |
Originally posted by JonsZX2SR:
This is an example of a layman looking at a cracked brake rotor and concluding that the drilled holes were the cause of cracking. An experienced engineer looks at a cracked rotor, analyzes the problem two levels deeper and determines the machining process was at fault. The layman concludes that drilled rotors are bad, while the engineer figures out how to properly manufacture a machined brake rotor.
That layman Rara Are you aware that not everyone is a High School Senior with a used Contour? You aren't the only engineer here. PLus, there are plenty of sites (Brake MFG's) that explain the effect CD has on brakes with the "Engineering Explanation" and feel it is not desirable.. FWIW, I have a BSME and MS myself, so I am a layman too.
So, where do you get "properly" cross drilled rotors?
Even Porche has problems with cast-in holes cracking.
Stress concentration is a problem. You can minimize it, but not eliminate it.
My name is Richard. I was a Contouraholic.
NOW: '02 Mazda B3000 Dual Sport, Black
BEFORE: '99 Contour SE Sport
Duratec ATX Spruce Green
PIAA 510's, Foglight MOD, K&N Drop-in
|
|
|
|
|
