Crossdrilled rotors part 3!!!

[yody]

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.

[rocketrod]

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]

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.

[rocketrod]

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 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]

Gotta throw down some half ass tech now and then.

[jjump59]

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]

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]

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]

okay, they don't cool down, i was just repeating wha tthe reps told me.

[David Pozzi]

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]

If it helped to cool, you would see a lot of flywheels/press. plates with holes all over them, i think.

[paul67]

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]

huh?

[yody]

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]

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]

WEll said!

[TitoJones]

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]

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!!!

[indyjps]

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]

Thats what I'm talking about.