[Beaverboy]

I'm running the Stillen cross-drilled front rotors with Stillen Metal Matrix pads. They look really good, came with a nice anodized coating, and they certainly help keep the brakes cool (I've got the little brakes on my wagon ). They cause more brake dust because the holes (even though they're chamfered) scrape the pads clean.

I've been keeping an eye on them for signs of spider cracking around the holes (which the chamfering is supposed to help avoid) and haven't been able to spot any yet (5 months of hard running in a very wet mountian summer and 5 autocrosses).

They actually hum when you get them really warm and stomp on them from speed. The sound is created from the backsides of the holes hitting the pads and it sounds baddass :twisted:

I had some folks try to talk me out of them based on loss of surface area... :roll: Don't make me break out the physics to prove how bunk that is (unless you'd really like to know) :idea:

/\ Here they are, installed, before I went and tore that anodized coating off.

\/ Of course, they're much more visible than any camera would lead you to believe

 

[Beaverboy]

You're gonna make me break out the physics!!! :evil:

:wink:

They do decrease the swept area of your rotors.. this is true.. but swept area has very little to do with braking performance (contrary to popular belief). The physics comes in the form of:

F = uF * N
Friction = (Coefficient of Friction) * (Normal force)

Friction, of course, is the resistance to sliding that two materials have against one another. With braking performance, friction is key. Friction creates heat, so the other key factor in braking performance is cooling.

Of course, there are other factors that effect braking.. like the diameter of the rotor. The larger the rotors the more torque the same amount of friction is able to produce. Torque is the real braking mechanic that matters when it comes to brake power. :wink:

The coefficient of friction (uF) is a number that depends on the surface. Since we're talking about brake pads against rotors, it's that coefficient of friction that matters... it's a constant regardless of how the two materials are pressed together. There are two versions of the uF though... a kenetic and a static, one for sliding friction and one for stopped friction... the stopped friction is almost always higher for any material combination... noone cares about how well their brakes hold statically though.. we're just talking about kenetic friction while trying to slow down a moving vehicle, so as far as we're concerned there's only one uF to deal with for any given pad material and rotor material combination. (sorry for the rambling)

Your normal force is the amount of force pushing the two materials together. The calipers transfer force from the brake booster and your foot to the pads to create a normal force.

You might have noticed that nobody mentioned pressure (force divided by area), because pressure isn't part of the formula. The suface area of the rotors doesn't matter when it comes to the friction, just the force from the calipers.

The problem:
The friction and heat production will be the same for a given force pushing the two materials together (whenever you bring any given car to a stop from a certain speed you create the same amount of heat energy in the brakes). Obviously that creates a problem if you want to stop a car with rotors the size of a CD in the same amount of time as one with 14" rotors.. The amount of heat that a 14" rotor can soak up and disburse would likely melt a rotor the size of a CD way before the vehicle ever came to a stop.

Well, in the case of cross-drilling, the loss of surface area and mass means the heat has to occupy what's essentially a smaller rotor, but the cooling effects of cross-drilled rotors (because you gain more surface area for cooling) more than compensates for the added heat saturation. Also, the more saturated with heat something is (in relation to the surrounding air), the more quickly it releases that heat, so while they may be hotter briefly, they give up the heat more readily so it makes them even more efficient at cooling.

When you cross-drill rotors, the pads don't get any additional cooling... so running pads than can handle higher heat is usually a good idea when running cross-drilled rotors.

Thanks, James. They're Rota Slipstreams.. they're priced well (~$110 each), they look good and they're light.