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Old 3 Aug 2007, 21:33 (Ref:1980117)   #31
AU N EGL
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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).

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."



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Old 4 Aug 2007, 01:22 (Ref:1980198)   #32
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I didn't need a PHD in brake discs, my post 20 said it in a lot less words.
I'll have to widen the door apature to get out of the room now !!!!!!
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Old 4 Aug 2007, 02:47 (Ref:1980233)   #33
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Originally Posted by GORDON STREETER
I didn't need a PHD in brake discs, my post 20 said it in a lot less words.
I'll have to widen the door apature to get out of the room now !!!!!!
Yes you did. but unfortnity some ppl did not understand your condenced statement.

It is always nice to read on the Brake manufactures pages, hidden as they many be, that Drilled or milled or cast in holes in rotor are for show, not performance and racing.


PHD = Phony Hole Disks.

I still like the one, "What is the braking co-efficent of a Rotor Hole?"
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Old 4 Aug 2007, 08:50 (Ref:1980295)   #34
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Or even my post - 18!

But I was quoting someone else - Gordon's was direct experience.


Turn it around another way - the pads for my four pot calipers have one radial groove across, that I presume is to let hot gases out. Less funky, more effective?
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Old 5 Aug 2007, 07:38 (Ref:1981210)   #35
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Listen to Gordon: the voice of age and experience! (Sorry Gordon, but true!).

Basics: Old engineer's saying: "When you have two cats sitting on the roof, which one falls off first? The cat with the smallest Mu" or if you like, coeiffient of friction.

Friction is not dependant on Surface Area: it is dependant on the interrelationship between the two materials and their effective mass.

Thus with a given Mu, what varies in a braking system is Pressure!

So why do larger tyres promote more adhesion? Why do larger disks/drums and pads/linings promote greater retardation?

Heat Dispersal!

Additionally, with tyres, a larger surface area allows a larger "Footprint" which allows less "Tread Shuffle", which allows greater faster heat dispersal. Heat in tyres is generated by the cords rubbing and twisting. less tread distortion, less heat.

If you change brake pads to a far "harder" pad, predal pressures increase, rapidly. Obvious.

Old pads used to suffer from "fade": a few hard applications and they burnt out: fun when approaching corners much rapidly!

Larger pad area and a greater friction coefficient means less "fade". It also means greater pedal pressure.

Small disks and small pads would be fine, if you can arrange to dissipate the heat quickly enough, which is usually pretty hard. Too much heat means spot hardening (very much so with cast iron) and grabbing and eventually failure.

Ventilated disks obviously assist in throwing off heat: but also incude inherrant weakness due to different rates of metal hardening and thus earlier failure. Same with cross-drilling.

Early experiences with my sparkling new Ford GT Cortina in 1966, running cross-ply tyres - which were standard in those days! - showed some of the problems!

In the wet, one disk would throw the surface water more quickly: leading to sudden grabbing! Add with aquaplaning from the captive layer of water at the tyre's leading edge, thereafter, all sorts of jollys took place!

The cure (Service Fix; oh dear how Henry loved these 'cos they were cheap!), was to cut two cross-grooves in the pads with two hacksaw blades held together in the saw frame!

Also, shifting the calipers to the back of the strut helped even more. (Ford had switched caliper position from the back to the front a la Lotus Cortina to aid cooling).

If you want greater cooling, then fit larger diameter disks; and thicker disks; and bigger calipers!

Don't drill!
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Old 5 Aug 2007, 11:32 (Ref:1981356)   #36
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I am not pro or anti holes, but we run AP discs which just have grooves.

However, if holes are so bad why would Porsche continue to use them? They aren't dummies after all.

Only considered opinions, not diatribes, sought as replies please.............
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Old 5 Aug 2007, 11:44 (Ref:1981372)   #37
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Probably, since they design the disk from scratch to use holes, use superior manufacturing and design methodology and finally material?

"Improving" any part has always been a temptation: often fraught with problems and sometimes disaster.
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Old 5 Aug 2007, 12:40 (Ref:1981418)   #38
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Originally Posted by Michael C Felth
Probably, since they design the disk from scratch to use holes, use superior manufacturing and design methodology and finally material?

"Improving" any part has always been a temptation: often fraught with problems and sometimes disaster.
What was that old joke? "Porsche 911, A poor design, over engineered to Perfection."

Just Kidding people. Porsches are great cars, Lots of fun to drive and race against.

Why do companies put holes is rotors? The customer wants them. Any engineer will show, and have in this thread, that holes are streesers and really do not help cooling.
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Old 5 Aug 2007, 15:56 (Ref:1981627)   #39
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Why do companies put holes is rotors? The customer wants them.
I don't thnik that holds for Porsche - and they use them on (some at least) works race cars too where there is no customer.
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Old 5 Aug 2007, 17:26 (Ref:1981728)   #40
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I don't thnik that holds for Porsche - and they use them on (some at least) works race cars too where there is no customer.
Porsche has works cars? I knew they have drivers on salary.

I though all the porsche race cars were customer cars? Some with a little more Factory help then others.

and if it is a 'works' car, they can afford to replace their rotors every race.

If they do that is great, maybe they now know something that almost evey other racing brake manufacture has moved away from except for marketing purposes.
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Old 5 Aug 2007, 17:48 (Ref:1981748)   #41
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The thing with Porsche works cars is that they would probably have new rotors fitted before they have a chance to crack. It's hardly like your average clubie who would run the same rotors for a few seasons if they can.
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Old 5 Aug 2007, 18:44 (Ref:1981782)   #42
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Back in 1973, I was at Silverstone for the Martini Trophy; six hours.

Porsche dominated to saythe least! (In the open sports-racers, turbocharged, forget the type number now).

Porsche monopolised the paddock and turned it into a sort of Porsche village!

The "Pit Stops" were merely a token. The three cars led from flag to flag.

I dread to think how much they spent on the cars, the transporters, the mobile workshops, drivers accommodation, team management van, etc.

With their budget chucking such minor parts after one race would be nothing!
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Old 5 Aug 2007, 18:44 (Ref:1981783)   #43
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The thing with Porsche works cars is that they would probably have new rotors fitted before they have a chance to crack. It's hardly like your average clubie who would run the same rotors for a few seasons if they can.
Ok - agreed.

But just because Porsche can afford to fit new discs every race/practice session/test - or whenever - is surely not a good enough reason on it's own.

There must be some competitive advantage which out-weighs the disadvantages, otherwise they wouldn't use them - surely?????????
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Old 5 Aug 2007, 19:45 (Ref:1981815)   #44
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There must be some competitive advantage which out-weighs the disadvantages, otherwise they wouldn't use them - surely?????????
If they thought like that they'd move the engine
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Old 5 Aug 2007, 20:43 (Ref:1981867)   #45
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Porsche has works cars?
As far as I am aware, yes, at least on this side of the atlantic, though not permanent from year to year.
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