The real deal about cross-drilled and Slotted Rotors

Darkiller · 11-01-2006, 07:45 PM

First, lets get some physics. Tell me how a heatsink with less mass will cool better? You do realize that a brake rotor acts as a large heatsink to transfer heat from the brake pads to the rotor. The heat generated from pads has to go somewhere and so it transfers to the rotor and caliper.

Porsche claims: "Discs are cross-drilled to enhance braking in the wet. The brakes respond faster because the water vapour pressure that builds up during braking can be released more easily."

They have said nothing about enhancing normal braking circumstances and the larger diameter rotors probably make up for the lack of material present in a smaller cross drilled rotor.

From Wilwood's website:

Quote:
Q: Why are some rotors drilled or slotted?
A: Rotors are drilled to reduce rotating weight, an issue near and dear to racers searching for ways to minimize unsprung weight. Drilling diminishes a rotor's durability and cooling capacity.

Slots or grooves in rotor faces are partly a carryover from the days of asbestos pads. Asbestos and other organic pads were prone to "glazing" and the slots tended to help "scrape or de-glaze" them. Drilling and slotting rotors has become popular in street applications for their pure aesthetic value. Wilwood has a large selection of drilled and slotted rotors for a wide range of applications.

As for the porsche rotors, a few notes from a forum I frequent:

Quote:
1) The holes are cast in giving a dense boundary layer-type crystalline grain structure around the hole at the microscopic level as opposed to drilling which cuts holes in the existing grain pattern leaving open endgrains, etc, just begging for cracks.

2) The holes are only 1/2 the diameter of the holes in most drilled rotors. This reduces the stress concentration factor due to hole interaction which is a function (not linear) of hole diameters and the distance between them.

3) Since the holes are only 1/2 as big they remove only 1/4 as much surface area and mass from the rotor faces as a larger hole. This does a couple of things:

It increases effective pad area compared with larger holes. The larger the pad area the cooler they will run, all else being equal. If the same amount of heat is generated over a larger surface area it will result in a lower temperature for both surfaces.

It increases the mass the rotor has to absorb heat with. If the same amount of heat is put into a rotor with a larger mass, it will result in a lower temperature.

3) The holes are placed along the vanes, actually cutting into them giving the vane a "half moon" cut along its width. You can see that here:

This does a couple of things:

First, it greatly increases the surface area of the vanes which allows the entire rotors to run cooler which helps prevent cracks by itself.

Second, it effectively stops cracking on that side of the hole which makes it very difficult to get "hole to hole" cracks that go all the way through the face rotor (you'll get tiny surface "spider cracks" on any rotor, blank included if you look hard enough).

That's why Porsche rotors are the only "crossdrilled" rotors I would ever consider putting on my car.

BTW, many of the above features are not present in older Porsche brakes. The above is for "Big Reds" and newer.

This is quite different from the standard drilled rotors you get from brembo/kvr/powerslot/"insert random ricer parts brand name here" brake rotors.

Further proof of the uselessness of cross drilled rotors are found here:

http://www.teamscr.com/rotors.htm

Quote:
Crossdrilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads from the 40’s and 50’s, not a whole lot. Rotors were first ‘drilled’ because early brake pad materials gave off gasses when heated to racing temperatures – a process known as ‘gassing out’. These gasses then formed a thin layer between the brake pad face and the rotor, acting as a lubricant and effectively lowering the coefficient of friction. The holes were implemented to give the gasses ‘somewhere to go’. It was an effective solution, but today’s friction materials do not exhibit the same gassing out phenomenon as the early pads.

For this reason, the holes have carried over more as a design feature than a performance feature. Contrary to popular belief they don’t lower temperatures (in fact, by removing weight from the rotor, the temperatures can actually increase a little), they create stress risers allowing the rotor to crack sooner, and make a mess of brake pads – sort of like a cheese grater rubbing against them at every stop. (Want more evidence? Look at NASCAR or F1. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it.)

