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View Full Version : Crossdrilled rotors part 3!!!



yody
11-08-2004, 11:15 PM
Well after attending the Sema show I have some new info(at least to me it was new) This is not for debate and i am sure people can prove some of these points wrong, and i don't intend to make this a 3 page post, just wanted to share what the vendors told me;
I talked to Baer brakes, Brembo Brakes, and 2 other small companies.
I would have to say speaking with 2 reps at Baer was a complete waste of time, they could only repeat the same useless info over and over, and obviously didn't know what they were talking about, and it was real easy to catch them in their ignorance. They proved the only thing they are good for is putting "kits" together(which they ARE good at) Basically what i came away with from the other companies was similar which was this;
A long time ago hot rodders on the salt flats drilled there rotors for a weight reduction. Then, they also helped vent the gases from the rotors/pads. Now degassing still takes place but not nearly as bad with the metallic pads. Where the holes/slots help is
1. the holes/slots gives the pad something to "bite" in to (questionable theory)
2. less mass equals faster cooling, so your rotors will last a little bit longer on the street if they can cool down faster between braking cycles. and perform better repeatedly
3. the holes will help get rid of the gases if there are any at all
4. the slots help sling out any built up brake dust/material(also questionable)
5. weight reduction
NOW for the bad part!
1. holes can crack at extreme temps and most likely will (but so do regular rotors)
2. You reduce the mass/area when drilling the rotor, so you lose braking pad area which isn't ideal. this is probably the biggest problem, if you are road racing you want all the braking surface possible.
so for the street they work good, but if you are going to be putting them to extremes, they might not be the best choice.
Now obviously some people knew these things and so forth. I am not putting this out their to be debated but feel free. This is just what I was told by the reps at the show. But in the long run, I don't think many of us here will see the difference between the 2. If you are building an extreme car that will be mostly track, holes/slots probably aren't for you, but if you are street, and occasional race maybe a few times a year, and you like the look of the holes/slots, go for it!
Now if I can remember/figuer out why race cars use slots? The brembo guy said it was for weight reduction, but i don't think they are losing much weight. I guess it will just be a black art for some of us. I am pretty happy with the info i came away with, even it isn't totally right, its good enough for me.:wtlw::wtlw::wtlw::wtlw::wtlw::wtlw::wtlw:

rocketrod
11-08-2004, 11:37 PM
FYI - This is from Wilwood's FAQs page

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.


Rod

dennis68
11-08-2004, 11:39 PM
I had to be the 1st to reply to this. I'm not sure if are coming around or still lost. I was almost with you until the heat issue came up. Drilling holes does NOT cool rotors off faster, it actually hinders the cooling prcoess as the holes "interupt" molecular compostion of the metal.

Glad that you have come to a compromise though.

Damn you Rod. :ripped: :ripped:

rocketrod
11-08-2004, 11:58 PM
Denny,
I agree with your earlier post....

Drilling holes does NOT cool rotors off faster, it actually hinders the cooling prcoess as the holes "interupt" molecular compostion of the metal.

but its to late :sleeping: :sleeping: to be using terms like "interupt" molecular compostion of the metal. Can't we just say crossdrilling reduces the surface area of the rotor and its ability to absorb heat.


Rod

dennis68
11-09-2004, 07:47 AM
Gotta throw down some half ass tech now and then.

jjump59
11-09-2004, 07:48 AM
We run a dirt car and have the drilled rotors for weight reduction only, never gave any thought to any of the other reasons. Less rotating weight gets off the corners quicker. I know most of you will say it's not enough weight to make a difference but even a 1 pound reduction on five different parts nets you 5 lbs. 5 lbs of rotating weight is a huge deal. When it comes to hubs, rotors, axles, ring gears, spools, driveshaft, wheels and tires we run the lightest stuff we can find that will last.

baz67
11-09-2004, 06:17 PM
Someone more educated then me explain the second law of thermodynamics to Cody. It does not cool faster whey you take mass away from the heatsink(rotor). Air is an insulator. That is why down keeps you warm in the winter.
Brian

baz67
11-09-2004, 06:20 PM
One more note. I tried to look at the rotors on the real race cars at SEMA and they only had slotted rotors. Not one had any holes in them.

yody
11-09-2004, 09:56 PM
okay, they don't cool down, i was just repeating wha tthe reps told me.

David Pozzi
11-09-2004, 11:51 PM
It stands to reason that if you drill the rotor the mass is reduced, less mass will heat up more when you brake, less mass will cool quicker whether the holes aid the cooling or not.

The result would be higher stress on the rotor due to the higher heat cycles and more stress risers to promote cracking.

If the rotor is way oversize to the application, the holes won't matter, if the rotor is pretty highly stressed to start with, drilling holes in it would shorten it's life.

