This is an excellent writeup Mike. Now you need to hit the gym and shed 40 lbs to compensate for the weight gain on your car.

 

Many of your points are accurate. I posted to remind everyone that there are laws of physics at work here.
You are correct that changing the weight of the rotor does not change its velocity, but you missed my point. Kinetic energy is MASS times velocity SQUARED. So any increase in mass dramatically increases the amount of energy required to accelerate it (and vice versa). (Another important factor in rotating kinetic energy is Moment of Inertia, but it is not a variable here.)
Since energy is not created, but only converted, this energy comes from the stored chemical energy in the fuel tank. The chemical energy is then converted to heat in the engine. If you reduce the mass that must be accelerated, and keep the energy the same (i.e. horsepower) then the vehicle will accelerate more quickly.
We recently conducted a "same day-same conditions" back-to-back test in Europe with a major tuner using 30+ lb wheels in a standing mile acceleration test. When these wheels were replaced with 20 lb wheels, the ET over the mile run dropped by 2.1 seconds. Data recorders on board confirmed your argument that 0-60 times were not affected much, primarily due to the fact that the powerful 2WD vehicle was EXTREMELY traction limited. However, at higher speeds 130-190+ mph, the vehicle accelerated much more quickly, due to the lower rotating inertia (remember mass times the SQUARE of the speed).
You were also correct about deceleration being traction limited as well. This is often shown by 80-0 or 60-0 data points. However, at high speeds, traction is less a factor and braking performance is largely determined by the braking torque available and often limited by the quantity of heat that can be generated and dissipated by the braking system. The braking system has to contend with the entire kinetic energy of the moving vehicle AND the kinetic energy of the rotating mass. Reducing that mass will improve braking performance and shorten stopping distances. It is simple physics.

As to what one can or cannot "feel", I tend to avoid those arguments.
However, dramatically changing unsprung weight can make a vehicle's driving experience change significantly. A lively, nimble feel can be transformed into a heavy and ponderous feel that can lead one to make unscientific conclusions.

I enjoyed your comments and challenges. Sorry for the long post.

 

Great writeup Mike. +rep for you~

 

Your original post was very clear and no reason why anyone would not understand, unless their physics background is limited.

I typically don't respond when some one argues a point that doesn't hold up. You very much feel the difference in breaking at speed as less pedal pressure is necessary before you get even close to traction limitation. To feel this you would have to drive similar cars with only the breaks being different.

I can understand how some may think this is difficult to feel; most of the driving population are very poor at peddle modulation if they even know what it is. I had a chance to dive on the oval at Milford and they use center lane with hands off to demonstrate this feature. Instructor informed me that less than one in 10 Americans get this on first try. He also stated that after about 15 minutes using this method makes all the difference in a drivers ability in many other areas.

Had another instructor tell me once that if someone gets in trouble, over 80% of the time it is caused by the feet. Threshold breaking; what the hell is that?

 

You are, of course, correct. But I lack the experience, skill and credentials necessary to relate such a story.
(Heck, I have enough trouble trying to explain physics).
But if one imagines trying to slow a 40 lb flywheel spinning at 2000 rpm, one can easily understand how eliminating that much kinetic energy will have a positive impact on brake "feel", deceleration, reduced stopping distance, driver fatigue, and the list goes on.....
Of course, one must also have an open mind.

 

I didn't miss your point at all, I am forcing you to be accurate in your arguments, as I try to be in mine. I will not be out-argued by incorrect statements.

Actually the moment of inertia is important, because while you tried to use angular velocity to support your argument what you really meant was angular momentum. It's much much worse to put a heavier WHEEL on the car rather than a heavier rotor, and at this point I'll leave it to the reader to figure out why.

Not at all, I enjoyed this discussion very much. I don't think we ever actually disagreed on the mechanics at work here; after all it is physics and it is not subjective.

I still stick by my original statement though that the OP could not have felt the difference in acceleration and will still put up $1000 for a blind test.

 

Have had approx 16+ hours track time in a stock 2009 C2S w/PDK and stock 19" wheels and stock red brakes.

My personal 2009 C4S has PCCB and Lightweight AMT Forged Alloy wheels.

The handling/braking difference between the cars is night and day.

And I would bet $10K that I could pick 10/10 tries which is which. Not sure how you would "blind" the test however

Pedal feel/modulation without doubt- PCCB superior. Driving dynamics, deceleration on initial high speed application, turn-in, steering crispness and responsiveness, ride quality, no doubt at all- lightweight wheels and PCCB's are far superior.

Side note- the wheels that Tech-1 Mike represents are in my opinion the best money can buy, bar none. Ultra-light. Tough finish. Very strong. Superb quality. Beautiful. Thanks you guys for the awesome product you make!

 

Two totally different cars.

 

OK, it is official that PCCB/OEM pads sucks compare with Cup steel rotors with Pagid RS29. I knew the Cup steel rotors felt great at Laguna Seca but most of my PCCB/OEM pads track time were at Thunderhill. I finally had a chance to try out the Cup rotors at Thunderhill two days ago and let me tell you this, they totally ROCK!! I can accelerate all the way to the end of the brake zone before turn 10, 14 and 1 and slam on the brake with total confidence unlike the OMG experience with PCCB/OEM pads where I have to ride on the brakes way before the brake zone to prime them.

Now the only question left to answer is how well does the PCCB perform with proper track pads like the Pagid P50 and RS29 compare with Cup steel rotors? That will be my next project

Here is a video of the Thunderhill brake test day

http://www.youtube.com/watch?v=1URJcmYui9I

 

I wouldn't go near a track in a GT3/RS without dedicated track pads (like RS29's)...or stainless steel lines, and Motul for that matter.

There is no such thing as a "Do-it-all" brake pad. Those who market such things are irresponsible, and those who believe it are foolish.

Everyone should have 2 sets of pads...one for the street, and one for the track. It's not like it's hard to change pads in porsche calipers.

 

Yup, you got that right. I love the pagid RS 29 but it noise like mad which does not bother me on street. I heard Pagid P50 pads noise too on PCCB. Will give feedback once I try out couple combination pads with PCCB

 

pretty incredible thread really appreciate it! can i have ur ceramics

 

Thanks. I already donated my PCCB to the Canadian Mountie

 

Mikymu,

Came across this old informative thread. Was wondering if you ever got to try out different pads (such as the P50) on the PCCB at the track?

 

Subscribed! Thanks for sharing your effort and great write up!