Anorak facts on drilled discs.
Discussion
As a firm believer in drilled discs for both weight reduction and heat dissipation (amongst other things), I recently decided to do a factual analysis of some one-off discs I have just done to justify my claims/opinions.
Discs are large at 332mm vented fronts and 324mm solid rears, so results would be %wise better on smaller discs and MUCH better on discs with alloy bells.
Taking into account all of the following
* Loss of surface area because you now have a hole
* The area of the chamfer of the holes drilled
* The minimum mass of cast iron at 7850Kg/m3
* The surface area of the drilled holes
* The number and diameter of the holes (obviously)
and ignoring
* Temperature differential across the disc
* Surface area of internal vanes
* material could be HEAVIER than stated
* % weight saving because it depends on "bell" material and thickness.
I can state that the discs I just did have a surface area increase, available to further dissipate heat of 40,000 mm2 or 17% on the front discs and 19,300 mm2 or 20.2% on the rears.
Each front disc is 485g lighter and each rear disc is 250g lighter, giving a total unsprung weight loss of 1470g or 3 1/4 Lbs. Not to be sniffed at!
I just gotta getta life!
8Tech.
Discs are large at 332mm vented fronts and 324mm solid rears, so results would be %wise better on smaller discs and MUCH better on discs with alloy bells.
Taking into account all of the following
* Loss of surface area because you now have a hole
* The area of the chamfer of the holes drilled
* The minimum mass of cast iron at 7850Kg/m3
* The surface area of the drilled holes
* The number and diameter of the holes (obviously)
and ignoring
* Temperature differential across the disc
* Surface area of internal vanes
* material could be HEAVIER than stated
* % weight saving because it depends on "bell" material and thickness.
I can state that the discs I just did have a surface area increase, available to further dissipate heat of 40,000 mm2 or 17% on the front discs and 19,300 mm2 or 20.2% on the rears.
Each front disc is 485g lighter and each rear disc is 250g lighter, giving a total unsprung weight loss of 1470g or 3 1/4 Lbs. Not to be sniffed at!
I just gotta getta life!
8Tech.
If you're seriously considering weight reduction as a factor you are IMO crackers.
The purpose is not to increase the cooling area or save weight, it is to give a path for gas to escape from the friction surface. The big problem with drilled discs is that they are vulnerable to stress raisers around the holes which lead to cracking and eventually the risk of structural failure. Grooved discs are far preferable from this point of view since they give the same degassing benefits without the cracking problems. Both of them do suffer from problems of increased noise and pad wear, and reduced pad contact area, so you are better off with plain discs unless degassing the pads is actually a problem for you, in which case I would always recommend grooved and never drilled discs.
The purpose is not to increase the cooling area or save weight, it is to give a path for gas to escape from the friction surface. The big problem with drilled discs is that they are vulnerable to stress raisers around the holes which lead to cracking and eventually the risk of structural failure. Grooved discs are far preferable from this point of view since they give the same degassing benefits without the cracking problems. Both of them do suffer from problems of increased noise and pad wear, and reduced pad contact area, so you are better off with plain discs unless degassing the pads is actually a problem for you, in which case I would always recommend grooved and never drilled discs.
GreenV8S said:
If you're seriously considering weight reduction as a factor you are IMO crackers.
The purpose is not to increase the cooling area or save weight, it is to give a path for gas to escape from the friction surface. The big problem with drilled discs is that they are vulnerable to stress raisers around the holes which lead to cracking and eventually the risk of structural failure. Grooved discs are far preferable from this point of view since they give the same degassing benefits without the cracking problems. Both of them do suffer from problems of increased noise and pad wear, and reduced pad contact area, so you are better off with plain discs unless degassing the pads is actually a problem for you, in which case I would always recommend grooved and never drilled discs.
+1The purpose is not to increase the cooling area or save weight, it is to give a path for gas to escape from the friction surface. The big problem with drilled discs is that they are vulnerable to stress raisers around the holes which lead to cracking and eventually the risk of structural failure. Grooved discs are far preferable from this point of view since they give the same degassing benefits without the cracking problems. Both of them do suffer from problems of increased noise and pad wear, and reduced pad contact area, so you are better off with plain discs unless degassing the pads is actually a problem for you, in which case I would always recommend grooved and never drilled discs.
Also, the main factor in cooling a disc is down to how thick it is. The fins between the rotor faces pull air through the rotors for better cooling. The greater the volume of cold air accurately channeled to the fins, the better the cooling. Having holes or groves on the disc has very little to do with it.
ian in lancs said:
why do they drill holes in the disks rather than the pads?
The pads are a lot weaker than the discs and would disintegrate around the holes. There's also the problem of blasting hot gasses and dust inside the callipers which would do them no good at all. Having the muck carried away by the disc is better all round.GreenV8S said:
If you're seriously considering weight reduction as a factor you are IMO crackers.
The purpose is not to increase the cooling area or save weight, it is to give a path for gas to escape from the friction surface. The big problem with drilled discs is that they are vulnerable to stress raisers around the holes which lead to cracking and eventually the risk of structural failure. Grooved discs are far preferable from this point of view since they give the same degassing benefits without the cracking problems. Both of them do suffer from problems of increased noise and pad wear, and reduced pad contact area, so you are better off with plain discs unless degassing the pads is actually a problem for you, in which case I would always recommend grooved and never drilled discs.
In 25 years I have never seen a cracked disc caused by the drilling, only cracked discs from wearing below minimum thickness or DIY drilled discs with holes too big and too close. Show me one!The purpose is not to increase the cooling area or save weight, it is to give a path for gas to escape from the friction surface. The big problem with drilled discs is that they are vulnerable to stress raisers around the holes which lead to cracking and eventually the risk of structural failure. Grooved discs are far preferable from this point of view since they give the same degassing benefits without the cracking problems. Both of them do suffer from problems of increased noise and pad wear, and reduced pad contact area, so you are better off with plain discs unless degassing the pads is actually a problem for you, in which case I would always recommend grooved and never drilled discs.
sniff diesel said:
The way I look at it is that most racing cars use non-drilled discs so therefor they must be better?
