Oh boy, I've had this discussion before...
I don't have all the physics details, nor do I care to look them, but here's
a brief summary.
A rotor has a couple of different jobs to do in the braking process. It's
first and most important job is to ABSORB heat. The heat comes from turning
the movement energy of the car into heat energy through the friction of the
pads onto the rotors. Energy can neither be destroyed or created, but it
can change forms. That heat energy has to go some place, so it is ABSORBED
by the rotor. After the heat is ABSORBED, it is then DISSAPATED into the
air via the surface area of the rotor.
You are right, more surface area = better heat dissipation, but you've
forgotten the number one job of the rotor. You've got to get the heat into
the rotor first before it can be dissipated. If you drill holes or slots in
a rotor, there is significantly less MASS to absorb heat, leading to warping
and cracking.
In addition drilled holes create stress points throughout the rotor. As the
brakes heat up, you get stress fractures around the holes. Shortly, the
rotor will be so cracked, it's unsafe to use. Here's just one picture of
MANY seen. Drilled rotors will also shred a pad like a cheese grater.
<http://www.r32skyline.com/Cracked_rotor_2.JPG>
So, you say, what about those slotted rotors then? Why not use those to
dissipate the gasses of the pads? The fact is, that most modern pads do NOT
gas. In the past, the gas came from organic pads that would boil under high
stress. The slots would help dissipate that gas and allow better pad to
rotor contact. However, today's pads made from metallic and Kevlar
materials do not gas.
Slots don't put the same stresses in a rotor that drilling does, but again,
you loose considerable MASS which reduces job number one...absorption of
heat.
A little more info I quickly found...
<http://www.shotimes.com/brakes/part2.html>
-A.
> From: ThoughtBoxLabs@aol.com
> Reply-To: ThoughtBoxLabs@aol.com
> Date: Fri, 28 Jun 2002 11:35:17 EDT
> To: knuckledragger@kcweb.net, Smokerbros@aol.com, autox@autox.team.net
> Subject: Re: Slotted and/or drilled rotors for '86 Civic Si?
>
> Slotted rotors: keeps gas released from rotors/pads under high temperatures
> from creating a "hydroplaning" effect between said rotors/pads.
>
> Drilled: more surface area for heat to dissipate.
>
> Will
>
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