The one glaring exception here is in the rare situation where the rotors are so oversized (look at any performance motorcycle or lighter formula car) that the rotors are drilled like Swiss cheese. While the issues of stress risers and brake pad wear are still present, drilling is used to reduce the mass of the parts in spite of these concerns. Remember – nothing comes for free. If these teams switched to non-drilled rotors, they would see lower operating temperatures and longer brake pad life – at the expense of higher weight. It’s all about trade-offs.

From Stoptech:

Quote:
Which is better, slotted or drilled rotors?

StopTech provides rotors slotted, drilled or plain. For most performance applications slotted is the preferred choice. Slotting helps wipe away debris from between the pad and rotor as well as increasing the "bite" characteristics of the pad. A drilled rotor provides the same type of benefit, but is more susceptible to cracking under severe usage. Many customers prefer the look of a drilled rotor and for street and occasional light duty track use they will work fine. For more severe applications, we recommend slotted rotors.

That almost sounds like an excuse to use cross drilled rotors, and for your street car which probably is never driven on the track, the drilled rotors are fine, but as Stoptech states, they will crack and are not good for severe applications.

From Baer:

Quote:
"What are the benefits to Crossdrilling, Slotting, and Zinc-Washing my rotors?

In years past, crossdrilling and/or Slotting the rotor for racing purposes was beneficial by providing a way to expel the gasses created when the bonding agents employed to manufacture the pads...However, with today’s race pad technology, ‘outgassing’ is no longer much of a concern...Slotted surfaces are what Baer recommends for track only use. Slotted only rotors are offered as an option for any of Baer’s offerings."

Then from Grassroots Motorsports:

Quote:
"Crossdrilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads from the '40s and 50s, not a whole lot. Rotors were first drilled because early brake pad materials gave off gasses when heated to racing temperatures, a process known as "gassing out." ...It was an effective solution, but today's friction materials do not exhibit the some gassing out phenomenon as the early pads. Contrary to popular belief, they don't lower temperatures. (In fact, by removing weight from the rotor, they can actually cause temperatures to increase a little.) These holes create stress risers that allow the rotor to crack sooner, and make a mess of brake pads--sort of like a cheese grater rubbing against them at every stop. Want more evidence? Look at NASCAR or F1. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it...Slotting rotors, on the other hand, might be a consideration if your sanctioning body allows for it. Cutting thin slots across the face of the rotor can actually help to clean the face of the brake pads over time, helping to reduce the glazing often found during high-speed use which can lower the coefficient of friction. While there may still be a small concern over creating stress risers in the face of the rotor, if the slots are shallow and cut properly, the trade-off appears to be worth the risk. (Have you looked at a NASCAR rotor lately?)

***wdb*** · 11-02-2006, 07:16 AM

Nice work.

stretch · 11-02-2006, 07:30 AM

Here's one of the most interesting thread on the subject, ever. Todd@TCE offered a $200 bounty to anyone that could prove that a cast rotor with holes exists- this argument came about after lots of Internet "experts" saying that they do, and that they would not fail the way drilled rotors do. Note that Todd@TCE sells slotted rotors just like any rotor dealer, but he'll get you blanks on request.
http://corner-carvers.com/forums/showthread.php?t=21273

I don't know anyone with racing experience and an education that uses anything other than blanks- the exception are those sponsored by bling-bling brake companies. That's not to say people don't race on slotted and drilled rotors, but the guys who race often almost always use blanks (and they'll have spares, too). Take my anecdotal experience for what it's worth, which isn't much, but I'm rather certain most brake vendors worth his/her weight will recommend blanks to you for track duty.

Drilled big brake kits (show car brake kits) work on some race cars because they're so oversized that they still get by. This is the case on sportbike rotors, too. Racing teams with these big brake kits often change their rotors after each race at the sponsor's expense. OEM-sized drilled rotors are downright unsafe for racing, towing, or even summer stop-and-go and sometimes come with a disclaimer warning of just that.

