drilled or slotted debate opener
Discussion
Here is a short piece written about the differences between cross-drilled/slotted and slotted only rotors. (taken from the Performance Brake Company)
Regarding which brakes are better stoppers......
Assuming all the other variables remain unchanged, cross drilled and slotted rotors will almost always deliver better braking figures than other types. The improved cooling effect of these rotors allows effective use of the brake system under extremely severe conditions (as might be seen in racing). The down side to this design is that when these rotors are forced to work to capacity for prolonged periods, they are subject to cracking around the drilled holes as a result of constant overheating and cooling. Until they destroy themselves these are the most efficient. For street use (long track racing), cross drilled rotors are perfect most of the time as the system is seldom used to capacity. The rotors should last a normal life span.
If the rotor is of only marginally adequate specification for the vehicle weight or use, cross drilled rotors can be problematic and require frequent "crack checking" and possible frequent replacement. Under these conditions, slotted only rotors are more desirable. Slotted rotors are less prone to cracking under severe service than drilled. What they lack in efficiency (if any) they make up for in longevity. You will seldom find a high performance short track racecar with drilled rotors, these cars are almost always fitted with slotted rotors for the reasons listed here.
If we had to stop hard one time from high speed, we would probably select a drilled and slotted rotor. If we had to do it over and over, we would almost certainly use slotted rotors.
I know there are different viewpoints on this topic but I do not like drilled discs IMAO
Regarding which brakes are better stoppers......
Assuming all the other variables remain unchanged, cross drilled and slotted rotors will almost always deliver better braking figures than other types. The improved cooling effect of these rotors allows effective use of the brake system under extremely severe conditions (as might be seen in racing). The down side to this design is that when these rotors are forced to work to capacity for prolonged periods, they are subject to cracking around the drilled holes as a result of constant overheating and cooling. Until they destroy themselves these are the most efficient. For street use (long track racing), cross drilled rotors are perfect most of the time as the system is seldom used to capacity. The rotors should last a normal life span.
If the rotor is of only marginally adequate specification for the vehicle weight or use, cross drilled rotors can be problematic and require frequent "crack checking" and possible frequent replacement. Under these conditions, slotted only rotors are more desirable. Slotted rotors are less prone to cracking under severe service than drilled. What they lack in efficiency (if any) they make up for in longevity. You will seldom find a high performance short track racecar with drilled rotors, these cars are almost always fitted with slotted rotors for the reasons listed here.
If we had to stop hard one time from high speed, we would probably select a drilled and slotted rotor. If we had to do it over and over, we would almost certainly use slotted rotors.
I know there are different viewpoints on this topic but I do not like drilled discs IMAO
The cross drilling is purely for aesthetics and adds no real brake cooling advantage but could arguably offer very slightly better initial bite, when we developed the discs with AP we discussed long and hard, but bowed to public opinion and went for both, however the crossdrilling that we had done had quite a shallow counter sink so as to keep the transition from disc to hole as smooth as possible. We have yet to find any cracks even after sustained track use.
Just had it confirmed that the LSV supply of AP brakes for the Monaro/HSV are both drilled & slotted (in line with HSV spec) and in future may well revert to just slotted. Warranty automatically comes with any genuine AP product sold. Not associated with LSV commercially, just a customer.
Porsche discs always seem to be drilled & they work very well.
I've always been led to believe that drilling helps disperse gases when the pads are applied & grooving helps to clean the surface of the pad.
I've actually had plain, drilled & grooved only discs (all AP) over the years & honestly couldn't tell the difference. Drilled tend to make the disc look bigger, which from a cosmetic point of view may appeal to some.
On the HSV they were grooved only & worked superbly, much better than the HSV (Harrop) 4 Pots.
Interestingly, all the different AP set ups squealed, regardless of anti-squeal shims,copper slip etc. etc.
I've always been led to believe that drilling helps disperse gases when the pads are applied & grooving helps to clean the surface of the pad.
I've actually had plain, drilled & grooved only discs (all AP) over the years & honestly couldn't tell the difference. Drilled tend to make the disc look bigger, which from a cosmetic point of view may appeal to some.
On the HSV they were grooved only & worked superbly, much better than the HSV (Harrop) 4 Pots.
Interestingly, all the different AP set ups squealed, regardless of anti-squeal shims,copper slip etc. etc.
A57 HSV said:
Porsche discs always seem to be drilled & they work very well.
Drilled discs crack. Thats fact. You may be able to smooth the outer section of the hole that is drilled, but you cant easily do that to the inner section.
I may not do huge miles, but I wouldnt buy a set of drilled discs. They just crack too soon under heavy usage.
As for Porsche, I'm pretty sure they cast their discs with the holes in them, so there are no sharp edges/stress points that will lead to premature cracking.
stevieturbo said:
Porsche discs always seem to be drilled & they work very well.
Drilled discs crack. Thats fact. You may be able to smooth the outer section of the hole that is drilled, but you cant easily do that to the inner section.
I may not do huge miles, but I wouldnt buy a set of drilled discs. They just crack too soon under heavy usage.
As for Porsche, I'm pretty sure they cast their discs with the holes in them, so there are no sharp edges/stress points that will lead to premature cracking.
Must do, as I don't know any Porsche owners who haved suffered from cracked discs.
Motorsport contacts that I know, always go for plain or grooved discs. I think drilled is a cosmetic thing really, must admit that I think drilled do look good but I'd choose performance over looks anytime.
I have the AP/Wortec six pot front, four pot rear set up which I can confirm are drilled and slotted. They are not old enough yet to show any signs of any wear, nor have they seen any track use, but from a day to day point of view in a big heavy car, they are all I ever wanted, and they look soooo nice. I think the whole debate swings on what you intend to do with the car, dont know of anyone on here who gets enough track time to warrant drilled discs having a chance of cracking. Good friend has a Porsche, solely for trackdays, which has the standard Porsche, drilled only discs
Mike
Mike
Most times when you "drill" something its for the purpose of reducing it's mass and I always thought thats how drilling started in brake discs whilst slots were more practical for cooling and removing gases. You can drill something (if done correctly) and save considerable weight for an inconsiderable reduction in strength. Have said that, any change of the surface of a brake disc will reduce the swept area which you'd surely have to make up elsewhere? I went for slotted Brembos on my alfa when I replaced them but due to their "unique" slot-design, felt considerable vibration through the wheel when braking hard, you could even hear it! Most if not all m/cycle discs are drilled but I suspect due to the reduced forces from the lighter machine, the discs are up to the job. Heard of few if any that have cracked. I cracked the solid steel ones on my '95 ex Police VS 5.0l though. They went all pretty colours and everything...
PB
PB
I can't imagine anyone drilling discs to save weight. I think the primary reason is to clear the gases away between the pad and disc, the secondary reason is for looks. Weight would only be an issue in extreme cases and in those cases you would just fit smaller discs. You don't need many holes/grooves to prevent the gas build up, many of the aftermarket discs have far too many and this reduces the pad contact area, increases noise and wear and physically weakens the disc all for no good reason.
I've always gone for just grooved discs for fear of cracking on drilled discs, on the last car i heavily modified Mk2 Golf GTi 16v i fitted grooved discs with the grooves going against direction of rotation if u understand me so cool air would be forced in & brake dust pushed out. Wether this makes any diff to brake performance or it's just a myth i'll let the more motorsport types tell me. 
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