XPost: comp.mobile.android, misc.phone.mobile.iphone   
   From: me@privacy.invalid   
      
   "Alan"  wrote in message news:t57i62$p1k$1@dont-email.me...   
      
   > Do the math on a single stop from highway speed.   
   >   
   > My 2012 BMW 135i MSport Edition weights 4,255lbs, so stopping it from   
   > 60mph must dissipate 694,273 Joules.   
   >   
   > We'll assume that all the energy of the stop remains in the rotors   
   > themselves, and that BMW has done it's job properly and that therefore the   
   > front and rear brake rotors are proportional in mass to the energy each   
   > must dissipate.   
   >   
   > So we can take the total mass of the rotors, the heat capacity of steel   
   > and calculate the change in temperature.   
   >   
   > The mass of those rotors is 34.47kg   
   >   
   > The specific heat of steel is about 420 joules per kilogram per degree   
   >   
   > 420J/kg*c   
   >   
   > So with 34.47kg, we get 14,477.4 J per degree C temperature change, and   
   > dividing the 694,273 joules we've got, we get a temperature change of...   
   >   
   > ...just about 48 degree.   
   >   
   > If it's a warm summer day, you're not going to want to touch the rotors,   
   > as they will now be above the boiling point of water (about 120°C)...   
   >   
   > (Provided you ignore the energy lost to the airflow and also dissipated by   
   > the mass of the pads and calipers themselves, as well was the energy   
   > conducted into the hub of each wheel)...   
   >   
   > ...but it's hardly going to be enough to deposit brake pad material onto   
   > the disk.   
   >   
   > This is why brake bedding procedures call for MULTIPLE retardations from   
   > highway speed in order to get the disks hot enough to deposit an even   
   > layer of pad material onto the surface.   
   >   
   > I know this, because in my amateur racing "career", I'm also a senior   
   > racing instructor, and because our students are driving road cars for   
   > sessions where the brakes actually WILL get to the temperatures that can   
   > cause that kind of deposition, we always tell them not to apply the   
   > parking braking when returning to the paddock after a session.   
      
   I remember many years ago watching a "for schools and colleges" TV science   
   programme at school about "energy". It talked about converting kinetic   
   energy into other forms. As an example, they rigged up a camera on a car,   
   pointing at the front brake disc. They they took it on a racing track (they   
   emphasised that this was off-road!), and repeatedly accelerated to 70 and   
   then braked hard to a stop. After a few cycles of this, the discs were   
   glowing cherry red. I was quite surprised that discs would get *that* hot,   
   and that brake pads would withstand contact with red-hot metal.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)