I went to the American Lemans races at Laguna Seca a while back, there was only one car with holes in the front rotors and I looked at a LOT of cars. All the rest had none, including the Ferrari.

kball
11-10-2004, 12:20 AM
If it helped to cool, you would see a lot of flywheels/press. plates with holes all over them, i think.

paul67
11-18-2004, 05:22 AM
I live in England and we drill the holes in the rotors to stop water build up on the face of the rotors whether sloted or not this was so the water had somewhere to go.

dennis68
11-18-2004, 06:50 AM
huh?

yody
11-18-2004, 07:45 AM
hey guys, another car that uses crossdrilled rotors is "big red" which is a car that many people here idolize. whats up with that, i thought that was the cats meow, super race car. hmmmmmmm.....

Fuelie Fan
11-18-2004, 09:35 AM
Mass is critical for thermal capacity: Q=m*c*dT
Assuming initial temps of ambient and final temps such that thermal stresses do not exceed maximum, and cp a property of the material, energy absorbtion is directly proportional to mass. (PS show me a document where the cp of a material changes appreciably when you drill holes in it. I would put the molecular integrity argument, as it relates to heat transfer, pretty low on the list of concerns behind mass change, area change, and airflow change)

How FAST this temperature changes is a function not only of how much heat (Q) you are trying to dissipate, but also the method you are using to dissipate it with. Surface area and air speeds are very important factors. Therefore you cannot assume that a more massive object always cool slower. You may add mass to an object by adding fins (think heat sink) and have it cool FASTER as a result.

Air is an insulator? Where do you think the energy is going to in the first place? The mythical ether? So if we put ducts pointing air to our rotors, do we do a better job of insulating them?? CONVECTIVE HEAT TRANSFER.

Now, it may be true that the airflow through the holes is not very efficient, and that it may in fact be better to have solid metal there and rely on transfer from the outer surface. BUT, this is not a slam dunk argument, certainly not one you can throw out into the world supported only by the thread count of your sheets. We don't know what the airflow speed through the holes is. Depending on rotor thickness you may have increased OR decreased surface area. These are unknowns.

I would rather hear NO tech than half-ass tech, personally.

yody
11-18-2004, 10:22 AM
WEll said!

TitoJones
11-18-2004, 12:27 PM
Yody-

Bid Red had the crossdrilled/slotted rotors for promo purposes (ie SEMA show) only. On race day, solid rotors are busted out.
Tyler

harshman
11-18-2004, 12:51 PM
Yody-

Bid Red had the crossdrilled/slotted rotors for promo purposes (ie SEMA show) only. On race day, solid rotors are busted out.
Tyler
ohhhhhhhhhhhh! SNAP!!! :seizure:

indyjps
11-18-2004, 02:33 PM
while we're on the heat dissipation issue wouldn't the drilled holes cause the rotor cool @ different rates across the face creating "heat risers" or hot spots?

dennis68
11-18-2004, 03:07 PM
Thats what I'm talking about. :bananna2:

protour_chevelle
11-18-2004, 03:29 PM
1. the holes/slots gives the pad something to "bite" in to (questionable theory)

I'll agree with that. Theres no question in pad life. Compare one pad on a solid rotor and the same pad(but new) on a drilled/cross drilled rotor. Its obvious which one is going to wear quicker. Reason? More bite. Now the real question is... To have bite or to have more surface area?

-Matt

CoryM
11-18-2004, 03:38 PM
My father (industrial heat transfer specialist) said about the same thing as FuelieFan. There are too many variables for each specific application to easily decide what would be better. Would take a lot of R&D to figure it out. Has no one out there EVER tried both slotted, and non-slotted on the same car, on the same track? Having seen a drilled rotor come apart on a rally car and cause a nasty crash I will never run them personally. BTW I have seen drilled rotors on a lot more rally cars than road race cars.
Cheers.

protour_chevelle
11-18-2004, 04:03 PM
BTW I have seen drilled rotors on a lot more rally cars than road race cars.
Cheers.

Thats very weird. The elements that a rally car faces would almost make you want to go with a solid rotor. But then again. After how many stages does the car go in for a complete over haul?

-Matt

CoryM
11-18-2004, 05:23 PM
Depends on what rally it is. WRC or similar you usually see about 50-70km between services. I am not sure what the rules in the services are but I would guess you are allowed to replace braking parts at any service. I think its done by a time limit rather than what you can replace. The rally cars rotors glow red very early compared to a road race rotor. Must be some major heat produced very quickly. Makes sense I suppose with less mass and more edges on a drilled rotor. They also appear very thin compared to an endurance type rotor so I would guess rotor life is not a concern for them. Some one here must be more of a rally fan than me and be able to clarify the rules.
Cheers.
CoryM

chicane67
11-21-2004, 01:50 AM
Yody~

Shut up. Your girlfriend can drink more than you and you want me to believe this new cross drilled theory as a law??

