Science...AKA "The Laws of Physic's" and "The laws of Thermal Dynamics"
in discussion here, so here are some of my thoughts...or IMHO...
Braking with automotive brakes using friction brakes is the conversion
of Kinetic Energy into heat
The heat is generated at the point of friction. That means where the
friction material touches the drum/rotor and since most have disc
these days...will reference disc brake terminology (though also applies
to shoe/drum setups
That heat goes someplace(s). Into the air right there, but a very
small amount. To the friction material, but friction material is NOT
a good conductor of heat...on purpose. Most of that heat goes to
the rotor...on a metal rotor. Plastic rotors are designed to absorb
that heat better/faster than the pad
The rotor must be able to absorb, but there needs to be a temp differential
No differential and it will *NOT* absorb and the larger the differential,
the faster it will absorb
The mass determines how much it can absorb quickly and hold that heat.
Then the metrics of rejection of that heat, which has thickness
of that path to the rejection spot/area/etc. The material characteristics
also come into play here, as is the shape (the why of a 'heat dam'
between the rotor and hub on 'some' single piece setups).
Thinner or less mass will have less thermal pathways to the rejection
area of design
An oblique moment #1...the centrifugal inertia must be taken into account
at this point by the designers, as the acceleration/deceleration
rates must be within design criteria (how fast it will spin up with
X amount of torque and stop with Y amount of braking). Dia of the
rotor, thickness (weight), etc
Part of that is the shape of the centrifugal vanes (straight, curved,
how many, etc). That also is factored by the dia of the rotor and
the expected dia of the tire vs the expected RPMs of that tire in
'normal' to top end usage
There is also a 'crush' component to that design, as when heated
to the upper end of design usage specifications...the metal will
soften (most are designed to the neighborhood of 1,400*F usage
top end) and the platters can be crushed (bent inwards to create a
wavy surface)
The platter thickness, the vent area thickness and the vanes (number
of them, placement of them, etc) along with expected material
removal during expected service life must be designed in
Part of (just part) the design is the density, porosity (voids), etc
of the cast iron (ditto plastic) rotor specification, as are the
spec's for the other components of any braking system
Oblique #2...the friction material likewise has a crush specification
and that is to OEM PSI's expected/spec'd to the mating components
Some aftermarket stuff for race or track can crush OEM friction
materials...AKA crumble at their high end temps
Once the heat is generated...it is absorbed by both the friction
material and rotor material. One of the 'whys' caliper pistons is
turned backwards (as asked of me by some) and is to limit the amount
of surface area touching the pad backing plate...to reduce the thermal
pathway back into the caliper
That YouTube link is correct, but not an absolute as too many will
think...hearing him say that the new binders out-gas "LESS", which
means they still do...to produce a high PSI area of gas to float the
pad friction material off of the rotor surface...AKA fade (just one
aspect of fade)
Back on the out-gas relief routs....
Holes were cool and still so much so because of the marketing employed
to sell them...
Drilled holes will crack. Cast in holes has less stress raisers
but they are still there (just less), but EXPENSIVE to cast them in
Holes do reduce the mass, so acceleration/deceleration inertia is
less...but...since less mass...less thermal capacity...in absorption,
moving it around and rejection
Slots has less material removed, so has more mass than holes. A good
thing in this regard, but less mass never the less
Oblique #3...the why of that cut/cast-in slot on most pads today.
Even OEM, or the better ones. That is an out-gas slot to allow
out-gases from floating the pad off the rotor surface, but that is
factored by the surface area in contact with the rotor...
Some longer pads has two or more of these out-gas slots cut/molded
into the friction material
Notice that they are NOT open all the way to the backing plate and
that is because once the friction material has worn down there...they
need all the surface area then can get in contact with the rotor...but...
a losing proposition, as the friction material will have less mass
to both hold and transfer that heat...they will over heat easily,
but braking is #1 to the designers to meet their design specifications
There is much, much more science to this and the above are just some
of the high points to get the science across...I hope... :B
I've gone back to OEM rotors for my Suburban. The cyro/slotted rotors
were cool and EXPENSIVE, but they too heat checked (micro cracks
that propagated into clean through cracks)...so why bother...
