I recently came across this SAE technical paper co-authored by two brake engineers at GM - The Effect of Rotor Crossdrilling on Brake Performance http://www.sae.org/technical/papers/2006-01-0691
Basically, it looks like they didn't necessarily set out to do a full performance comparison, but incidental to their testing on three brake systems they found they had enough data to write it up as a paper.
It'll cost you $14 do download a copy, but it's well worth it if you are interested in the subject. Hopefully at least some of you are willing to give it a read and start a discussion about it. Below are my thoughts to kick it off.
First off, the surprising conclusion is that drilling the rotors actually does increase the rate of cooling for a given a rotor, by up to about 20% in one case. Unfortunately however, that gain comes with some very serious drawbacks, including the possibility that your rotors may actually run hotter despite of it.
I believe one of the most significant considerations in interpreting the paper's data is that they used semi-metallic brake pads for all the testing. From what I've read, these typically have a max operating temp rating of about 500~550 deg C. Secondly, I understand that the primary functional mechanism of this type of pad is abrasion rather than adhesion. By contrast, the higher temp rated race type pads are more biased towards an adhesive mechanism relying upon a transfer layer of material deposited on the rotor.
Where this comes in to play is in interpreting the test data that shows higher apparent friction and deceleration gain for the plain rotor at low temp, the cross-drilled rotor at higher temp, and generally greater temperature fade stability for the cross-drilled. At low temp, the greater surface area of the plain rotor is the dominant performance factor, as one would expect. At higher temps however, the breakdown of the pad is the dominant consideration and the cheese grater effect of the cross-drilled rotors helps to compensate for this by providing increased mechanical friction.
If however, this test were to be repeated with a higher temp rated adhesive type race pad, I'd expect the results to be exactly the opposite. That is at lower temps where you would expect the race pad to perform poorly, the cross-drilled rotors would provide an abrasive mechanism and increased performance. But at higher temps, not only would the lesser surface area of the cross-drilled rotor become the dominant consideration, but the abrasive mechanism of the holes would inhibit proper formation of the transfer layer.
Another interesting aspect of the paper is the radical base performance difference between the three brake systems used in the test. "System 3" appears to be an overall poor performer in comparison with the other 2 systems. In the case of System 3, the cross drilling appears to be a pure bling change, as it is rather consistently outperformed by the plain rotors, despite their being smaller in overall diameter. System 1 was a better performer, but quite unbalanced. Almost all the work on this system was being done by the front rotors. System 2 looks to be the best of the lot, and consequently showed the least radical performance deviations between the plain and cross-drilled rotors.
I believe the base performance difference between System 1 and System 2 helps in interpreting the results showing increased cooling efficiency with the cross-drilled rotors. On System 1, tests at 50kph, 100kph, and 160kph, showed an increase of cooling values (hA) of 8.8%, 12.1%, and 20.1% on the front and -3.2%, 1.9%, and 8.5% on the rear. For System 2, the values at 50kph, 110kph, and 140kph are 7.8%, 10.4%, and 12.1% for the front and 4.1%, 7.7%, and 13.4% rear. The other key piece of data is the raw hA numbers. At 160kph, the average hA for the front rotors of System 1 is 19.26 for the plain rotors and 23.13 for the cross-drilled. By comparison, the System 2 values at 140kph are 23.35 for the plain rotors and 26.18 for the drilled. The conclusion I draw from this is that while cross-drilling does improve the cooling rate, the degree of benefit is inversely proportional to the size of the rotor. From a pure cooling rate consideration, the plain rotors of System 2 outperform the cross-drilled rotors of System 1. In practical terms, using cross-drilled rotors would seem to make the most sense only after you have run out of options for increasing the rotor diameter.
Pad wear on cross-drilled rotors was also shown to be up to up to 50% greater under hard use, and about 25~30% greater under street use. What was not explicitly tested was the effect of this increased wear on temperature. As I understand it, the thinner the pad, the higher the pad temp. My suspicion is that at some point, the diminished heat capacity of the pad due to wear more than offsets the gain in cooling capacity of the cross-drilled rotors. I further suspect that this may be the operating mechanism behind many of the catastrophic failures of cross-drilled rotors at the track. Basically, they will start off strong, but the accelerated rate of pad wear soon drives the rotor into an overheated condition.
