ghoffman

This is for the engineering geeks out there like me! Try this little calculator that lets you know the stress concentration factors of a plate with holes. Put in the radius of the hole, the distance to the hole from another hole or edge and see how much faster the holes will make your rotors fail. We all know all rotors will fail, it is a matter of when and this will give you a factor of 2-4x less life before cracking. BTW, slots do a similar concentration factor. This is especially true in a brittle material like cast iron. Use the case of an infinite plate with 2 circular holes. For the case of 2mm radius holes 25mm apart, the stress is 2.91 times the "usual" rotor stress. Or to put it another way, you willl crack 2.9 times sooner.
http://www.fatiguecalculator.com/finders/findkt.htm

Besides, the coeffcient of friction of a brake pad on air is very low as compared to cast iron.

wtknght1

RogueVette

I have been trying to convince people of this for years.

They usually say well if it wasn't the best why does Porsche use them...

gonzalezfj

Rogue you took the words right out of my mouth (well, keyboard, maybe).

As a retired mechanical engineer, I have posted many times on the degradation of cast iron around holes that act as stress concentration points.

As you said, Porsche brakes are usually brought up as a counter-argument. Fact is Porsche rotors crack just the same as any other "drilled" rotor. Some manufacturers cast their holes in place instead of drilling them. Cast holes are a little better, but still have all the disadvantages of holes.

Frank Gonzalez

joe@zip

As far as cast holes, I don't believe that anyone can prove that full holes are actually cast into rotors. In fact, if you can prove this, there is a reward... http://www.corner-carvers.com/forums...ad.php?t=21273

ghoffman

The only reason that that matters anyway is that the grain flow around the hole would be better, but stress concentration factors still exist regardless of grain flow. In aircraft structures, whenever you put a hole in a structure, you have to make up for it with a "doubler" around the hole, one way or another.

Gary2KC5

I agree with everything said and I do not have the expertice you gents have but...

my stock C6Z06 rotors last 3X longer than stock C5 rotors...even with the non-direction (disfunctional) side.

maybe it's because they are .25" thicker, but heads up I've no complaints about the drilled rotors (which I thought I would never say). It's the pad wear that I've got a beef with.

BTW: I have tried the RB 2p rotors and they are much better than the stock peices but I only got to use them for about 5 laps (before being rained out) on the full course at Autobahn CC. still in the 5 laps (at about 7/10's learning the south part) it groved the friction rings (with the ET900 pads). 1st time I've seen pads wear rotors more than the pads

Gary2KC5

BTW: I don't think anyone thinks holes in the rotors are good...they just aren't as bad as we thought.

Allthrottleandsomebottle

Hehehe.....I have been around engineering/casting/warranty for a while......and Porsche Eng says with some truth:
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.

Don't get me wrong...........my NAPAs work fine as is, so if it ain't broke,well, you know the rest

ghoffman

Actually, the smaller holes do not help, they make the stress concentration factor larger! Run the model, use say 25mm for the hole spacing and try different hole radii and you will find that the smaller the hole, the bigger the stress concentration factor! This is logical, just as increasing the radius of a crankshaft journal fillet reduces cracks in the corner. The smaller the radius, the more it approaches a sharp notch.

AU N EGL

John Shiels

still drilling mine tonight! 1/2" or 3/4"?

Mojave

On Corner-Carvers, they are offering a reward to anyone who can prove the holes are cast in. So far, no one has produced any evidence that this is true. Do you have any pics of a rough-cast rotor with the holes in it?

JohnD60

I thought I wasn't running my car that hard at the DE's and was tired of the rusty NAPA rotors so I went to something drilled and sloted. Four 20-minute sessions later:



I'm painting NAPA rotors now....

C5 XTC

Except if you have Z51 rotors it is impossible to find replacements that are not drilled (at least I cannot find any). Have any suggestions? C5 XTC

Mojave

Well, you could buy C5 or Base C6 PAB's (pad brackets) and run the smaller C5/base C6 rotors. Then you can use the $20 napa rotors.

Wicked Weasel

that calculator seems like a lot of work to prove something I already know

AU N EGL

Yah I think we all agree on that one. But there are some very annal ppl ( many europeansand one BBK manufature in particular) who just dont get it.