We are not driving racing cars with exotic materials, we are driving road cars and I do not see too many undrilled performance option discs to replace the stock drilled discs on Ferrari's, Porsches's Lamboughini's or suchlike.Nearly 4 Lbs is a fair old weight to lose on unsprung weight and a lot cheaper than a Carbon Fibre bonnet for the same weight saving.
8Tech said:
In 25 years I have never seen a cracked disc caused by the drilling, only cracked discs from wearing below minimum thickness or DIY drilled discs with holes too big and too close. Show me one!
I don't have one here to show you, but I've seen discs with cracks originating at the drillings which clearly are compromising the strength of the disc, and discs *have* been known to fail on the track usually with catastrophic results. It's a known problem and one which you would do well to avoid.The quick GT3 guys get rid of the standard disks with holes cast in them and move to alcon or similar slotted disks as the standard ones crack radiating from the holes.
Friend had an AP 355mm drilled disk on his Evo 6 crack from the bell to the outer edge in 2 places so the disk was in 2 separate pieces - he switched to slotted too.
Many race cars don't have exotic materials yet seem to choose slotted...
Friend had an AP 355mm drilled disk on his Evo 6 crack from the bell to the outer edge in 2 places so the disk was in 2 separate pieces - he switched to slotted too.
Many race cars don't have exotic materials yet seem to choose slotted...
Edited by m12_nathan on Saturday 8th November 20:58
Yes, but again we are talking RACE cars, not road cars. Just doing a Google Images search on the net on cracked discs will show an equal number of cracked discs on both drilled and non-drilled discs but every image I saw for as far as I went, the drilled discs have overheated or were not chamfered.
If you use a stock drilled aftermarket ROAD brake disc and then go racing with it, it really is not a fair test to blame the holes when it overheats and fails. It would have probably cracked even if it wasn't drilled.
Slotted discs are better for sustained high temperatures.
Exerpt from major professional disc brake manufacturer/supplier:-
Some rotors have been returned with more serious structural cracking, however the number equates to less than 0.2 per cent of rotors shipped. And of these, most were used in motor sport applications, something they were not designed or warranted for. To be frank, most of these owners would have destroyed non-drilled rotors under the same conditions. They were simply asking too much of their standard braking system and should have upgraded to our purpose-built motor sport rotors and possibly bigger calipers as well.
So I still stand by my original post, for the road, drilled is the way to go. If you drive beyond the capabilities of your vehicles brakes or race, then buy a racing brake set-up.
8Tech.
If you use a stock drilled aftermarket ROAD brake disc and then go racing with it, it really is not a fair test to blame the holes when it overheats and fails. It would have probably cracked even if it wasn't drilled.
Slotted discs are better for sustained high temperatures.
Exerpt from major professional disc brake manufacturer/supplier:-
Some rotors have been returned with more serious structural cracking, however the number equates to less than 0.2 per cent of rotors shipped. And of these, most were used in motor sport applications, something they were not designed or warranted for. To be frank, most of these owners would have destroyed non-drilled rotors under the same conditions. They were simply asking too much of their standard braking system and should have upgraded to our purpose-built motor sport rotors and possibly bigger calipers as well.
So I still stand by my original post, for the road, drilled is the way to go. If you drive beyond the capabilities of your vehicles brakes or race, then buy a racing brake set-up.
8Tech.
[quote]for the road, drilled is the way to go
[/quote]
I disagree. For the road standard setup is fine. Drilled/grooved are noisy. Whats wrong with getting more air to a standard vented disc?
I have never had issues with a standard BMW setup, on road, or track. The M5 was lacking in some areas, but when allowances were made could be overcome.(On road)
[/quote]
I disagree. For the road standard setup is fine. Drilled/grooved are noisy. Whats wrong with getting more air to a standard vented disc?
I have never had issues with a standard BMW setup, on road, or track. The M5 was lacking in some areas, but when allowances were made could be overcome.(On road)
8Tech said:
So I still stand by my original post, for the road, drilled is the way to go. If you drive beyond the capabilities of your vehicles brakes or race, then buy a racing brake set-up.
8Tech.
None of the reasons you've given for this view stand up to inspection, and you seem to have decided this arbitrarily.8Tech.
It is widely known that drilled discs are more prone to cracking than plain or grooved discs.
Chamfering the drillings does not solve the problem.
Discs with the holes cast instead of drilled are better but still suffer the problem.
Grooved and plain discs still suffer the problem but to a vastly smaller extent than drilled discs.
Drilling the discs to save weight is bonkers.
So, under what circumstances do you think there are actually tangible benefits to drilling, and in what respect is drilled *ever* better than grooved?
8Tech said:
Each front disc is 485g lighter and each rear disc is 250g lighter, giving a total unsprung weight loss of 1470g or 3 1/4 Lbs. Not to be sniffed at!
You can't add up all the saved unsprung mass. Any saving in unsprung mass is specific to the suspension/brake/wheel assembly as the spring/damper assembly controls that mass. Also to a lesser extent the axle from which the mass was saved, factoring in the ARB's.From AP's web site
http://www.apracing.com/info/info.asp?section=Disc...
I believe them!
Steve
Frik said:
GreenV8S said:
Discs with the holes cast instead of drilled are better but still suffer the problem.
Does anyone actually make disk that are literally cross drilled? Can't imagine they'd last very long at all.Gassing Station | BMW General | Top of Page | What's New | My Stuff