BrakeExpert

This post is about why blank rotors are better than cross drilled or slotted rotors in the name of performance.

The reason for the creation of cross drilled rotors initially was to remove the "gasses" from the brake pads. HOWEVER, most of your modern brake pads (Axxis metal, AEM semi-ceramic) do NOT produce gasses when heating. This was on bad brake pads used in the 1950s and 1960s. Back then, asbestos was also used, and we dont use that either.

The other reason is so called heat dissapation. I don't have my physics and thermo books with me, but the logic is that the holes in the rotor are suppose to allow the brake pad to cool. So...air gets into the rotor from the inside of the vents. If you have a back rotor which is solid, air gets into these holes how? If your stopped, you are leaving air inside these holes sandwiched between the pads, thus creating air with a rising temperature. Its increasing in pressure from the heat, which I guess you "could" call a gas that would affect braking. So the cross drilled rotors do not remove any gasses formed by brake pads (because there are none created anymore) but could possibly inhibit the creation of "hot spots".

Cross drilled rotors have LESS contact area because of the holes.
But if the rotor is cooler, its better, right? Well no, because these rotors are not cooler. The heat is generated from the pad/rotor contact. What removes heat the most effectively? When stopped or moving, the pad transfers heat into the rotor because its made of cast iron. the rotor has a lot of surface area and even vanes in it. But the little holes allow air in this surface contact, and you can transfer more heat into a solid big ass chunk of cast iron more than you can into the air. Don't believe me? Touch some steam at 150 degrees, then touch a piece of hot metal which is at 150 degrees. Which burns your hand? the metal. So let the heat transfer into the metal, because since it has so much more surface area, dissipates better.

Safety!!
Cross drilled rotors can crack! I have seen them!!


Even racing teams will recommend AGAINST cross drilled rotors:


Companies that sell cross drilled rotors that are redrilled may not be structurally sound. I have actually seen pictures of rear Integra rotors that have had hairline cracks turn into the rotor actually breaking apart!

Do your homework. Even Porsche and Ferrari will admit that the cross drilled rotors they use are for looks. So if you are one of those kids who thinks the little holes look cool, get a name brand drilled rotor like Ferrari does. The REASON Ferrari's 'holed' rotors are alright to use is because they are CAST with the holes in them, so they are not actually drilled into cast iron rotors. Cheap drilled rotors are not safe, and even the good ones are not necessary. Why do Ferrari do it? People THINK they want it, and it sells. If you don't believe me, go into the business world. You will learn that pretty soon, you can sell utter s-word if people THINK its better.

Information I gathered from http://www.pdm-racing.com/prod....html
says:


"KVR Crossdrilled Rotors

Why should you upgrade to cross drilled rotors?

Simply stated, the function of any vehicles brake system is to stop the vehicle. This is accomplished by absorbing the kinetic energy stored in the moving vehicle, and converting it into heat. The friction caused by the brake pad rubbing on the rotor is the source of this heat. The more quickly and efficiently that heat can be absorbed and dissipated, the more quickly and efficiently the car will stop.

There are several contributing factors to this heat reduction. One of the most common sources of heat is from the gases produced by the bonding agents of the brake pad burning off. Under severe braking, this can actually produce a boundary layer of gas that pushes the pad away from the rotor, which can lead to excessive brake fade. The cross-drilled holes or slots in a rotor provide an escape path for these gasses (de-gassing or out-gassing are common terms), and allow the pad to stay in contact with the rotor. As well as de-gassing, cross drilling or slotting will provide better wet weather braking as water is swept through the holes, or down the slots.

A vented rotor can be viewed as an air fan. When in motion, the vents draw air from the center of the rotor outward. This air flow, over an increased internal surface area, effectively dissipates rotor heat. Cross drilling adds to this air flow, as well as providing additional rotor surface cooling. "

This company is just telling you that the rotors may be cooler, however they fail to mention that the holes really do create a more than substantial decrease in surface area, thus less braking, thus less heat created, thus the less heat CREATED will leave the rotors cooler, the holes barely do anything! Its the less braking lowering the temperature!

Slotted rotors-
Find me a company that uses stock slotted rotors. They remove brake dust, but if you study braking systems, you find that with modern cars, flat blank rotors and semi-ceramic pads, the brake dust causing the rotor to slip on it is almost non-existent. But the brake dust doesnt need all those lines. Notice how most front brake pads (and most back) have that line down the middle to give essentially two bite points. If OEM or racing companies found it to be a benefit, they would do it.

PROOF OF IT ALL:

Find me an F1 car as of now that uses cross drilled or slotted rotors.
They all use carbon fiber rotors and ceramic pads. Are they drilled or slotted? No.