Allthrottleandsomebottle

Hum..........I will look into the cast vs. drilled in all my spare time
Food for thought.........the book and cals are good, but time and time again, nothing beats real world testing.
I used to also work in the heavy duty off road world and dealt with 70 lb disc brake rotors yep, 70!. And for the record.............NO STINKIN slots or holes, just a nice blanchard grind

Allthrottleandsomebottle

More reading...........from Centric Parts( Stoptech is the performance engineering and manufacturing division of them )
Cracking
Cracking is primarily due to heat cycling that weakens the cast iron discs. The exact mechanism of this failure is disputed. Cast iron discs are formed with the excess carbon being precipitated in the form of carbon plates or flakes dispersed throughout the ferrite (iron) matrix. What is believed to happen is that when discs are operated above about 900º F, the carbon becomes more flexible or "fluid" in its shape partly due to the thermal expansion of the enclosing ferrite matrix. Then, as the disc cools relatively rapidly back below about 900º F the carbon is trapped in a changed more random shape then when it was first cast. This creates internal stress on the part and continuously transforms the disc by relieving the stress through the cracking. The cracks begin by appearing between carbon flakes. Nodular or ductile iron would resist this cracking due to the excess carbon being precipitated in a spheroidal form, but it, like other alternative materials do not have the mechanical properties needed to function ideally in a brake disc application. In discs that are cast to resist cracking through chemistry and controlled cooling at the foundry, cracking will still occur, but more slowly and take the form of heat checks on the surface. In some cases cracks will begin at the periphery of the disc and propagate inwards. In this situation, propagation can be delayed by drilling small holes at the end of the cracks (stop drilling). We do not recommend this however, because if the cracks continue to propagate unnoticed, catastrophic mechanical failure will result. Replace disc at the first sign of cracks at the outer edge of any size. A historic note, the original purpose of the curved or angled vane disc was to prevent cracks from propagating by imposing a solid vane in the path of the crack. The cooling function was secondary.
Cryogenic treatment : A thermal process in which metallic components are slowly cooled to near Kelvin temperature and then equally slowly returned to room temperature. Proponents claim that the grain structure is refined by the process. There is considerable doubt about the effectiveness of the process. Evidence is largely anecdotal and to date no scientific and quantitated engineering studies have been published.


Drilled or cross-drilled rotors
Discs that have been drilled through with a non-intersecting pattern of radial holes. The objects are to provide a number of paths to get rid of the boundary layer of out gassed volatiles and incandescent particles of friction material and to increase "bite" through the provision of many leading edges. The advent of carbon metallic friction materials with their increased temperatures and thermal shock characteristics ended the day of the drilled disc in professional racing. They are still seen (mainly as cosmetic items) on motorbikes and some road going sports cars. Typically in original equipment road car applications these holes are cast then finished machined to provide the best possible conditions by which to resist cracking in use. But they will crack eventually under the circumstances described in another section (see Cracking). Properly designed, drilled discs tend to operate cooler than non-drilled ventilated discs of the same design due the higher flow rates through the vents from the supplemental inlets and increased surface area in the hole. That's right, inlets. The flow is into the hole and out through the vent to the OD of the disc. If discs are to be drilled, the external edges of the holes must be chamfered (or, better yet, radiused) and should also be peened.

Darrick Dong; Director of Motorsports at Performance Friction: "Anyone that tells you that drilling makes the disc run cooler is smoking crack."

Power Slot: "At one time the conventional wisdom in racing circles was to cross-drill brake rotors to aid cooling and eliminate the gas emitted by brake pads. However, today’s elite teams in open wheel, Indy and Trans Am racing are moving away from crack prone, cross-drilled brake rotors in favor of rotors modified with a fatigue resistant slotting process."

Stop Tech: "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." (Note that even though Stop Tech sells both drilled and slotted rotors they do not recommend drilled rotors for severe applications.)

Wilwood: "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."

From Waren Gilliand: (Warren Gilliland is a well-known brake engineer in the racing industry and has more than 32 years experience in custom designing brake systems ...he became the main source for improving the brake systems on a variety of different race vehicles from midgets to Nascar Winston Cup cars.) "If you cross drill one of these vented rotors, you are creating a stress riser that will encourage the rotor to crack right through the hole. Many of the rotors available in the aftermarket are nothing more than inexpensive offshore manufactured stock replacement rotors, cross drilled to appeal to the performance market. They are not performance rotors and will have a corresponding high failure rate"

From Baer: "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."

Grassroots Motorsports: "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?)

AP Racing: "Grooves improve 'cleaning' of the pad surfaces and result in a more consistent brake performance. Grooved discs have a longer life than cross-drilled discs."

also from AP: "Cross drilled...can compromise disc life. Radiused drilled...mainly used for aesthetic reasons on road applications."