Thinking About Drilled Rotors?

[rcwaldo]

Gas slotted rotors are what CART and Nascar are using. For big ovals (Michigan, Fontana, Motegi) CART allows its teams to use carbon rotors and doed the IRL. Gas slotting not only, vents, but has a self cleaning feature as well. If I had a choice, I would use gas slotted rotors for DE's etc.

[Sniady]

From what I heard, Porsche started using drilled rotors, not to cool off the rotors, but the holes were used for brake dust to have a quciker escape route. I guess when you brake as high speeds you want your brake dust not to hang on the pads.

Again, thats what I heard. I guess it does make sense?

[GeorgeK]

OK, some trivia:
Porsche started drilling rotors for the one-off "bergspider", a car that weighed under 400 Kgs, that used berylium and titanium and unobtainium all over. They drilled the rotors only as aweight savings measure, but they were surprised that the car braked BETTER in wet conditions. That is how it all started (for Porsche at least).
Now I am still waiting to see a case of a *broken* drilled Porsche disc (not the result of some backyard company)
GeorgeK

[arrivederci]

Two things (all speculation on my part):

1) I've heard that porsche originally used cast rotors, but now they are all drilled (Zimmerman included).

2) My feeling is that the cast/drilled are another attempt to reduce unsprung weight, among other things. The brakes on 930s, 993s, etc are large enough, with enough mass, that they have the ability to dissipate the heat even with holes. The braking systems on the cars with drilled rotors are more than adaquate for the 911 in stock form (track use included).

Also realize that Porsche has used slotted rotors on the later model 928s...with big reds. That car is significantly heavier than the 911, therefore they -needed- the additional mass in the brakes to aide in cooling.

[Bill Verburg]

Here's the reason all rotors will crack if abused. Each app will have a different level at which this occurs.



When two bodies slide against each other, frictional heat is generated and the resulting thermoelastic deformation alters the contact pressure distribution, the data shown here is on a properly functioning system in ~4mins of normal street use

[Bob D]

Wow - how did you dvelop that!
It looks like you even modelled the position of the internal vanes.
Very cool - so to speak.

[slakjaw]

hi


i was under the impression the drilled rotors were drilled to let or help water escape the braking surface, not to help cool.

in fact i was told that non drilled rotors will stop better because there is more surface for the pad to contact.

is this true??

71 911
K

[masraum]

surface area doesn't really matter. Frictional forces don't take surface area into account, just the force and the coefficient of friction. larger pad area only helps under extreme conditions because it allows the pad to absorb more heat and so is more difficult to overheat.

[cegerer]

The 73 911RSR came with 'drilled' rotors, right out of the 917 parts bin. -- Curt

[Bill Verburg]

The image posted above is a high speed thermograph strobed to a normally functioning automobile rotor. It shows the development of localised hot spots due to thermoelastic deformation. They all do that.

Here are some other interesting shots. These are composites showing temperature averages in low and high g stops


These are stress patterns, again in low g stops


And high g stops


One more again strobed showing the effect of holes


No cracks were found after 200 high stops, none after 200 low g stops, but after only 50 combination of hi and low g stops cracking developed

[TimT]

Holy Schmoley Batman, I checked out the cost of that thermal analysius program from the Univ of Michigan....... $8000

Interesting stuff though on the thermal stress and deformation the discs see

[Bill Verburg]

Yeah I forgot to credit the work to J.R. Barber at the U of Michigan.

The other interesting fact is the the TEI hot spots are antisymmetrical, hot spot on one side w/ a corresponding cold spot on the other.

The same info also applies to clutches.

[Wil Ferch]

To Bill:
I respect your vast knowledge of Porsche brakes...but your assertion that cryo treatment is a "crock" is just plain false. I deal in the cryo industry as part of my work, and there is a metallurgical change that takes place for most types of metals when subjected to minus 300 degF temps. It usually changes the microscopic grain structure at the boundary level... and adds a degree of toughness to the host metal. The real trick is knowing how to / when to do this properly....and how much benefit to expect from the process.
---Wil Ferch

[Geary]

Hi Wil

The cryogenic debate will probably be around forever. I'm trying to keep an open mind, but this is what I've learned in the past year:

1) Some of the finest heat treaters in the world have moved AWAY from using the extremely low temperature (-100C to -300C) pre-tempering step in the heat treatment of high nickel alloys (typically used for automotive gears and such), and are now favoring mildly low temperatures of around -10C in place of the cryogenic step. Ultra-low temperatures were eliminated NOT because the heat treaters felt that the cryogenic specialists could later do a better freezing job (out-of-sync), but rather because there were no measureable differences in the final product.

