Blank rotors
03-30-2011, 04:21 PM
#41
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Seems a little contradictary.
Please explain why loss from drilling would not equate loss in surface area?
When your talking about a hole vs. thin lines. Either way material is being taken away.
It's easy to say loss of surface area is loss of surface area is loss of surface area. I'm certain the holes equate to some loss of surface area.
Please explain why loss from drilling would not equate loss in surface area?
When your talking about a hole vs. thin lines. Either way material is being taken away.
It's easy to say loss of surface area is loss of surface area is loss of surface area. I'm certain the holes equate to some loss of surface area.
03-30-2011, 04:32 PM
#42
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My follow up to you in red;
Originally Posted by Brakemotive
You're wrong about the less material = heat build up. A typical 20+ lb rotor will usually lose about .1 lb of iron during the drilling process. Cross-drilling increases the wetted surface area of the rotor thereby improving itscooling ability. Convection heat transfer is second only to radiation in terms of energy conversion. Conductive heat transfer which you're implying is negatively affected by drilling; is responsible for the lowest percentage of energy conversion. The more airflow through the vanes, the cooler they will run.
So, you're saying that if I take a torch to a 10 lbs. block of material it will heat up faster than if I take a torch to a 9 lbs. block of material that has holes cut in it? Not quite the same as a brake rotor, but the principal remains.
No you've totally twisted my example into something that makes no sense - I'm saying a 19.9lb iron disc with cross-drilled holes spinning at speed will shed heat faster than the same 20.0lb iron disc without cross-drilled holes spinning at the same speed.
The reduction in surface from drilling has no affect on braking force - your master cylinder bore, pedal fulcrum, line pressure, number of caliper pistons, their size and their position relative to the center of the hub is what determines braking force.
Yes it does. Not in the actual pressure being put on the rotor by the pad, but the actual force that creates the stopping power is reduced by the decrease in swept area.
No it doesn't - as I said in the post above, the pad compounds coefficient of friction is what determines the rate of deceleration; which is why chamfering a pad which reduces its effective surface area dramatically (more so than drilling a rotor) has no negative impact on rate of deceleration.
What physics support your claims?
Originally Posted by Brakemotive
You're wrong about the less material = heat build up. A typical 20+ lb rotor will usually lose about .1 lb of iron during the drilling process. Cross-drilling increases the wetted surface area of the rotor thereby improving itscooling ability. Convection heat transfer is second only to radiation in terms of energy conversion. Conductive heat transfer which you're implying is negatively affected by drilling; is responsible for the lowest percentage of energy conversion. The more airflow through the vanes, the cooler they will run.
So, you're saying that if I take a torch to a 10 lbs. block of material it will heat up faster than if I take a torch to a 9 lbs. block of material that has holes cut in it? Not quite the same as a brake rotor, but the principal remains.
No you've totally twisted my example into something that makes no sense - I'm saying a 19.9lb iron disc with cross-drilled holes spinning at speed will shed heat faster than the same 20.0lb iron disc without cross-drilled holes spinning at the same speed.
The reduction in surface from drilling has no affect on braking force - your master cylinder bore, pedal fulcrum, line pressure, number of caliper pistons, their size and their position relative to the center of the hub is what determines braking force.
Yes it does. Not in the actual pressure being put on the rotor by the pad, but the actual force that creates the stopping power is reduced by the decrease in swept area.
No it doesn't - as I said in the post above, the pad compounds coefficient of friction is what determines the rate of deceleration; which is why chamfering a pad which reduces its effective surface area dramatically (more so than drilling a rotor) has no negative impact on rate of deceleration.
What physics support your claims?
03-30-2011, 04:56 PM
#43
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http://www.youtube.com/watch?v=_e0eCdyKsmg
03-30-2011, 05:11 PM
#44
Not to spoil anyone's fun, but neither the Rahal-Letterman GT2 BMW M3 (nor any of the FIA GT4's) run drilled rotors on the track. The above is a picture of a show car -- and an old one at that by the looks of the calipers. The current cars run AP Racing Radi-Cal's. They MAY have used drilled rotors occasionally for qualifying (depending on the track), but I can 100% guarantee they do not start a race on drilled -- ever. Every time I've seen the car in person it had J-Hook rotors on it.