If they helped the fastest cars in the world, wouldn't they use them? Its basic calculations that show the lack in surface area does not make up for the possible loss in temperarure. They use brake cooling air ducts insted.

BIG BRAKE KITS:
Some have asked if the big brake kits are worth it. This is sort of a relative question, but the simple answer is no. Regarding the big ones with drilled rotors, if you know that they are cast that way, at least they wont crack. I will still advice against them.
In terms of a big brake kit, I have seen some for Civic DX models. Civics have the small pad, small caliper, and a 9.5" rotor. The big rotors are 12" in diameter, ok so the overall diameter is close to that of an Acura RL (1999). But the sweeping area (the area that the pad can grab) is still the same if they use the same caliper and same pad. If you have the same pad and caliper, you are using the same rotor surface, just farther out, so it will increase braking from stock. However, if you were to change knuckles, etc, and get Acura RL caliper (larger piston than your civic DX piston), RL pads (much bigger and taller), and RL rotors 11.8" but much more surface area is touched, then you have a better brake setup because you have OEM parts, and a better grip on more area of the rotor. The downfall is added weight (since big brake kits are usually 2 piece and lighter) but the benefit is that you have so much more stopping. Ok, so the big brake kit will have less unsprung and rotational mass (so a little better accelleration but less braking), but they tend to run over a grand, and you can use OEM parts to build a better setup for half that.

IN CONCLUSION:
Don't buy slotted or cross drilled rotors, blank are better, and stop better. Physics people, get me my formulas and help me out here.

If you must get rotors with designs on them, get the slotted ones by a good company, and DON'T get blank rotors redrilled with little holes all over them. IF you absolutely must have the rotors with holes cause you like em, get them from a company that casts the rotors like that. I have seen rotors break and this is for your safety. On many cars that I have seen, it seems the solid rear discs that are cross drilled are more prone to cracking than the front, this is on Integras and Civics, so don't think that the S2000 rotor is any less likely!

REMEMBER......
Your car will only stop as fast as your tires will allow. All the braking in the world wont work if your tires are bald and on ice!!

cenix

good writeup i like it, and very true.

regarding cross drilled rotors: one thing i might want to add or discuss was, a reason why a lot of auto manufacturers were adding drilled rotors to their lineup was not for any additional dry braking benefit, but rather, to help evacuate water better in wet driving/braking conditions. the manufacturers themselves know this type of setup is not a particularly advantageous setup for hard racing applications, but that it also provides a "sporty" look to the cars themselves as you've mentioned.

Stealth Operator

Agree that cross drilled rotors aren't optimum, but beg to differ that proper engineered correct directional 2-piece slotted rotors are worse than blanks. In fact, I don't believe there are any general 2 piece-blanks.

Carbon ceramic is in another discussion pool altogether. But for normal steel rotors on our cars, the biggest problem is heat under competition/extreme use. And no single piece blank will be more optimum than a directional 2-piece slotted rotor.

Neutered Sputniks

The only real benefit to slotted rotors is that they keep the pad surface clean - so if you braze your pads, it cleans that off. That's it. And that's, really, pretty minor.

Blanks > Slotted > Cross-drilled

CourageOO7

All of that is true when comparing cross drilled to slotted rotors but your F1 example doesn't apply. F1 cars weigh less than 1500lbs. Stopping a GT car is totally different. Yes, F1 cars have a massive power/weight ratio advantage...but they are so much lighter that getting heat into the ceramic brakes is actually a difficult task. I suspect that, even at 800bhp and 1500lbs, F1 brakes benefit more from the increased friction area than they would from the additional ventilation...I mean, they are open wheel cars, no more ventilation needed.

I was just pulling up examples of cars with what are considered excellent brakes but short of ceramic. The 911 turbo (996), C6 Z06, Ferrari's, etc...
I was looking for a car without drilled brake discs. I cannot find one. I think this is partly aesthetics. They do look cooler. But, made right, I wonder if there's some benefit to drilled brake discs. I bet initial bite is better....pad life certainly degrades which should mean increased friction, right? I wonder if a high quality drilled disc actually does perform better on a car weighing some 3000lbs.

ZDan

I hesitate to even get into this, because I don't think drilled/slotted rotors are the be-all/end-all of braking, and many of them *are* purely for styling, and for the S2k and my 240Z, I only run blank-faced rotors.

BUT, there is a lot to take issue with here:
The brake pads I use create dust and, surely, *some* gases when being used at the track. For sure, as the pad and rotor are abrading each other away, material *is* being removed from them, and it's gotta go somewhere. Not to say I think holes or slots are strictly necessary to evacuate it.

That may be part of the idea, another part is that holes in the rotors increase surface area (as long as hole diameter is less than twice the rotor face thickness), increasing the rate at which the rotor can dissipate heat.