Slots are still a stress point, and I've seen rotors with stress cracks originating from the slots. Note that this rotor is cracking everywhere (even a blank rotor can do this), but it is clearly at its worst where the slot is:

Cross-drilled rotor failures are very, very easy to find. These pictures were taken after just TWO 20 minutes track sessions:

flatthump · 11-02-2006, 07:34 AM

^^those got a little hot..lol. When I was in an Auto-tech program, my favorite teacher spent two days talking about slotted and cross-drilled brakes and the waste of money they are on a track...

***BoostJunkie*** · 11-02-2006, 07:48 AM

I think dimpled rotors should also be thrown in here. Performance Friction uses dimples on most of their 2-piece rotors, and their products are widely used in various racing series. Any thoughts?

stretch · 11-02-2006, 07:58 AM

Quote:

Originally Posted by BoostJunkie

I think dimpled rotors should also be thrown in here. Performance Friction uses dimples on most of their 2-piece rotors, and their products are widely used in various racing series. Any thoughts?

Dimpled rotors should wear roughly as well as slotted rotors, which is to say much better than drilled rotors but still not as good as blanks.

The best question to ask, in my opinion, is why not use blanks? Does anyone really buy the argument of wiping debris away, and if so, how often has that helped you? There is no good reason not to use blanks. They're cheaper, more durable, and have the greatest heat capacity.

Darkiller · 11-02-2006, 08:12 AM

make this a sticky

Rmiller · 11-02-2006, 08:23 AM

I disagree that drilling would result in a decrease in cooling capacity. It would result in a decrease in heat capacity due to the decrease in material, but drilled rotors will have greater surface area and thus greater capacity to cool.

Mykl · 11-02-2006, 08:52 AM

Quote:

Originally Posted by stretch

Dimpled rotors should wear roughly as well as slotted rotors, which is to say much better than drilled rotors but still not as good as blanks.

The best question to ask, in my opinion, is why not use blanks? Does anyone really buy the argument of wiping debris away, and if so, how often has that helped you? There is no good reason not to use blanks. They're cheaper, more durable, and have the greatest heat capacity.

I've never had slotted rotors, so I can only go off of "hear say" but apparently slotted rotors cause the pads to bite a little harder when you get on the brakes. Why this may be, I have no idea.

I only use blank rotors, and I recommend that others do the same.

There are two other reasons why you see drilled rotors on motorcycles... weight and weather. Sportbike manufacturers are doing every thing they can to make their bikes go faster and handle better, so that little bit of weight that far out on the rotating mass of the front wheel probably makes a difference. The weather thing is a no brainer, unlike auto brakes they're literally out in the open at the front of the vehicle. The water needs to go somewhere.

Mykl · 11-02-2006, 08:54 AM

Quote:

Originally Posted by Rmiller

I disagree that drilling would result in a decrease in cooling capacity. It would result in a decrease in heat capacity due to the decrease in material, but drilled rotors will have greater surface area and thus greater capacity to cool.

With all those holes the air isn't flowing through the entire rotor like it should with a ventillated design.

Rmiller · 11-02-2006, 10:38 AM

Quote:

Originally Posted by Mykl

With all those holes the air isn't flowing through the entire rotor like it should with a ventillated design.

That's possible, but I didn't read anything about that. I just stated what I know about physics. Less mass & more surface area both made an object easier to cool (the decreased mass will require less energy to heat as well). Look at intercoolers. Much lighter than a giant block of aluminum with a hole drilled through it would be, yet more effective at cooling.

For the record, I would also choose blanks if I had the choice.

**Turninconcepts.com** · 11-02-2006, 12:04 PM

Quote:

Originally Posted by Mykl

With all those holes the air isn't flowing through the entire rotor like it should with a ventillated design.

DINGDINGDING!!!

