[Supras] stock brake rotors

[Walker, Brian (Rich. Dist)]

But I was a thermo major :)
Heat transfer is derived from surface area and temp. differential. Since
you've got huge differences in temp during braking (rotor vs. pad face),
there's a lot of heat transferred to the rotor through the outside
surface. Since's there's a much smaller temp difference for cooling
(rotor temp vs. air) you need more surface area to cool (accomplished by
lots of air across the vented center)
You're still talking surface area, just not the area that's seen on the
outside of the rotor. A reason BBK's have thick rotors, more surface
area for cooling, thickness of the rotor does nothing for braking power.

Just wanted to point out that surface area does matter, a lot.
Brian


-----Original Message-----
From: Jeff Mohler [mailto:speedtoys.racing at gmail.com] 
Sent: Friday, April 11, 2008 1:31 PM
To: Walker, Brian (Rich. Dist)
Cc: Khalid Almufti; Supras at supras.com
Subject: Re: [Supras] stock brake rotors

Everyone is told that..but..it just isnt true.

Theyre machined..it makes no difference how the hole got there, the
outside edge of the hole is where it will split under enough expansion
stress.


Now..why are they there??

1:  Looks sporty.
2:  IF theyre there on a true race car, theyre dropping mass that way,
but the braking system is already designed to run cool enough to NOT
FAIL the rotor by cracking.

I you look at how airflow moves through a rotor, it becomes instantly
clear why holes dont help.

If you consider that it takes 3 seconds to get a rotor to 1500d in
braking, and maybe a mile to get it down to where it was
before..."surface area" doesnt aid in cooling on a track whatsoever.
Its that large volume of airflow moving thru the rotor core.  Holes
reduce the pressure differential at the hub where air enters the rotor
core, and drops the CFM that it flows.

GO find a thermodynamics major and talk about drilling holes in a
rotor...not supraforums.com or some vendor that wants to -sell- you
this process.   I will be the first to talk you out of it on a track
car if you bought rotors from me.

GO to a club racing event like SCCA or NASA, find 3 cars out of 500 with
drilled rotors that were not given to them for free as a
sponsorship deal.   Bet ya cant.


And..if its so good, why such small holes, and why so few?  (Cuz it
doesnt work)


On Fri, Apr 11, 2008 at 7:48 AM, Walker, Brian (Rich. Dist)
<Brian.Walker2 at vdot.virginia.gov> wrote:
> I had been told Porsche casts these features into their rotors. One  
> would expect that a 6 figure car would have all the best in 
> technology,  is the drilled/slotted feature one of those aesthetic 
> things they throw  in?
>
>  I'd imagine cost from pad wear isn't of concern on those, maybe it's 
> to  aid non-street pads on cars typically used on the street?
>
>
>  -----Original Message-----
>  From: Jeff Mohler [mailto:speedtoys.racing at gmail.com]
>  Sent: Friday, April 11, 2008 10:40 AM
>  To: Walker, Brian (Rich. Dist)
>
>
> Cc: Khalid Almufti; Supras at supras.com
>  Subject: Re: [Supras] stock brake rotors
>
>  On Fri, Apr 11, 2008 at 7:04 AM, Walker, Brian (Rich. Dist)  
> <Brian.Walker2 at vdot.virginia.gov> wrote:
>  > I'm no expert but honestly, I don't think there is a performance 
> gain
>
>  > from those type of rotors. Of course, with slots and drilling you 
> are
>
>  > increasing surface area slightly, which would mean they can shed a

> > little more heat.
>  ---
>  Yes, you lose BRAKING surface area.
>
>  Nobody has proved that they make rotors cooler.  The goal is pad face

> temps, which go UP when you lose rotor surface area.
>
>  You're also decreasing contact area for the pad  >  slightly, which 
> would mean a little less braking power (we're talking
>
>  > very small amounts)  >From the research I've done, pads nowadays 
> don't
>
>  > release a gas layer  like they used to which was the main reason 
> for  > slotting/drilling, to  remove or scrap that layer away. When 
> you have  > a drilled/slotted piece  (provided it's added after
manufacturing vs.
>  > cast into the piece) you're  also adding areas prone to cracking.
>  ---
>  Drilled rotors crack no matter HOW the holes got there if you get 
> enough  energy into them.  Its how a round piece of metal expands.
>  Has anyone ever seen any evidence of a rotor casting WITH holes?
>
>  No..it doesnt exist.
>
>  If you put holes in pizza dough, does it stop splitting as you expand

> it  on the outside edge?  No..thats how a round rotor expands, and 
> fails  with enough energy thrown at it.
>
>
>  >  for street use, most people use slotted/drilled for street because

> > they  look cool. There are some processes out there like cryo 
> treating
>
>  > that  are supposed to reduce the possibility of cracking, I've 
> never  > tried  them though.
>  ---
>  Nope.   Cryo hardenes the rotor, which adds some lifetime to it, but
>  unless the rotor costs like $130, its not worth the $50 to do it.
>  It'll still fail the same way..its still steel.
>