I am not bashing any products just trying to keep brake knowledge out there.

To your above ?, they use a spacer bracket which brings the caliper back to fit the larger rotor. If you want my opinion, pm me. I had a 996c2 with a bbk from stoptech, a 996tt with a bbk from Brembo, and now my 997tt with Brembo 2pc rotors. I have also owned 4 cars with the rotora bbk.

 

It's pretty much the only option available for the non-Turbo / GT2 / or GT3 996 cars, so it's hard to argue "apples to apples".

I personally don't like the idea of using a larger disc with the factory caliper because of the odd wear you get with a pad that was designed specifically for the smaller (318mm) disc.

I'd ask them specifically what brand/manufacturer disc they are using in that particular application.
The quality of the disc and the metallurgy makes just as much, if not more, of a difference that increasing the diameter.
Also, what is the size of the air gap?
It is possible to have a disc that is larger in diameter, with more surface area, but LESS overall mass and heat capacity than the OEM disc.
That would then have adverse effects as well in a high performance environment.

 

Well, I clicked on the Turbo's handle, and hit the "all posts by Turbo" link. I spot checked like 10-20 posts and found a very interesting pattern. Do the check yourself, and you'll see this:

Post after post after post has something directly related to Rennstore and MANY mention Craig by name. I don't know who Turbo is, but he REALLY likes Rennstore AND Craig.

-td

https://www.6speedonline.com/forums/2122746-post1.html

https://www.6speedonline.com/forums/2132932-post3.html

https://www.6speedonline.com/forums/2108710-post1.html

https://www.6speedonline.com/forums/2098302-post159.html

https://www.6speedonline.com/forums/2059103-post18.html

https://www.6speedonline.com/forums/2231169-post27.html

https://www.6speedonline.com/forums/2205793-post20.html

https://www.6speedonline.com/forums/2205797-post19.html

https://www.6speedonline.com/forums/2311797-post2.html

https://www.6speedonline.com/forums/2308679-post1.html

https://www.6speedonline.com/forums/2236852-post10.html

 

I would not use a caliper spacer unless it was a Porsche part. In my limited brake knowledge, calipers, rotors & pads need to work together in symphony. Messing with that engineering seems dangerous. If I got a larger rotor, I'd want a larger pad & caliper to match. There is a big red brake kit listed for sale in the parts forum for those looking.

After all this debate, I'm leaning toward the PF rotors when mine need to go.... Good input from Vivid - this is why we need to support the OFFICIAL board sponsors.

 

Here is the PFC rotor. http://www.vividracing.com/catalog/p...5-p-60667.html

$710 for the pair. 2pc rotor.

 

Tim,

I'm a little late in this thread, but I disgree on some things you say. I'll present them as things for you to ponder on:

If a car was designed from the factory w/ solid-faced rotors and you replaced them w/ aftermarket cross-drilled rotors of the same size (i.e. w/o increasing diameter or thickness), then you will get worse performance overall. However, if a car was designed from the manufacturer to have cross-drilled rotors, then you will get better cooling, fade resistance, consistent pedal feel.

Longevity for cross-drilled rotors will be far less than solid-faced rotors and it will eat through pads at a more rapid rate in performance scenarios (i.e. track days). However, longevitity and pad wear will be comparable on the street (unless you drive like a maniac on the street).

Actually, cross-drilled rotors will have more surface area than the solid-faced counterpart (comparing same diameter/thickness). Conversely, cross-drilled rotors will have less mass than the solid-faced counterpart.

The aluminum hat serves very little purpose in heat dissipation. However, an aluminum hat does allow the rotor disc to expand and contract uniformly under varying temperature. Otherwise, the rotor may bow or develop cracks easier (during track days).

Before you reply w/ what all the gurus and experts say, I'll present a couple of things that should make you ponder.

The first thing, I take my observation from my ownership experience w/ the E46 M3 -- so take it with a grain of salt if you care, it's all conjecture on my part. The U.S. E46 M3 come equipped with solid-faced rotors, however, Euro models come equipped with 2-piece cross-drilled rotors of the same size. While I would NOT use those 2-piece cross-drilled rotors for track days, the CSL (and ZCP/Competition package) M3 come equipped with larger diameter, 2-piece cross-drilled rotors in the front. The ZCP/Competition package cars weigh about as much as a regular M3 (3400lbs), but come equipped w/ larger 13" 2-piece, cross-drilled rotors. Now, consider that a 996 C4S weighs 3200lbs and has 13" cross-drilled rotors. Now, also consider the location of the engine and the affect of brake bias. I would say, at least for the C4S, that the front brakes are sized ok for track days, but please read more below as there are trade-offs.