Chris
Chris
Last edited by Chris_B; 03-30-2011 at 05:17 PM. Reason: Added photo
03-30-2011, 05:16 PM
#45
My follow up to you in red;
Originally Posted by Brakemotive
You're wrong about the less material = heat build up. A typical 20+ lb rotor will usually lose about .1 lb of iron during the drilling process. Cross-drilling increases the wetted surface area of the rotor thereby improving itscooling ability. Convection heat transfer is second only to radiation in terms of energy conversion. Conductive heat transfer which you're implying is negatively affected by drilling; is responsible for the lowest percentage of energy conversion. The more airflow through the vanes, the cooler they will run.
So, you're saying that if I take a torch to a 10 lbs. block of material it will heat up faster than if I take a torch to a 9 lbs. block of material that has holes cut in it? Not quite the same as a brake rotor, but the principal remains.
No you've totally twisted my example into something that makes no sense - I'm saying a 19.9lb iron disc with cross-drilled holes spinning at speed will shed heat faster than the same 20.0lb iron disc without cross-drilled holes spinning at the same speed.
lol, alright. I can see where the cross-drilled rotor could shed heat faster than a blank rotor if you're moving fast enough, but you're leaving out the fact that they will heat up faster. Taking material away (no matter how much or how little) decreases the mass that can absorb heat, and the heat that's built up when the pad touches the rotor occurs much faster than the heat that's shed by the very small amount of increased air flow (I would love to see the analysis of air flow through holes in a rotor, I honestly can't see them making a large impact, but would like to see proof regardless).
The reduction in surface from drilling has no affect on braking force - your master cylinder bore, pedal fulcrum, line pressure, number of caliper pistons, their size and their position relative to the center of the hub is what determines braking force.
Yes it does. Not in the actual pressure being put on the rotor by the pad, but the actual force that creates the stopping power is reduced by the decrease in swept area.
No it doesn't - as I said in the post above, the pad compounds coefficient of friction is what determines the rate of deceleration; which is why chamfering a pad which reduces its effective surface area dramatically (more so than drilling a rotor) has no negative impact on rate of deceleration.
I can see where you're going with this. The torque capacity of disk brakes is determined by the friction coefficient, clamping force, and the mean radius (which doesn't change with drills/slots). If you decrease the area and keep the force the same, the only thing that changes is the pressure on the rotor. Could this be one of the reasons that pads wear faster with drilled/slotted rotors? Also, increased pressure on the rotors could increase stresses in the rotors.
What physics support your claims?
Originally Posted by Brakemotive
You're wrong about the less material = heat build up. A typical 20+ lb rotor will usually lose about .1 lb of iron during the drilling process. Cross-drilling increases the wetted surface area of the rotor thereby improving itscooling ability. Convection heat transfer is second only to radiation in terms of energy conversion. Conductive heat transfer which you're implying is negatively affected by drilling; is responsible for the lowest percentage of energy conversion. The more airflow through the vanes, the cooler they will run.
So, you're saying that if I take a torch to a 10 lbs. block of material it will heat up faster than if I take a torch to a 9 lbs. block of material that has holes cut in it? Not quite the same as a brake rotor, but the principal remains.
No you've totally twisted my example into something that makes no sense - I'm saying a 19.9lb iron disc with cross-drilled holes spinning at speed will shed heat faster than the same 20.0lb iron disc without cross-drilled holes spinning at the same speed.
lol, alright. I can see where the cross-drilled rotor could shed heat faster than a blank rotor if you're moving fast enough, but you're leaving out the fact that they will heat up faster. Taking material away (no matter how much or how little) decreases the mass that can absorb heat, and the heat that's built up when the pad touches the rotor occurs much faster than the heat that's shed by the very small amount of increased air flow (I would love to see the analysis of air flow through holes in a rotor, I honestly can't see them making a large impact, but would like to see proof regardless).
The reduction in surface from drilling has no affect on braking force - your master cylinder bore, pedal fulcrum, line pressure, number of caliper pistons, their size and their position relative to the center of the hub is what determines braking force.
Yes it does. Not in the actual pressure being put on the rotor by the pad, but the actual force that creates the stopping power is reduced by the decrease in swept area.
No it doesn't - as I said in the post above, the pad compounds coefficient of friction is what determines the rate of deceleration; which is why chamfering a pad which reduces its effective surface area dramatically (more so than drilling a rotor) has no negative impact on rate of deceleration.
