Re: misconceptions on drilled rotors and SS lines

["Rich Wagner" <drsavage@domain.elided>]

Mark wrote:

>Cross-drilled rotors will NOT improve your braking.

Whoa, careful here.  There are situations under which cross-drilling will
improve braking.  However, under normal road conditions, this statement is
true.


>Cross drilling is done to lighten the unsprung weight on race cars.

Well, cross-drilling does reduce mass, but not dramatically.  More
importantly, it provides a mechanism through which the braking surface (the
part of the rotor currently in contact with the pad) can be cleansed of
foreign matter, like water, oil, and dirt.  A solid rotor that is fouled
requires a few revolutions of pad contact before it's clean and full
friction is restored.  A slotted or drilled rotor is clean before it
transits from one side of the pad to the other.  The foreign matter is
pushed into the voids, and is expelled as soon as that part of the rotor is
clear of the caliper.   What this means is, with a drilled or slotted
rotor, there is little delay between brake application and
response--especially in adverse conditions.  To really reduce unsprung
weight, you've got to spend the money and go with aluminum, titanium, or
carbon rotors.


>Actually brake effectiveness will decrease  and wear will
>increase as you have reduced the surface area of the rotor.

"Effectiveness" is a hard word to quantify or qualify.  Drilling will
indeed reduce the braking surface.  What this means is that, to provide the
same braking force--the same deceleration--will require more pedal effort
with a drilled rotor than it does with a solid rotor.  Unless a ridiculous
amount of material has been removed, you will be able to lock the brakes
either way, so absolute braking power is the same.  However, with the
drilled rotors, the brakes will feel heavier; the pedal will be less
sensitive, making modulation more difficult; and the feel of a locked brake
will be less obvious.  Reduction in feel is not a positive attribute in a
brake modification.


>Heat is actually dissipated by heat sink action on the rotor, 
>and again, you have reduced the mass of the heat sink, not what 
>you really want to do.

Decelerating a 2,500 lb car from 60 MPH to 0, assuming the front brakes
dissipate 60% of the energy, puts 90,000 ft-lb of energy into each of those
rotors.  That will raise the temperature of a 10 lb steel rotor by about
360 degrees F.  Lighter rotors would get even hotter.  If the rotors really
acted as heat sinks, you would have no brakes left after two applications. 
In a few situations, notably a low-speed panic stop, the rotors really do
act as heat sinks.  However, under normal or even racing operation, the
heat generated under braking is dissipated largely by forced convection and
radiation.


>Cross drilled rotors are just like rear spoilers or wings on a street car,
>for looks only. 

Indeed, the brakes on the GTV6 are absolutely superb--something which I
expect is probably true for all of Alfa's models.  Howard, if you feel that
the brakes of your GTV6 just aren't effective enough, I would suspect that
there may be something wrong with them.  With the standard engine, I can
attest that the brakes exhibit no fade even when driving the car flat-out
on a road course in the middle of summer.


Rich Wagner
Montrose, CO
'82 GTV6 Balocco

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