The second thing, there is an very interesting SAE article available for purchase from www.sae.org that is appropriately called, "The Effect of Rotor Crossdrilling on Brake Performance." The article reference number is 2006-01-0691. I have purchased this article, but it's is DRM locked. Let me know next time we meet and I will print you a copy for your viewing pleasure.

Anyway, this article was written by 2 engineers at GM. There are pikchurs and grafs, but there are alot more words, about 25 pages worth. They tested 4 brake systems:

Brake System 1 - Cross-drilled rotors, sliding aluminum calipers front/rear (twin piston front, single piston rear) w/ 17" wheel envelope in front and 16" wheel envelope in rear. Semi-metallic high-performance street pads. This was tested on a high-performance sedan.

Brake System 2 - Cross-drilled rotors, fixed calipers (opposed pistons) front/rear w/ 18" wheel envelope. Similar semi-metallic high-performance street pads. The specs are suspiously for a Porsche...

Brake System 3a - Cross-drilled rotors, sliding aluminum calipers front/rear (twin piston front, single piston rear) w/ 18" wheel envelope in front and 17" wheel envelope in rear. Non-Abestos Organic street pads. The specs are suspiciously for a Corvette...

Brake System 3b - Solid-faced rotors, sliding aluminum calipers front/rear (twin piston front, single piston rear) w/ 18" wheel envelope in front and 17" wheel envelope in rear. Just like 3a, BUT the rotor diameters are smaller (4% smaller front, 8% smaller rear) Non-Abestos Organic street pads. The specs are suspiciously for a Corvette...

There's alot of scientific/technical mumbo-jumbo that goes over my head so I'll skip to the conclusion w/ the 2 points that pertain to my reply:

1. Brake system 1 and 2 saw "improved heat rejection capability" depending on speed. Brake system 1 saw an improved heat rejection capability in the front rotors between 8.8% and 20.1%

2. Solid-faced rotors exhibit approximately twice the fatique life over the cross-drilled equivalent (i.e. cracking) AND the attachment to the rotor hub influenced (increased) the stress field *cough* Cue 2-piece, floating hat rotor design marketing here *cough*

There are other points besides these two, but if I posted anymore, then it would be plagarism.

///Michael

 

This is definitely not the case. I have been in the field of professional motorsports and the performance aftermarket for almost 20 years as an engineer and entrepreneur, but more importantly, as an enthusiast. I have a copy of the SAE paper you are referring to and can read different conclusions from the limited work done by those two researchers. While their findings were valid for the testing they did, you must fully consider the conditions they tested under. .8g braking? Even my wife exceeds that (on a good day)! Performance tires are capable of more, with DOT-R tires well above 1.2g's longitudinally (as a point of reference, I've worked with cars generating well over 4g's of braking, but I digress...). Different things happen at the higher pad pressures. Cross-drilled rotors absolutely add to the initial pad bite. It's been blind tested with many professional race drivers and they can always tell the difference. However, a passive street driver will not.

This is absolutely true, with heavy emphasis on track days. Street driving, even the occasional blast, won't have much affect on rotor life, but pad life will suffer to some degree. There are a lot of other variables in play here, like how many holes, how large, are they radius-chamfered or angle-chamfered, etc., etc. The SAE paper only looked at one example.

The opposite is true as far as surface area goes. Cross-drilling REDUCES surface area. The reduction in surface area is offset by the additional leading edges, which create the increased pad bite at higher pedal efforts. The loss of mass is pretty negligible -- a 1/2 lb. per rotor give or take, depending on rotor geometry. The other big benefit for more aggressive drivers is the reduction of pad glazing when using cross-drilled rotors. This is especially helpful when using lower quality pads with copious amounts of low temperature binders.

