zoned019

Okay, David (faithofadragon), and I were having a little discussion/debate on AIM about rotors..

Neither of us managed to convince each other of our points, so I decided to post here and see what the rest of you all think.


I said that drilled rotors will dissipate heat, thus giving better braking.

He claims that holes in the rotors won't dissipate any heat, only the gases created by braking/from the pads.

I agree with what he said, but also think it will keep the rotor dramatically cooler.

Let's hear some reasons why we are right/wrong, I'll post up what I think + why too.

hamhead

Drilled does remove heat from the system, although it is the most unimportant way that brakes remove heat from the system. Drilled means less usable surface area for generating friction and less material to DISSIPATE the heat away from the pads. They are also less reliable because of fatigue and stress resulting from the reduced material.

They do dissipate heat, but they don't give better braking in most instances. So your both right and your both wrong. tongue.gif

zoned019

Well, I was specifically talking about rotors which were cast with holes, not discs that are drilled afterwards.

How can you say that the heat it dissipates is unimportant? I realize that it makes less of a surface for friction, but don't you think that having a lesser, but cooler, surface is better than the whole hot surface?
Basically you're saying the holes are ineffecient? I don't think that you are really decreasing the surface area of the disk by that much...but you are making it a lot cooler.

hamhead

Even brakes that are cast with holes can result in fractured brakes, but this really wouldn't happen unless your on the track really pushing a car, not day to day. Thats not the topic though. tongue.gif

The Rotor's surface is where the pads contact and generate friction to slow the vehicle down. Since it is this friction that causes the stopping you ideally want as much as possible right? The more friction you have the better your stopping will be. This is reason #1 why BIGGER brakes are the best way to improve a vehicle's stopping ability. More surface area on the pad and the rotor = more friction = better stopping.

The rotor, by way of thermal tranfer, dissipates heat. This lessens the amount of heat at the contact area because it is moved throughout the whole rotor. The bigger the rotor the better here as well. The more metal it has the more metal the heat can be diluted into.

Now comes the step where the rotor takes the heat it DISSIPATED from the pads and gets rid of it for good. By radiating it to the surface - either the faces or inside the vanes. It is here where cool air interacts with the hot metal to cool it off and remove the heat. This is where drilling helps. However, think of it like this:

So the benefits of a cross drill:
Removal of particles and enhanced heat removal.

The downsides:
Less usable surface area for generating friction, less material to dissipate the heat away from the pads, and reliability.

zoned019

So you're saying that the holes are ineffecient, that is to say that the amount of surface area lost (by the holes in the rotors) reduces more braking power than the lesser temperature increases braking power?

Since that sounds confusing I'll say it like this and it will make more sense..
Surface area lost reduces braking power by (i'll just pick a number) 10
Decrease in temperature increases braking power by 5

is that what you're saying?

Because I would think that the decrease in temperature would help more, even with the decrease in power from loss of surface area.

Hmm, this is sort of a parallel to the "more air vs colder air" debate of CAI's.. tongue.gif


Now, faithofadragon on the other hand says that holes in rotors won't even help decrease the temperature..

hamhead

Correcto. Which is why I said your both right and both wrong in different aspects. Of course I could be wrong totally, but this is what I have taken to be the truth about these things.

If you were doing an extremely windy road course, then the extra cooling would come in handy. If you were doing a road course with a few turns, and longer straightaways where the brakes would cool down, then regular pads would be more useful.

Mad-Machine

Rotors with cast in holes are much less likely to crack than those drilled. They are DESIGNED for the holes and thus the internal webbing and material can be properly placed to make up for the loss in strength.
Porsche and Ferrari both do this for their low end brakes. Unlike what Ham said, cast in holes rarely break in motorsports as there are no unknown weakspots or stress fractures from the drilling.

Now.. if you want efficent cooling with holes.. you need proper ducting of cool air. Much like a CAI, you need to run a tube from the front bumer (like say where the fogs are) back to a SEALED (or as close as you can get to sealed) backing plate between the brakes and the underside of the car. To do it correctly, you need to force air into the Hub of the brakes (where the vanes empty) so that the air will flow out from the centre of the rotor to the edges. It will also leak out the holes. The holes will increase the surface area in respect to COOLING, but not in respect to the friction area.

Of course now in motorsports the big thing is ceramic brakes.. but those cost as much as our Tiburons did new.

And yes, both holes and slots will decrease the amount of outgassing your pads do. While not really necessary for the minimal amount of braking most people do.. they come in very handy on winding circuts and for those that live in the mountains. The more often you brake, the more you need holes or slots. It is a compromise, but even brakes with 10% (your number) less surface area will make up for later as you use them often and repeatedly with little chance between for proper cooling.

Dmitry

also one big advantage of drilled and/or slotted rotors is that it helps water to get out of pads/rotor contact so water will escape faster. so in rainy weather - they may be better.