**Turninconcepts.com** · 11-02-2006, 12:05 PM

MR. DARKILLER!!!!!

I LOVE YOU!!!!!

Thank you thank you thank you!!!

ender18d · 11-02-2006, 12:25 PM

I didn't see your "no frills" rotors on your site anymore??? I don't need them yet, but was planning on a pair....

Quote:

Originally Posted by Turninconcepts.com

MR. DARKILLER!!!!!

I LOVE YOU!!!!!

Thank you thank you thank you!!!

**Turninconcepts.com** · 11-02-2006, 12:37 PM

Quote:

Originally Posted by ender18d

I didn't see your "no frills" rotors on your site anymore??? I don't need them yet, but was planning on a pair....

We were having supplier problems so I pulled them for now. Starting monday I will have considerably more time to work on things like that so we will see if we can get them back on through a different supplier.

As it is I've been doing some research, and want to try an experiment I've had in mind for a while, but to pull it off I'm gonna need some rotors. And a track whore... perhaps Myles since I know he's got the equipment, and know how to check something out for me...

11-01-2006, 07:45 PM	#1
Darkiller Spec C Club Car: 2006 STI BLUE Fav Mod: Fp20g and protune Location: 1000 oaks, SoCA Posts: 1,499 Join Date: Dec 2005 Trader Rating: (0)	The real deal about cross-drilled and Slotted Rotors First, lets get some physics. Tell me how a heatsink with less mass will cool better? You do realize that a brake rotor acts as a large heatsink to transfer heat from the brake pads to the rotor. The heat generated from pads has to go somewhere and so it transfers to the rotor and caliper. Porsche claims: "Discs are cross-drilled to enhance braking in the wet. The brakes respond faster because the water vapour pressure that builds up during braking can be released more easily." They have said nothing about enhancing normal braking circumstances and the larger diameter rotors probably make up for the lack of material present in a smaller cross drilled rotor. From Wilwood's website: Quote: Q: Why are some rotors drilled or slotted? A: Rotors are drilled to reduce rotating weight, an issue near and dear to racers searching for ways to minimize unsprung weight. Drilling diminishes a rotor's durability and cooling capacity. Slots or grooves in rotor faces are partly a carryover from the days of asbestos pads. Asbestos and other organic pads were prone to "glazing" and the slots tended to help "scrape or de-glaze" them. Drilling and slotting rotors has become popular in street applications for their pure aesthetic value. Wilwood has a large selection of drilled and slotted rotors for a wide range of applications. As for the porsche rotors, a few notes from a forum I frequent: Quote: 1) The holes are cast in giving a dense boundary layer-type crystalline grain structure around the hole at the microscopic level as opposed to drilling which cuts holes in the existing grain pattern leaving open endgrains, etc, just begging for cracks. 2) The holes are only 1/2 the diameter of the holes in most drilled rotors. This reduces the stress concentration factor due to hole interaction which is a function (not linear) of hole diameters and the distance between them. 3) Since the holes are only 1/2 as big they remove only 1/4 as much surface area and mass from the rotor faces as a larger hole. This does a couple of things: It increases effective pad area compared with larger holes. The larger the pad area the cooler they will run, all else being equal. If the same amount of heat is generated over a larger surface area it will result in a lower temperature for both surfaces. It increases the mass the rotor has to absorb heat with. If the same amount of heat is put into a rotor with a larger mass, it will result in a lower temperature. 3) The holes are placed along the vanes, actually cutting into them giving the vane a "half moon" cut along its width. You can see that here: This does a couple of things: First, it greatly increases the surface area of the vanes which allows the entire rotors to run cooler which helps prevent cracks by itself. Second, it effectively stops cracking on that side of the hole which makes it very difficult to get "hole to hole" cracks that go all the way through the face rotor (you'll get tiny surface "spider cracks" on any rotor, blank included if you look hard enough). That's why Porsche rotors are the only "crossdrilled" rotors I would ever consider putting on my car. BTW, many of the above features are not present in older Porsche brakes. The above is for "Big Reds" and newer. This is quite different from the standard drilled rotors you get from brembo/kvr/powerslot/"insert random ricer parts brand name here" brake rotors. Further proof of the