An aluminum hat dissipates heat MUCH better than the cast iron hat it replaces. It's a simple heat transfer calculation, which is backed up by track testing. Aluminum also does, as you say, allow the rotor to expand with a little less resistance. This is OK up to a point, beyond which rotor coning becomes an issue. This is why a two-piece strap drive system is best, followed by a two-piece full-floating system, then by a two-piece bolted system. All are better than one-piece iron rotors at all times (except for price!).

I definitely appreciate your personal experience. It's what we all need to make parts better. I rely on that type of input, controlled testing and input from professional drivers.

By the way, comparing an M3 brake system to a Porsche 911 brake system does not fly. The rear weight bias of the 911 changes the entire equation, making any comparion invalid. The rear brakes on the Porsche, since the engine is behind the rear axle, do considerably more work than the rear of an M3.

Great paper, but the devil is in those pesky details. If I can get some research funded, I'd love to do a series of tests using more realistic data for the more serious driving enthusiast.

 

I have this, and I suggest you re-read it. It is HIGHLY biased towards cross-drilling. AND it does not account for slotted or dimpled rotors or other engineered solid rotors. ONY completely solid ones (see the picture on page 24) and p.23 where they admit they didn't even look at slotted or dimpled rotors). It is unclear what types of venting/vaning the solid rotors they used have - if any!

Because of these fundamental issues, it is easy to find snippits favorable to cross-drilled rotors in the article - but only as they are compared to generic solid rotors. Again, ONLY completely solid rotors, and not race rotors with slots / dimples / vanes / vents, etc. [Note that there is plenty of discussion about the vaned design of the x-drilled - but almost no discussion on the type or design of the solid rotor. Clearly a Brembo (or other) solid rotor will operate better than less "engineered" models.] Despite the fact that they used junk solid rotors, there were still a number of points favorable to the non-crossdrilled rotors they tested!Also, keep in mind that their test does not accurately represent real world use. For example, the tests were not dramatically affected by pad build up in the holes. This will severely change the results, since the ENTIRE premise of the article is that the holes help. In fact - they indicate that OVER 90% of the hole will clog under use! [Brake system 1, full. Brake system 2, 90%.] They also state that this can have really bad results. [This also illustrates that new pad technology does NOT need the holes for venting gas.]

I draw your attention to one of the most important conclusions - design affects performance. System 1 and 2 showed better performance with x-drilled, but system 3 showed better performancewith solid. But, what would happen with a properly designed rotor?

The closing statements of this article details the many negative affects of using cross drilled rotors. But I want to emphasize that the data presented is only a few data points regarding completely solid vs crossdrilled rotors. NOT crossdrilled vs track/slotted/dimpled/etc rotors.

Also, please refer to the more recent technical articles on the SAE site. There is a great one on Race Braking Technology (ca 2008). That one states you can improve brake cooling "through rotor design, cooling ducts ... and in some cases rotor crossdrilling can improve cooilng, but at the expense of lining wear rates" (not "in all cases"). This paper also talks in depth about brake fade.

There are a few other papers out there, but I have not read any more relevant that I can remember.


=========================================
=========================================
I wanted to clear up a few things you stated, so others will not be confused:

I'm pretty sure this means that the solid rotors lasted TWICE AS LONG, not 1/2 as long. The actual quote is: the solid rotors "tended to enjoy a fatigue life approximately twice as long as the crossdrilled version."

This is referring specifically to cross drilled rotors, and indicates that only the rotor plate attached to the hub exhibited stress fractures at the holes - AND the type of hat had a significant effect on the cracking.

Only base Corvettes use floating calipers (C5 and Z51). The fast ones use fixed calipers, just like us. (Z06, ZR1) However, IIRC, even the base Z51 uses a slotted rotor, or at least a properly vented one.

As for the M3 statement, cross drilled weigh less, and cost less - which is why they come with most cars. Not because they perform better.

Brake bias is very complicated. More than just where the engine is. Brake bias is a function of many parts, including ride height, pad compound, piston size, car weight, rotor size, etc. Bigger brakes are not always necessary for adequate stopping. With brakes being unsprung weight, you want them big enough to get the job done - not some huge monobloc monsters just for show.

Well, that's my $0.02, more like I wrote a nickel's worth.

-td

 

ummmmm....... what?!?!

 

slotted/dimpled > cross drilled.

-td