XPost: comp.mobile.android, misc.phone.mobile.iphone   
   From: robin_listas@es.invalid   
      
   On 2022-05-08 12:36, NY wrote:   
   > "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.   
      
   I worked for a company where we made industrial test beds (I was the   
   programmer). One of those was for testing brakes. During testing at the   
   brake factory where this machine was installed, the chap there actually   
   brought the disk to that hot. The testing procedures included braking   
   from 40, 60, & 80 down to 0, several times in a row, while graphing disk   
   temp, fluid pressure, torque...   
      
   He said a good pad material would not degrade or "fade" suddenly when   
   too hot, but would lose the friction coefficient gradually as   
   temperature increases, thus warning the driver of the danger.   
      
   --   
   Cheers,   
          Carlos E.R.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)