SEMA was a blast. You gotta come out for an "unedited" weekend.......

As for the new rotor theory, I see your question as a realistic quandry. But I dont believe anything has really changed in the true "emperical" data of this question. Yeah, everybody had a clueless answer to what you had asked them, but they are still in it for the allmighty buck.

The only questioned truism in this thread is the FACT that in Europe the common application for a drilled rotor is for the "wet" invironment applications. But.......the limitations for them is still limited to good PM. If you own it, maintain it.

jjump59
11-21-2004, 07:26 AM
saw an S55 the other day, you know the one built by AMG with the 469 horse engine. Drilled rotors, it was factory acording to the owner. i think that was my first oem sighting. are there others?

baz67
11-21-2004, 08:51 AM
Yes there are others. This is the marketing guys over the engineering guys.
Brian

zbugger
11-21-2004, 11:21 AM
Cory, how many times do you see the Rally drivers running throught puddles? Has anyone here ever had their brakes hydroplane? I know for a fact that it's not fun. My brakes are out in the open, with my front runners, and it's happened to me. In this situation, the holes give a place for the water to go so the pad can make contact with the rotor. If I got drilled rotors, it would be for looks. As my car will be my daily driver, I want pad life to be long. Solid rotors for me.

CoryM
11-21-2004, 11:49 AM
Would have to see what rotors the rally cars use on paved rallys. I will look next time Im at a rally or watching on TV. I definatly have lost brakes due to water before. Last year I drove a 79 flatdeck with 2000lbs on back through a tirewash (20ft long, 16" deep puddle to wash the mud off truck wheels leaving job site) Left the job site and hit the brakes to turn a tight corner out of the alleyway and suprise! no brakes. Luckily I was able to gear it down hard enough to get slowed down. I had to heat the brakes for about a mile before getting brakes back. Thinking back to last time I was at a sprint car race, they all use drilled or star shaped rotors (aluminum or titanium). Then again, they dont use the brakes while racing ;) .
Cheers.

Johnjan
12-10-2004, 10:10 AM
I got drilled rotors cuz they look kewl. The rest of the arguments pro and con are academic.

baz67
12-11-2004, 12:14 PM
I hope you are not serious. Us adults spell it, because and cool.
Brian

TonyL
12-11-2004, 01:05 PM
play nice. we dont want to reduce a technical thread to a insult war...

Q ship
12-11-2004, 08:47 PM
I just read this today at the FRRAX forum, the quote is from Bob Bishop. I feel it is a well thought out response to this question.
Any depression or void in the friction area surface will provide a place for water to more rapidly be swept away. But, normal braking pressure and heat will dry the rotor surface very quickly. Depending on your speed, the slots/holes could save you a couple of feet to many feet. But remember, if your rotors are wet, the road surface is probably wet also. In this case, you cannot use maximum braking force anyway (whether or not you have ABS). But, in a laboratory controlled environment with wet rotors and pads, I believe that I could demonstrate greater deceleration for the first ˝ second of brake application.

Certain pad compounds at certain temperatures (and pressures) will tend to have an excess amount of their spent friction material fuse to the rotor surface, but not evenly. This will feel like a warped rotor once it has built up to a certain thickness variation. Slotted rotors can help to minimize this.

Certain pad compounds at certain temperatures will tend to “glaze” over. This greatly reduces their CF and produces brake fade. Slotted rotors usually eliminate this. Hawk has stated that their compounds, when used within their recommended heat range, do not exhibit either characteristic, and therefore do NOT benefit from slotted rotors. They also stated that slotted rotors will wear their pads out MUCH faster; think of knife blades scraping across the face of the pads. They recommended that slotted rotor users limit the # of slots to 3 per rotor face to do the least harm to pad life.

The only time out-gassing should be a consideration would be when one is forced to use a pad WAY outside of its temperature range. The only scenario I can think of is using new (green) OEM type pads in very mountainous driving, or pulling a heavy trailer. If you are from a flat area and in street driving had never severely overheated this pad compound, you could do so in a very hostile environment. Modern pads which have been bedded-in have to be severly overheated to outgas. By the time outgassing would occur, the material would have already experienced temperature fade.

In true high-speed road racing where pad life (and rotor life) are not major considerations, PROPERLY slotted rotors are generally beneficial.

Cross drilled rotors only make sense where one MUST minimize rotor weight, does not have the budget to obtain optimally sized rotors for their application, and will not need repeatability. Drag racing comes to mind. I am not commenting on light weight vehicles such as motorcycles. Of course, you can always add enough weight to an x-drilled rotor design that you overcome the immediate problem of rotor cracking. This is not to say that you can obtain an equally strong x-drilled rotor for the weight. The x-drilled rotor will have to weigh more than the equivalent plain rotor to achieve equal strength. The heavier rotor will afford greater heat sinking, but if you need rotors with greater heat sinking, you could increase the weight of your plain rotor design and end up with a stronger piece. If you accept my premise, then you can see how one could always juggle the various parameters to achieve any one or any combination of: greater strength, greater heat sinking, and lower weight with plain rotors. This assumes that other variables remain constant, such as rotor material, cooling vane efficiency, etc.