I can see where you're going with this. The torque capacity of disk brakes is determined by the friction coefficient, clamping force, and the mean radius (which doesn't change with drills/slots). If you decrease the area and keep the force the same, the only thing that changes is the pressure on the rotor. Could this be one of the reasons that pads wear faster with drilled/slotted rotors? Also, increased pressure on the rotors could increase stresses in the rotors.
What physics support your claims?
03-30-2011, 05:29 PM
#46
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Being generous, lets say that conductive heat transfer is responsible for 25% of heat dissipation in your brake system. For this example lets use a rotor weighing about 20.80 lbs after drilling. The identical rotor casting with no machining weighs in at 21.00lbs (yes I actually pulled two rotors and weighed them), that means a whopping 2/10 of a lb was removed in machining.
If we take the .2 lbs and divide it by the total mass of the blank rotor we get; (.2/21 = .009528) or .95 %. Now using typical daily driving conditions where a rotor is at about 600*F, if we multiply that by .25 (25%), that means a total of 150* is dissipated through conduction heat transfer. Now if we take that value and multiply by .0095238 (the reduction in mass, see above) we get 1.43*. That means that for this particular rotor we are losing a total of 1.43* in cooling ability due to the loss of mass from drilling. It has been conclusively proven that rotor cross-drilling can reduce max temps by as much as 200* due to improved airflow and increased wetted surface area. Which would you rather have? The ability to transfer 1.43* or the ability to transfer 200*?
03-30-2011, 05:30 PM
#47
The reason pads wear a bit quicker when running on drilled and/or slotted rotors is that the additional leading edges are working harder, causing slightly higher local temperatures and additional wear. That is also why they create additional friction (beyond Newton's simplified law).
Decreasing contact surface area (within limits) has a negligible effect on rotor stress. It does, however, increase the contact surface temperatures. This will lead to faster pad and rotor wear. Plus the fact the pad will reach it MOT (Maximum Operating Temperature) faster when pushed hard, leading to fade. A properly engineered braking system will have all of these factors (and several more) taken into account before the sizing is finalized.
Chris
03-30-2011, 05:36 PM
#48
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Not to spoil anyone's fun, but neither the Rahal-Letterman GT2 BMW M3 (nor any of the FIA GT4's) run drilled rotors on the track. The above is a picture of a show car -- and an old one at that by the looks of the calipers. The current cars run AP Racing Radi-Cal's. They MAY have used drilled rotors occasionally for qualifying (depending on the track), but I can 100% guarantee they do not start a race on drilled -- ever. Every time I've seen the car in person it had J-Hook rotors on it.
Chris
Chris
03-30-2011, 06:28 PM
#50
Regardless of what's used on race day the benefits of cross-drilled rotors on road going vehicles is well established. Ferrari, Aston Martin, Mercedes, Porsche, BMW, Lamborghini, Lotus, Lexus, Audi, Bugatti all use cross-drilled rotors for reasons other than aesthetics.
03-30-2011, 06:40 PM
#51
Please...
Being generous, lets say that conductive heat transfer is responsible for 25% of heat dissipation in your brake system. For this example lets use a rotor weighing about 20.80 lbs after drilling. The identical rotor casting with no machining weighs in at 21.00lbs (yes I actually pulled two rotors and weighed them), that means a whopping 2/10 of a lb was removed in machining.
If we take the .2 lbs and divide it by the total mass of the blank rotor we get; (.2/21 = .009528) or .95 %. Now using typical daily driving conditions where a rotor is at about 600*F, if we multiply that by .25 (25%), that means a total of 150* is dissipated through conduction heat transfer. Now if we take that value and multiply by .0095238 (the reduction in mass, see above) we get 1.43*. That means that for this particular rotor we are losing a total of 1.43* in cooling ability due to the loss of mass from drilling. It has been conclusively proven that rotor cross-drilling can reduce max temps by as much as 200* due to improved airflow and increased wetted surface area. Which would you rather have? The ability to transfer 1.43* or the ability to transfer 200*?
Being generous, lets say that conductive heat transfer is responsible for 25% of heat dissipation in your brake system. For this example lets use a rotor weighing about 20.80 lbs after drilling. The identical rotor casting with no machining weighs in at 21.00lbs (yes I actually pulled two rotors and weighed them), that means a whopping 2/10 of a lb was removed in machining.
