[s-cars] G60/brake threads... first-order system factors?

[QSHIPQ at aol.com]

R+C
Comments inserted:
In a message dated 10/29/2003 10:03:13 AM Central Standard Time,
uberseehandel at yahoo.com writes:
>Your reply to Leigh is innaccurate in several
>significant points.
OK

>The Porsche rotors are not cross drilled, the holes
>are cast in, this is described by Porsche, and SHW, as
>'pierced' in English - think nipples as an aide
>memoire.

That can raise the bar wrt the point of failure, but the downsides to
crossdrilled rotors have to do with the heat stress and heat distribution at the hole
as well as the surrounding material.

>The microline cracks between piercings are not a
>problem most of the time, its not a fault, its a
>physical outcome of being heat cycled. What is a
>problem is when cracks run to the outside edge of the
>rotor, then its discard time.

We can freely disagree on this point.  It IS a fault, in fact, specifically
it's a *failure* of trying to engineer  a hole drilled thru a heat sink to make
it able to properly disapate heat.  You don't see the same cracking on a
solid disk, rarely do you see it on a slotted disk.  From an engineering
perspective, that puts the failure pretty much in direct correlation with the hole thru
the heat sink.   I won't address "most of the time", I'll address only
failure.  Cracking is failure and can lead to catastrophic failure.  Your point is
valid in a rotor that is inspected "most of the time".  That rarely happens in
street cars.

>Contrary to your assertions, the Porsche factory does
>used pierced rotors in race applications, I have some
>current season Supercup rotors sitting here (R
>996.351.409.9J, R 996.351.410.9J, R 996.352.405.90,
>and R 996.352.406.90), additionally I have several
>sets of 993 GT1 long distance racing rotors, similarly
>pierced. These are all rotors supplied and used by the
>Porsche boys - if you doubt me, pick up the phone and
>call Weissach, I'll even give you a DDI number or two
>if you still have doubts.

Using your own quote, "most of the time" the rotors used by porsche boys
(defined as the porsche street cars going to the track, as is the context of this
thread) are not of the "pierced" type.  Several race car chassis use CD
rotors.  IMO, these are cars that have already addressed the heat issues, and have
optimized the chassis to brake ratio (heat, balance, weight, weight transfer,
cog, etc..)  Putting this discussion back to miss piggy, we know that just
based on weight and weight transfer comparing her to the street 993tt, I would
expect this rotor to crack it's piercing much more frequently than the street
cars.  I do know that if you read any of the porsche lists, comparing
specifically where street cars go to the track, cracking of these covetted crossdrilled
rotors is a problem.  Whatever Supercup and GT1 uses seems irrelevent.  We
could put forth the math of this pretty easily starting with static weight and
weight distribution, then maximum weight transfer under braking.

>If you examine the Supercup cars, you will see that
>their brake ducting is by way of attachment to the
>lower suspension arms, not through unibody.

Anyway you can get it there.   My point is that even looking at a 325i from
10 years ago, BMW made a concerted effort to put cool air to the brakes.
Optimally?  Not my claim, but certainly puts audis efforts in perspective.

>Because I have more experience than most when it comes
>to retarding large heavy cars from high speeds
>repeatedly, I have gone down a path where I have
>learned that large mass rotors = peace of mind. I
>should point out that I am talking about stopping from
>over 200 mph, which is a speed rarely reached on track
>days and never reached legally on a regular basis
>outside of Germany, so my experience of brake
>behaviour under stressful conditions is not typical.
>Big rotors and large pad contact areas are what equals
>rotor and pad longevity. The problem with ducting air
>in a street car is the large cross sectional area of
>the air pipe and its consequent ragility in everyday
>driving. Experience has shown that anything less than
>a diameter of 4 inches is hardly worth the effort.
>After some time in a wind tunnel (with a rolling
>floor), I hope to have a roadgoing solution.

My point to this has already been put forth.  First, ducting on G60's isn't
really necessary for normal street driving.  Audi has proved that already, MOST
of the time, heat isn't a problem.  Hence our crappy brakes.  Let's also not
forget that a G60 pad has a LOT of surface area.  Big rotors and large pad
contact area AREN'T necessary, unless you have the First Order and Priority,
which is a HEAT problem.  You can address the heat several ways, including using a
larger heat sink.  A better way to address this IMO/E is to get air to the
heat sink.  We can easily agree that miss piggy lacks massively in this
department.  In fact, so much so that, although 4in ducting may be "ideal", less could
certainly be more than nuttin.  A scoop doesn't require a diameter nor does
it carry those limitations.  Porsche did this too.  I figured out a way to do
it on the triangulated swaybar cars already.  It works.

>Given the difficulties of feeding sufficient volumes
>of air to the right part of hot rotors on everyday
>cars, the high mass rotor/big swept pad area solution
>is practical for most cars and their owners.
>Once Composite Ceramic brakes enter the picture, the
>situation changes as the mass comes down radically.

Maybe.  Ceramic brakes can also put more heat where you don't want it.
Specifically, the issues with ceramic brakes in street cars will be high heat and
hot shutdown> no  airflow causing radiant heat related problems.  Back full
circle.

Scott Justusson