This theoretical analysis does not attempt to comment on what rotors are commercially available for your application, or your intended usage. Compromises must be made with the ideal.

During the C6 press introduction days, one of the GM engineers acknowledged that the x-drilling of the Z-51 rotors was for cosmetic appeal only.

Original post. (http://frrax.com/rrforum/index.php?act=ST&f=2&t=3085&)

baz67
12-12-2004, 01:17 PM
Nice find Q ship! I like that last sentance.

I agree Tony, but the use proper grammer and spelling should be used. He bought drilled rotors because he thinks they look cool. That is fine as long as he is happy. Keep the immature stuff on the sites that accept it.

Brian

JMarsa
12-12-2004, 01:49 PM
I guess the question is why?

http://www.chevrolet.com/corvette/index.jsp?feature=corvette_z51

--JMarsa

teamsleep13
12-12-2004, 08:13 PM
If you slot your own rotors take Carroll Smith's suggesttion from "Engineer To Win" and don't radially slot them. Use a ball end mill as normal, but slot them from one side to the other across the face of the disc on either side of the disc hat. Make sure to keep the slot away from the I.D. of the disc too. It's easier to do, saves time and works just the same.

Carroll doesn't totally oppose the drilling of the rotors, as long as they are done properly(Brembo pattern), watched for stress cracks and when found, thrown away outright. He doesn't think they do much for performance, but they do lighten up the unsprung weight(which helps alot with the small Formula Ford type cars he raced alot)

AP racing has been promoting their 'J-hook' rotors for NASCAR use, and claim that they will work better than the normal slotted rotors. I haven't used them but AP racing is one of the leaders in brake technology and I have a certain trust in them.

I am just waiting for the carbon ceramic material to be sorted out. Mercedes brakes seem to be the best(most durable and most stopping power) for carbon ceramic, then Porsche and then Ferrari. Once the details of production and realiability get nailed down, we should be able to get them from companies like Brembo, AP racing and Alcon at a price that will be higher than iron rotors, but will most likely never had to be replaced if abuse isnt severe. Hopefully it is sooner rather than later.

Thats my take, ciao.

Hunter

CAMAROBOY69
05-26-2005, 10:25 AM
At Gingerman raceway on Tuesday, one of the only bad things I saw happen was on one of the Mustangs there. He ended up cracking his rotor in 3 or 4 different places. He had drilled rotors with Baer brake pads. He was yelling over and over again at everyone. "This is why you DONT buy drilled rotors!!!"
I said "ok you sure have convinced me" :lmao:

68protouring454
05-26-2005, 10:53 AM
adam i wonder why he bought them in the first place?? dumbazz
jake

CAMAROBOY69
05-26-2005, 10:58 AM
I dont know that I would call him that. He was a pretty cool guy and probably thought he could go a bit longer before damage happened. I really had no idea either that it could happen that fast. He only went out a few times. He really didnt feel like talking much after that so I didnt ask many questions.
Last I knew he and his friends were going to try to get new disks installed in time before the enent ended. That was 2 hours before it ended and we were already leaving. Not sure if he ever got it going again. Last I heard him say was that the rotors he was about to buy were $220.00 a piece up front.

68protouring454
05-26-2005, 12:44 PM
makes no sense if he knew he would ruin them on the track no matter when it happend first time or not, especially when you say the rotors are that much money, some people
jake

Pro-touring towncar
06-02-2005, 06:34 PM
very informitive but I have a question. Solid as in completely solid or solid vented simalure to OEM?
I am wanting to do a big brake conversion on the towncar but wonder if I should use the std rotors or powerslots.

Tim

baz67
06-02-2005, 08:34 PM
Tim,

Solid means vented in this case. No holes or slots put into the rotors. You need vented rotors. I do not even know they make solid non vented rotors for the applications we use.

zman1969
07-29-2005, 02:52 PM
heck just drive through deep water to cool them down! :poke:

Supercharged 86
07-29-2005, 03:29 PM
I'm coming into this late so forgive if this was mentioned before, but in racing applications drilled rotors often crack. If done properly the holed should actually be cast into the rotor as opposed to drilled as the name implies. For street use it doesn't matter though. And also for street use I think drilling is just cosmetics and reduces the surface area more than the holes can make up for most of the time. I do like slots though :icon996: And they now make slotted rotors with the holes drilled inside the slots them selves which could prevent many of the problems. They don't look good nice cosmetically though.