2) Brembo spent thousands in the testing of cryogenically-treated brake rotors in attempts to improve the performance of their own products, and found absolutely no difference in performance / longevity of either rotors or pads.

We read the occasional post from someone claiming incredible benefits derived from cryogenics (so I try to keep an open mind), but until the benefits are scientifically demonstrated and explained, I gotta remain skeptical.

[RallyJon]

Hey Geary, what's your background/credentials? You write like you have some insider perspective here...

[Bill Verburg]

Will, You are of course entiteled to your opinion on the matter. I have based mine on extensive research and feedback from the manufacturers of brake components

This from Stoptech is typical

In simple terms. In the formation of pearlitic iron, during the cooling of the foundered part the metal passes thru a time temperature gradient that will form a certain amount of Austenite. Thru careful control of the cooling process the amount of Austenite will be negligible. Further cooling after passing the Austenite threshold forms primarily pearlite and some martensite. Cryogenic treatment converts austenite into martensite. Martensite is a more compact better organized structure and therefore stronger. Austensite is a face centered cubic structure and is weaker than matensite which is body center cubic structure. The iron rotor made by StopTech benefits very little from Cryo because of the negligible amount of Austenite present, but all cast iron can benefit from cryogenic treatment.

From Brembo

Lamellar graphite grey cast iron is the most common type and is used for the examination of production of discs and drums in braking systems. As can readily be observed in lamellar the micrograph analyses, the graphite is present in the form of small plates that structured grey seem like threads as they are only seen in section.
These various cast irons all have a rather similar composition and therefore the actions taken to obtain the different structures involve the additives and, above all, thermal treatment conditions during the processing and cooling stages. In order for the product to be considered good quality for disc production it must be homogeneous from the micrographic standpoint: thin plates of graphite dispersed in the perlite.

I do not dipute the fact that cryo treating changes the physical properties of the treated material, I do not believe(just my opinion mind you) that these changes are necessary or desirable for brake applications.

Among the physical changes purported to cryo treatment are increased hardness, is this a good thing where the C_f may be altered from a carefully engineered nominal initial value.

Another attribute is the stress relievement at Kelvin temps. What happens when the roptors are reheated to 4-900&deg temps during braking.

Admittedly cryo tratment is cheap and probably does no great or permanent harm and may be desireable in other applications(Jerico transmissions treats their gears cryonically). But the fact that none of the major manufacturers , Brembo, Stoptech, Alcon, Coleman, AP endorses the technology should tell everyone something(again just MO)

[beepbeep]

Originally posted by Geary
Hi Wil

The cryogenic debate will probably be around forever. I'm trying to keep an open mind, but this is what I've learned in the past year:

1) Some of the finest heat treaters in the world have moved AWAY from using the extremely low temperature (-100C to -300C) pre-tempering step in the heat treatment of high nickel alloys (typically used for automotive gears and such), and are now favoring mildly low temperatures of around -10C in place of the cryogenic step. Ultra-low temperatures were eliminated NOT because the heat treaters felt that the cryogenic specialists could later do a better freezing job (out-of-sync), but rather because there were no measureable differences in the final product.

2) Brembo spent thousands in the testing of cryogenically-treated brake rotors in attempts to improve the performance of their own products, and found absolutely no difference in performance / longevity of either rotors or pads.

We read the occasional post from someone claiming incredible benefits derived from cryogenics (so I try to keep an open mind), but until the benefits are scientifically demonstrated and explained, I gotta remain skeptical.

Very interesting info!

P.S. -300C must be kinda hard to achieve , being under zero Kelvin and all... ;-)

[Geary]

Very interesting info!

P.S. -300C must be kinda hard to achieve , being under zero Kelvin and all... ;-)

Oops ! Absolutely correct, Goran. That should have been -100F (dry ice treatment) to -300F (liquid nitrogen treatment). Only the -10C figure was correct.

[Wil Ferch]

I wonder if the cracking of cross-drilled rotors ( whether cast in place or drilled), would be substantially improved if one were to "polish" the holes ...say with a dremel and a cratex tip..to remove the smaller stress risers from the manufacturing process that makes the holes in the first place... ( and if you had a spare X00 hours to do this , I guess )
--Wil Ferch

[Randy Webb]

Bill, I read Wil's post as saying that there are benefits to the process in general (indisputable), but one has to evaluate the magnitude of benefit for a particular application. It's obvious from your post that the benefit is nil for Porsche brake rotors.