uselessness of cross drilled rotors are found here: http://www.teamscr.com/rotors.htm Quote: Crossdrilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads from the 40’s and 50’s, not a whole lot. Rotors were first ‘drilled’ because early brake pad materials gave off gasses when heated to racing temperatures – a process known as ‘gassing out’. These gasses then formed a thin layer between the brake pad face and the rotor, acting as a lubricant and effectively lowering the coefficient of friction. The holes were implemented to give the gasses ‘somewhere to go’. It was an effective solution, but today’s friction materials do not exhibit the same gassing out phenomenon as the early pads. For this reason, the holes have carried over more as a design feature than a performance feature. Contrary to popular belief they don’t lower temperatures (in fact, by removing weight from the rotor, the temperatures can actually increase a little), they create stress risers allowing the rotor to crack sooner, and make a mess of brake pads – sort of like a cheese grater rubbing against them at every stop. (Want more evidence? Look at NASCAR or F1. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it.) The one glaring exception here is in the rare situation where the rotors are so oversized (look at any performance motorcycle or lighter formula car) that the rotors are drilled like Swiss cheese. While the issues of stress risers and brake pad wear are still present, drilling is used to reduce the mass of the parts in spite of these concerns. Remember – nothing comes for free. If these teams switched to non-drilled rotors, they would see lower operating temperatures and longer brake pad life – at the expense of higher weight. It’s all about trade-offs. From Stoptech: Quote: Which is better, slotted or drilled rotors? StopTech provides rotors slotted, drilled or plain. For most performance applications slotted is the preferred choice. Slotting helps wipe away debris from between the pad and rotor as well as increasing the "bite" characteristics of the pad. A drilled rotor provides the same type of benefit, but is more susceptible to cracking under severe usage. Many customers prefer the look of a drilled rotor and for street and occasional light duty track use they will work fine. For more severe applications, we recommend slotted rotors. That almost sounds like an excuse to use cross drilled rotors, and for your street car which probably is never driven on the track, the drilled rotors are fine, but as Stoptech states, they will crack and are not good for severe applications. From Baer: Quote: "What are the benefits to Crossdrilling, Slotting, and Zinc-Washing my rotors? In years past, crossdrilling and/or Slotting the rotor for racing purposes was beneficial by providing a way to expel the gasses created when the bonding agents employed to manufacture the pads...However, with today’s race pad technology, ‘outgassing’ is no longer much of a concern...Slotted surfaces are what Baer recommends for track only use. Slotted only rotors are offered as an option for any of Baer’s offerings." Then from Grassroots Motorsports: Quote: "Crossdrilling your rotors might look neat, but what is it really doing for you? Well, unless your car is using brake pads from the '40s and 50s, not a whole lot. Rotors were first drilled because early brake pad materials gave off gasses when heated to racing temperatures, a process known as "gassing out." ...It was an effective solution, but today's friction materials do not exhibit the some gassing out phenomenon as the early pads. Contrary to popular belief, they don't lower temperatures. (In fact, by removing weight from the rotor, they can actually cause temperatures to increase a little.) These holes create stress risers that allow the rotor to crack sooner, and make a mess of brake pads--sort of like a cheese grater rubbing against them at every stop. Want more evidence? Look at NASCAR or F1. You would think that if drilling holes in the rotor was the hot ticket, these teams would be doing it...Slotting rotors, on the other hand, might be a consideration if your sanctioning body allows for it. Cutting thin slots across the face of the rotor can actually help to clean the face of the brake pads over time, helping to reduce the glazing often found during high-speed use which can lower the coefficient of friction. While there may still be a small concern over creating stress risers in the face of the rotor, if the slots are shallow and cut properly, the trade-off appears to be worth the risk. (Have you looked at a NASCAR rotor lately?) This ad is not endorsed by this member. Please register or login to hide this ad.

