Are Cross Drilled and Slotted Rotors worth it?

Buckman said:

Rotors 101 and other misc. brake info:

First let me state that these are all my opinions, not to be taken as gospel. I have compiled this information with the help of numerous sources including a performance shop, a specialty car builder/road racing shop, numerous members of forums including CrazyAl who has ran a brake dyno for hundreds of hours in grad school, as well as excerpts from the SAE published "Advanced Brake Technology".

There is a lot of information out there on rotors, brakes, pads and braking systems in general. Enough that it is easy to become confused, and to give in to the latest "trend" from companies trying to hawk their wares. I am going to try to debunk some of this information and provide a clear case of what we, the normal driver, should look for when purchasing them. This post is by no means fully compiled, it is up to the reader to do further research, and from there come to their own conclusion.
To some readers some of this text will look familiar. I used a portion of a previous text and expounded on it a bit.

Cross-drilled:
In the 1970's drilling was used to relive a problem called "outgassing", which was caused by poor brake pad formulations. Since the 80's, better brake pad formulation has eliminated the need for drilled rotors. Contrarty to popular belief cross drilling is actually worse for heat handling than a solid rotor. The holes are pointing the wrong direction for cooling. Cooling holes need to be in the radial direction (like the spokes on a wheel), not axial. Some racing rotors are cross drilled for weight savings purposes, but these are known to be less durable than solid rotors. "Street" rotors are cross-drilled simply becacuse it looks cool. Drilled rotors are also prone to cracking.

Slotted:
Slotted rotors were also developed in the early years of performance disc brakes. The theory was that the groove would scrape the pad surface, reducing glazing. Slotted rotors do not warm up any faster than solid rotors. And furthermore, if they did, they would certainly not dissipate heat better. The claim that they come up to temperature faster yet cool more effectively at the same time is a paradox. The slots in a slotted rotor are far too small to have an appreciable effect on surface area or mass, and therefore they have no significant affect on cooling or on weight savings. However, they do create stress concentrations (areas where cracks can form) on your rotor surface

Zinc Plating:
This is just another fancy name for galvanizing. Sure, you can plate a rotor. However, the first time you brake, the brake pads will scour the plating off the braking surface. You might be able to keep the hub of the rotor from rusting, but you cannot affect the braking area itself.
The fact is that most rotors (even non-premium OEM replacements from the auto parts store) are either zinc, chromate, or chrome plated. The plating cannot stop most of your disc rusting simply becasue the main braking surface of your rotor (which is the most visible part of it) looses its coating after your first drive with the new rotors."
If cross-drilled rotors really were better than solid rotors, you would see much more of them on vehicles from the factory. 95% of this "cross drilling helps heat dissipation" comes from overseas vendors trying to sell people their brake products. Auto makers such as Ford spend a lot of money enigneering their brake systems. They know what works and what doesn't.
As for the "logic" of the situation, that is what the marketing people are counting on to sell brakes. First off, look at one of these street-style drilled rotors like a Powerstop. Those holes are not even 1/4" in diameter and there aren't very many of them . Do you really think that an appreciable amount of air is going to move through those holes?
Why the holes do not cool appreciably: When a brake disc is spinning, the hollow passages inside the rotor (the vanes and channels) act like a centrifugal pump. Air from the center of the disc (the hub area) is pulled through those channels and is blown out the "rim" of the rotor. This is how rotors cool themselves down. The interior surface area of these channels is very large, and the vanes themselves act like heat sink fins to carry heat into the air. When the disk is spinning, it literally pumps air through itself to cool itself down. Cheaper rotors have straight vanes in them becasue this design is easier to manufacture. Premium brake rotors have directional or curved rotors. Why? because just like the rotor in a proper pump, curved vanes are more efficient at moving the air through the rotor. When the rotor is spinning, air is moving in a radial direction (like spokes on a bicycle wheel). The drilled holes in a rotor are perpendicular to this direction--they are oriented axially, not radially. Consequently they are pointed the wrong direction for air to be flowing through them. If that doesn't make sense, think of it this way: Pretend you are trying to drive out of a small driveway that has a T-intersection with a very busy street. You cannot move very quickly becasue you have to wait for an opening in traffic before you can get out. Furthermore, the holes have very low surface area compared with the rest of the rotor. Think about the total effective area of a brake rotor: both sides (braking surface) plus the area of all those channels inside the rotor. Now think about the surface area of the inside of a hole. The additional surface area gain by drilling holes is a tiny fraction compared to the rest of the rotor.

Wilwood installation notes:
"Caution on drilled rotors: There is a common mis-perception that rotors are drilled to improve cooling. The reduced mass of a drilled rotor will dissipate its retained heat quicker, but it also builds up heat at a much faster rate. The decision to use drilled rotors should be solely based on the merits of the lower rotating and unsprung weight, and not for improved cooling. It is not wise to use drilled rotors in sustained high heat on hard braking tracks unless the team budget affords a high frequency of rotor and brake pad replacement"

Concerning warping of rotors:

Warped rotors usually happen when a rotor is heated unevenly, if they get too hot, or if they undergo a sudden temperature change.

Changing your pads won't help warping. When you brake, you are converting the forward motion of your car into heat. Changing pads won't change that. Different pads might let you brake harder or with less dust, but pad changes won't affect how how much heat gets transferred into your rotors. That is purely a factor of your car's weight and how fast you were going when you stopped.

The most common cause of rotor warping on the street is if the rotor is hot from braking and then water gets splashed on it. Avoid puddles after braking hard. If you're driving in the rain, take it easy on the brakes.