Some of the other data and conclusions from the paper on cross-drilled rotors:
Basically, it looks like they didn't necessarily set out to do a full performance comparison, but incidental to their testing on three brake systems they found they had enough data to write it up as a paper.
It'll cost you $14 do download a copy, but it's well worth it if you are interested in the subject. Hopefully at least some of you are willing to give it a read and start a discussion about it. Below are my thoughts to kick it off.
First off, the surprising conclusion is that drilling the rotors actually does increase the rate of cooling for a given a rotor, by up to about 20% in one case. Unfortunately however, that gain comes with some very serious drawbacks, including the possibility that your rotors may actually run hotter despite of it.
I believe one of the most significant considerations in interpreting the paper's data is that they used semi-metallic brake pads for all the testing. From what I've read, these typically have a max operating temp rating of about 500~550 deg C. Secondly, I understand that the primary functional mechanism of this type of pad is abrasion rather than adhesion. By contrast, the higher temp rated race type pads are more biased towards an adhesive mechanism relying upon a transfer layer of material deposited on the rotor.
Where this comes in to play is in interpreting the test data that shows higher apparent friction and deceleration gain for the plain rotor at low temp, the cross-drilled rotor at higher temp, and generally greater temperature fade stability for the cross-drilled. At low temp, the greater surface area of the plain rotor is the dominant performance factor, as one would expect. At higher temps however, the breakdown of the pad is the dominant consideration and the cheese grater effect of the cross-drilled rotors helps to compensate for this by providing increased mechanical friction.
If however, this test were to be repeated with a higher temp rated adhesive type race pad, I'd expect the results to be exactly the opposite. That is at lower temps where you would expect the race pad to perform poorly, the cross-drilled rotors would provide an abrasive mechanism and increased performance. But at higher temps, not only would the lesser surface area of the cross-drilled rotor become the dominant consideration, but the abrasive mechanism of the holes would inhibit proper formation of the transfer layer.
Another interesting aspect of the paper is the radical base performance difference between the three brake systems used in the test. "System 3" appears to be an overall poor performer in comparison with the other 2 systems. In the case of System 3, the cross drilling appears to be a pure bling change, as it is rather consistently outperformed by the plain rotors, despite their being smaller in overall diameter. System 1 was a better performer, but quite unbalanced. Almost all the work on this system was being done by the front rotors. System 2 looks to be the best of the lot, and consequently showed the least radical performance deviations between the plain and cross-drilled rotors.
I believe the base performance difference between System 1 and System 2 helps in interpreting the results showing increased cooling efficiency with the cross-drilled rotors. On System 1, tests at 50kph, 100kph, and 160kph, showed an increase of cooling values (hA) of 8.8%, 12.1%, and 20.1% on the front and -3.2%, 1.9%, and 8.5% on the rear. For System 2, the values at 50kph, 110kph, and 140kph are 7.8%, 10.4%, and 12.1% for the front and 4.1%, 7.7%, and 13.4% rear. The other key piece of data is the raw hA numbers. At 160kph, the average hA for the front rotors of System 1 is 19.26 for the plain rotors and 23.13 for the cross-drilled. By comparison, the System 2 values at 140kph are 23.35 for the plain rotors and 26.18 for the drilled. The conclusion I draw from this is that while cross-drilling does improve the cooling rate, the degree of benefit is inversely proportional to the size of the rotor. From a pure cooling rate consideration, the plain rotors of System 2 outperform the cross-drilled rotors of System 1. In practical terms, using cross-drilled rotors would seem to make the most sense only after you have run out of options for increasing the rotor diameter.
Pad wear on cross-drilled rotors was also shown to be up to up to 50% greater under hard use, and about 25~30% greater under street use. What was not explicitly tested was the effect of this increased wear on temperature. As I understand it, the thinner the pad, the higher the pad temp. My suspicion is that at some point, the diminished heat capacity of the pad due to wear more than offsets the gain in cooling capacity of the cross-drilled rotors. I further suspect that this may be the operating mechanism behind many of the catastrophic failures of cross-drilled rotors at the track. Basically, they will start off strong, but the accelerated rate of pad wear soon drives the rotor into an overheated condition.
Some of the other data and conclusions from the paper on cross-drilled rotors:
- There is no practical difference in wet weather performance.
- Pad outgassing is not factor
- Thermal fatigue life is significantly shorter for cross-drilled rotors. In the case of poorly balanced and possibly under-sized System 1, by as much as 50%.