11-02-2006, 07:30 AM	#3
stretch Silver Member Car: Aspen 2005 STi Fav Mod: TurnInConcepts.com Coilovers Location: Silver Spring, MD Posts: 1,780 Join Date: Mar 2006 Trader Rating: (2)	Here's one of the most interesting thread on the subject, ever. Todd@TCE offered a $200 bounty to anyone that could prove that a cast rotor with holes exists- this argument came about after lots of Internet "experts" saying that they do, and that they would not fail the way drilled rotors do. Note that Todd@TCE sells slotted rotors just like any rotor dealer, but he'll get you blanks on request. http://corner-carvers.com/forums/showthread.php?t=21273 I don't know anyone with racing experience and an education that uses anything other than blanks- the exception are those sponsored by bling-bling brake companies. That's not to say people don't race on slotted and drilled rotors, but the guys who race often almost always use blanks (and they'll have spares, too). Take my anecdotal experience for what it's worth, which isn't much, but I'm rather certain most brake vendors worth his/her weight will recommend blanks to you for track duty. Drilled big brake kits (show car brake kits) work on some race cars because they're so oversized that they still get by. This is the case on sportbike rotors, too. Racing teams with these big brake kits often change their rotors after each race at the sponsor's expense. OEM-sized drilled rotors are downright unsafe for racing, towing, or even summer stop-and-go and sometimes come with a disclaimer warning of just that. Slots are still a stress point, and I've seen rotors with stress cracks originating from the slots. Note that this rotor is cracking everywhere (even a blank rotor can do this), but it is clearly at its worst where the slot is: Cross-drilled rotor failures are very, very easy to find. These pictures were taken after just TWO 20 minutes track sessions: Last edited by stretch : 11-02-2006 at 08:16 AM.

11-02-2006, 07:16 AM	#2
*wdb* dances with roads Posts: 4,957 Join Date: Apr 2005 Trader Rating: (8)	Nice work.

11-02-2006, 07:34 AM	#4
flatthump Enginerd Car: The Spaze Mobile Fav Mod: Tuning Location: South O.C., Ca Posts: 12,757 Join Date: Jan 2006 Trader Rating: (5)	^^those got a little hot..lol. When I was in an Auto-tech program, my favorite teacher spent two days talking about slotted and cross-drilled brakes and the waste of money they are on a track...

11-02-2006, 07:48 AM	#5
*BoostJunkie* Sold My Soul For Mods Car: '05 CGM STi Fav Mod: Track Time Location: Bethesda, MD Posts: 6,441 Join Date: Jun 2004 Trader Rating: (12)	I think dimpled rotors should also be thrown in here. Performance Friction uses dimples on most of their 2-piece rotors, and their products are widely used in various racing series. Any thoughts?

11-02-2006, 08:12 AM	#7
Darkiller Spec C Club Car: 2006 STI BLUE Fav Mod: Fp20g and protune Location: 1000 oaks, SoCA Posts: 1,499 Join Date: Dec 2005 Trader Rating: (0)	make this a sticky

11-02-2006, 08:23 AM	#8
Rmiller STI Driver Car: 06 STI Fav Mod: Stock! Posts: 261 Join Date: Feb 2006 Trader Rating: (0)	I disagree that drilling would result in a decrease in cooling capacity. It would result in a decrease in heat capacity due to the decrease in material, but drilled rotors will have greater surface area and thus greater capacity to cool.

11-02-2006, 12:05 PM	#13
Turninconcepts.com Authorized Vendor Posts: 4,691 Join Date: Jul 2003 Trader Rating: (50)	MR. DARKILLER!!!!! I LOVE YOU!!!!! Thank you thank you thank you!!!