Another common cause is cheap brake rotors. Some rotors are made of better materials and better designs than others. When you are buying rotors, try and get some with a warranty. That way if they do warp, you get new ones free. Also, if you get the chance to look at them in the store, look for ones with vanes that are curved or angled as opposed to straight. Directional-vane rotors keep cooler than straight-vaned rotors so they rotors are less likely to warp.

Keep in mind that some cases of "warping" are really not warping at all...and instead is caused by having pad material transfer itself to the rotors. This happens when the rotor is very hot and you keep the car stopped with your foot on the brake.

Concerning Hawk Pads:

These offer better high-temp performance compared to stock but they are also very good "cold". Be careful: some competition brake pads are only effective once they are warmed up. That is OK on the track, but it is dangerous on the street. A ceramic type pad would also be good. They also have the side-effect of having less dust.

Size of rotors:

If you want a slotted/drilled rotor and you want it to be at least as effective as a solid rotor, then it must be made larger to compensate for the material lost by the drilling--such as the Brembo, Baer, and similar 'big brake' kits. Those products are excellent. A drilled rotor will have similar or worse performance than a solid rotor of the same size.

SAE test:

The SAE test #J2430 is the current Brake Dyno standard used for cars & light trucks. This test method studies brake torque (this is what most people when when they discuss brake effectiveness), fade, cooling curves, and so on. The paper SAE J2430 Recommended Practice and Its Application for Characterizing Aftermarket Brake Friction Material Effectiveness by Trainor et al. describes an experiment in which several aftermarket brake rotors and pads were tested against OEM parts under this standard. Unfortunately, the paper does not specify brand names. However the results are very clear:

1) Drilled and/or slotted rotors do not have a significant impact on brake torque (braking "power"), hot or cold.
2) Drilled and/or slotted rotors either have no measureable effect on fade, or they demonstrate worse fade characteristics than OEM.
3) Drilled and/or slotted rotors have the same cooling curve as OEM.
4) Various aftermarket PADS do demonstrate significant increases in brake torque, hot and cold.

Keep in mind that this paper tested rotors of the same size.

Why do high performance cars have cross drilled/slotted rotors?

Cars like Ferrari's have very effective brakes becasue they are large. The cross-drilling lightens the rotor to reduce the inertial load of the big brake disc and it looks good too.

The cross-drilling does not give a Ferrari its awesome braking. The giant rotors and calipers do that. The cross-drilling allows them to use such a large rotor without incurring a big performance loss due to added rotating weight. Keep in mind the faster maintenance schedule on high performance car such as high end Porsches and Ferraris.

I called a well known specialty car maker and avid road course racer, in a nutshell he told me:

Slotted: For outgassing. Today's pads much better now, outgassing is almost eliminated, no need for slots

Drilled: Really for looks only. Used to be to save weight, not needed anymore, and actually compromises the integrity of the rotor.

Drlled and slotted looks cool, but not needed.

If you want to improve your braking, here are your options:

1) You can increase clamping pressure of your calipers. This could mean a smaller bore master cylinder, calipers with a larger effective piston area, a higher brake pedal ratio, or stainless flex lines. These things all work to increase the pressure at the brake pad.
2) You can change the composition of the brake pad to increase friction where it contacts the rotor. A more agressive pad will deliver more brake torque. This is also a trade-off however. Aggressive pads cause more rotor wear.
3) You can increase the diameter of the rotor. A larger diameter rotor has more brake torque than a smaller diameter rotor.
Fooling with slots, dimples, and holes with modern pads has no appreciable effect on brake torque. If you want a brake upgrade on a budget, get some more agressive pads and the stainless lines. If that's not good enough, you need to start looking for a "big brake kit" instead.
Now then, drilling does reduce the weight of the rotor, which reduces unsprung rotating weight. And that is a good thing. However, drilled rotors are also prone to cracking and are more likely to fade. You have to decide wether or not it's worth it based on the kind of driving that you do. Also note that "street" drilled rotors, especially the OEM-replacement type (like Power Stop) are drilled so little that you'd never notice the weight that was saved. These people are just out for your money.

My personal choice: Hawk HPS pads, braided lines, high performance brake fluid with a higher boiling point.
I hope this helps clarify some of the mystique surrounding this topic. In the end when the time comes for you to consider upgrading, it is up to the reader to do the research and come to your own conclusions.

Other interesting links:
Wilwood http://www.wilwood.com/Centers/Information/index.asp
Stoptech http://www.stoptech.com/tech_info/tech_white_papers.shtml
SAE Advance Brake Technology http://www.sae.org/technical/books/R-352
Hawk HPS pads http://www.hawkperformance.com/performance/hps.php


Special thanks to CrazyAl and the performance shops I have spoke with regarding this subject.


Notes:
Good info on brake fluid here:

http://www.stoptech.com/tech_info/wp...fluid_1a.shtml


Info on braided lines here:

http://www.stoptech.com/tech_info/wp...teel_Q&A.shtml


Brake fluid:

http://www.stoptech.com/products/brake_fluid.shtml

http://www.raceshopper.com/castrol_srf_brake_fluid.shtml

Braided lines:

http://www.stoptech.com/products/sta...al_lines.shtml

http://www.holley.com/division/Earls.asp

http://www.steeda.com/products/stainless_steel_brake_hoses.php

Steeda and Stoptech lines fit both GT and 4.0 You will need to check Earl's

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