If we take the .2 lbs and divide it by the total mass of the blank rotor we get; (.2/21 = .009528) or .95 %. Now using typical daily driving conditions where a rotor is at about 600*F, if we multiply that by .25 (25%), that means a total of 150* is dissipated through conduction heat transfer. Now if we take that value and multiply by .0095238 (the reduction in mass, see above) we get 1.43*. That means that for this particular rotor we are losing a total of 1.43* in cooling ability due to the loss of mass from drilling. It has been conclusively proven that rotor cross-drilling can reduce max temps by as much as 200* due to improved airflow and increased wetted surface area. Which would you rather have? The ability to transfer 1.43* or the ability to transfer 200*?
The only way I can see that these holes can have any appreciable cooling affect is if there is significant air flow between the outside surface of the rotor and the inside surface (in the vented region). This would mean that there's an appreciable pressure difference between these two surfaces.
I'm here to learn, along with many others. You make a lot of valid points, but I've never seen conclusive facts about how much greater the cooling capacity is with drilled rotors; I've only read things that point to the contrary (along with talk from RR and auto-x guys on what they use on this board) and see what race teams use (sometimes the J-slots or a set of slots opposite to each other, maybe 6 in total, to aid in bite and cleaning the surface of the pad).
03-30-2011, 10:59 PM
#54
You won't find any Autozone rotors on my cars. They don't even know where they come from. Unfortunately for me, I do...
03-31-2011, 07:49 AM
#56
The benefits are primarily in the initial "bite". The extra leading edges from cross-drilled hole increase the effectiveness of the pad as well as raise its temperature more quickly. This advantage starts to normalize after a second or two, but the difference can be felt in most cars. I've felt it and I have test results from vehicle data acquisition and brake dyno results that back it up. We used to hacksaw slots into race pads to get more initial bite on tracks where we had trouble keeping heat in the brakes. Again, more leading edges equate to just a little more bite.
I have also worked with professional racing drivers who can definitely feel the difference. Once during qualifying at Daytona, drilled rotors were put on the car. Before the morning race warmup, the crew swapped them for slotted rotors in preparation for the 24 hour race -- but neglected to tell the driver. He then blew right through the back straight chicane when there was much less bite. Race pads take longer to heat up and he was surprised there was not enough heat in them yet due to the rotor change.
Are there additional stress risers? You bet. But that makes little difference on the street. I've been in the industry for over 20 years and hardly ever see (properly manufactured) drilled rotors that cracked before they wore out -- unless severely abused or tracked hard. I have seen MANY poor quality rotors (bad casting, substandard materials, wrong thermal post-processing, incorrect drill patterns, poor finishing, etc.) like the typical eBay garbage crack, but that has nothing to do with the drilling question.
Decreased pad life? Yes, a little anyway, but not severe at all. Most street pads are so low in friction that the extra bite that the drilled rotors gets out of them is well worth the trade-off. Pads are relatively cheap and quick and easy to change out, so I've never found it to be an issue.
Contrary to some of the points laid out in this thread, reducing the surface area has absolutely no effect on brake torque. But it does cause the pad and rotor to heat up a bit quicker from cold. The effective torque arm can be viewed at the center of pressure for the pad for calculation purposes. This does not change by drilling the rotor, so no torque is lost.
Also, drilling a rotor typically removes .35 - .70 pounds, depending on the size of the rotor, its design and plate thickness. This is relatively insignificant as far as temperature rise is concerned. Yes, the rotor will run slightly hotter, but good luck actually measuring the change without an on-board data acquisition system.
Chris
I have also worked with professional racing drivers who can definitely feel the difference. Once during qualifying at Daytona, drilled rotors were put on the car. Before the morning race warmup, the crew swapped them for slotted rotors in preparation for the 24 hour race -- but neglected to tell the driver. He then blew right through the back straight chicane when there was much less bite. Race pads take longer to heat up and he was surprised there was not enough heat in them yet due to the rotor change.
Are there additional stress risers? You bet. But that makes little difference on the street. I've been in the industry for over 20 years and hardly ever see (properly manufactured) drilled rotors that cracked before they wore out -- unless severely abused or tracked hard. I have seen MANY poor quality rotors (bad casting, substandard materials, wrong thermal post-processing, incorrect drill patterns, poor finishing, etc.) like the typical eBay garbage crack, but that has nothing to do with the drilling question.
Decreased pad life? Yes, a little anyway, but not severe at all. Most street pads are so low in friction that the extra bite that the drilled rotors gets out of them is well worth the trade-off. Pads are relatively cheap and quick and easy to change out, so I've never found it to be an issue.
Contrary to some of the points laid out in this thread, reducing the surface area has absolutely no effect on brake torque. But it does cause the pad and rotor to heat up a bit quicker from cold. The effective torque arm can be viewed at the center of pressure for the pad for calculation purposes. This does not change by drilling the rotor, so no torque is lost.
Also, drilling a rotor typically removes .35 - .70 pounds, depending on the size of the rotor, its design and plate thickness. This is relatively insignificant as far as temperature rise is concerned. Yes, the rotor will run slightly hotter, but good luck actually measuring the change without an on-board data acquisition system.
Chris
Thanks this is a good and understandable explanation.
Would u recommend J pattern for street use?
So your opinion is cross drilled is better than slotted?
If one wanted some of the benefits of cross drilled is it possible to pattern a rotor with a cross drilling pattern on 2 sides opposite of each other? or maybe a pattern at 9, 12,3,6 o clock positions?
I've noticed a few glazed pads in the past its true. And I agree I see some value in extra bite. I wouldnt want to go through pads every 10K tho....
03-31-2011, 10:42 AM
#57
Chris
Thanks this is a good and understandable explanation.
Would u recommend J pattern for street use?
So your opinion is cross drilled is better than slotted?
If one wanted some of the benefits of cross drilled is it possible to pattern a rotor with a cross drilling pattern on 2 sides opposite of each other? or maybe a pattern at 9, 12,3,6 o clock positions?
I've noticed a few glazed pads in the past its true. And I agree I see some value in extra bite. I wouldnt want to go through pads every 10K tho....
Thanks this is a good and understandable explanation.
Would u recommend J pattern for street use?
So your opinion is cross drilled is better than slotted?
If one wanted some of the benefits of cross drilled is it possible to pattern a rotor with a cross drilling pattern on 2 sides opposite of each other? or maybe a pattern at 9, 12,3,6 o clock positions?
I've noticed a few glazed pads in the past its true. And I agree I see some value in extra bite. I wouldnt want to go through pads every 10K tho....
Cross-drilled vs. slotted: For regular street and occasional aggressive use, it's hard to beat cross-drilled. But if a driver attends track days and/or is aggressive in canyons, it is possible he may push a rotor too far for drilled. For that guy, slotted is the preferred.
I would never recommend varied or "unbalanced" drill patterns, if that is what you are suggesting. Rotors need to expand and contract evenly and uniformly. Irregular patterns interfere with this natural phenomenon, leading to all sorts of vibration issues. If you are looking for a less-aggressive drilling pattern, go for a combo drilled & slotted rotor. Just make sure the slots are done correctly (ramped out at each end, or "diamond" cut) and the holes are radius-chamfered (not a straight angle chamfer).
Also make sure that the hole pattern is such that the pad is contacted all the way across its face. Too many cheap drilled rotors out there have patterns that run the holes concentrically over the pad face, so you have intermittent areas that see no holes while others see every hole that comes by -- NOT good! Of course, I think I just eliminated 90% of the drilled rotors on the market, but you most often get what you pay for.
Chris
03-31-2011, 10:55 AM
#58
BTW, that system on the RL GT2 M3 is well north of $15k, not $5k.
Chris
03-31-2011, 11:25 AM
#59
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I haven't read here that anyone is recommending that solution, just blank rotors versus other patterns. The racing references are data points for lessons learned over the years. Some of those lessons are applicable to faster street vehicles and others are not.
BTW, that system on the RL GT2 M3 is well north of $15k, not $5k.
Chris
BTW, that system on the RL GT2 M3 is well north of $15k, not $5k.
Chris
03-31-2011, 11:54 AM
#60
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I'm here to learn, along with many others. You make a lot of valid points, but I've never seen conclusive facts about how much greater the cooling capacity is with drilled rotors; I've only read things that point to the contrary (along with talk from RR and auto-x guys on what they use on this board) and see what race teams use (sometimes the J-slots or a set of slots opposite to each other, maybe 6 in total, to aid in bite and cleaning the surface of the pad).
As far as evidence to support my claims check out this paper - http://papers.sae.org/2006-01-0691
The toptech white papers are great resources too - http://stoptech.com/tech_info/tech_white_papers.shtml