Yes, but that doesn't imply less braking for a given clamping force.

What if the pad is at 900 degrees, the rotor is at 800 degrees, and the air is at 100 degrees?

Solid rotors can also crack, and I have seen them. Everybody knows that if you track even solid rotors long enough, cracks are inevitable. Of course holes give them a good convenient place to start... Agree that drilling rotors is a bad idea, and they will crack MUCH sooner. But to someone who has cracked a buncha blank-faced rotors, to see the alarmed statement that "cross drilled rotors can crack!" seems kinda funny.

Linkie to Porsche and Ferrari "admitting" to this? Porsche claim that it is for improved braking in the rain, btw, which jibes with my understanding of why motorcycle rotors are all drilled (direct exposure to rain).

You're declaring that it is pure marketing doesn't make it so. I am not ready to categorically accept that the holey ceramic rotors on Ferrari, Porsche and Corvette ZR1 models are *purely* marketing just on your say-so.

Totally disagree with this point. The braking will be the SAME, for a given level of clamping force (i.e., brake line pressure, i.e. effort at the pedal), whether there are holes in the rotor or not. The lost hole area is made up for with increased average pressure over the remaining rotor area. Contact area does NOT directly impact frictional force. It's determined by clamping force times coefficient of friction, regardless of whether there are holes in the rotor or not.

F1 carbon/carbon brake rotors not having any surface features does not constitute "proof of it all" for cast-iron or even Ferrari/Porsche/ZR1 ceramic rotors.

Swept area goes up with the increase in working radius.

I only get blank rotors, but they don't "stop better". Drilled or slotted will stop just as well (possibly better in the rain). They just cost more and will crack sooner (drilled for sure, slotted, probably).

Agree.

Agree. Getting the best tires for your usage is going to benefit braking performance WAY more than any cheese-grater/food-processor-blade looking brake rotors!

Voodoo_S2K

So many easily debatable points make here, especially given the self-proclaimed title of "BrakeExpert".

BrakeExpert

There is more to discuss, but I felt I'd get the ball rolling.

There are companies that admitidaly put these on for aesthetics. Porsche admits the use of them for consistent lap times. But yes, markieting is a big factor, a huge factor. If someone wants to buy a 300,000$ car and they are convinced they want drilled/slotted disc, you betcha the dealer would spend a few bucks to give that guy the rotors he wants. If I were buying a car like that and wanted a certain part a different way or color, and money were no object, I'd pay to have it done. I'm not saying that its ALL marketing, as I said, there are companies who will make the slots sharp to allow brake dust to evacuate, and the holes cast in to reduce the risk of cracking on a roadcar's normal generation of heat into that disc.

There are 2 piece rotors with blank rotors, I personally run them, although my aluminum hats and blank curved-vane discs were bought at two different times and I assembled. them. I will be back later to defend what I've said, and this post was old and from the old h-t, I'll reword some things. We'll talk physics.

Sorry if I sound cocky or like a know it all, I don't mean to.

As for the cross drilled verse blank cracking, yes, both rotors can crack given ENOUGH heat.

BrakeExpert

But as stated the bigger issue of cross drilled. This is effects of heat.

Say you hit the brakes from 65mph on a road and come to a stoplight. At the light, those holes are allowing the pad to touch the air inside that hole. This is transfering the heat from the pad to the air. If it were a solid disc, it would transfer the heat into the rotor. The rotor is a bigger heatsink, so much bigger that not having the holes is more effective (someone wanna get me a formula to help?) What does happen is that at that stoplight, as the brake cools, the part of the pad with the hole will not expand or contract at the same rate as the rest of the pad. This is on a very small scale, but this is the same thing that occurs when you bed in pads and get em hot and then come to a standstill. The part of the rotor exposed to the air cools at a different rate than the part coverd by the pad. This part can be thicker ever so slightly that when you take off and hit the brakes very soon after, this part is raised and the pad still wears it. This can create warped rotors. If you draw rings on the rotor where the holes are, you can see on some cars with cross drilled rotors that run really hot, that these rings are actaully noticable, and these holes can cause the rotor's thickness to vary more.

Maybe I'm not clarifying properly, but cross drilled discs can increase warpage.

One thing drilled/slotted discs can do is because of that reduction in contact, the brakes can warm up faster. For some on the street, this may be preferable, but I don't feel that pro outweighs the cons.

I apologize again if the first post sounds too childlike, it was meant more as a rant to the ricers at the time, less than scientific.

ZDan

For one thing, I could give a rat's arse what happens while I'm stopped at a traffic light after braking from 65mph, driving on the street. That is such a light-duty situation, nothing remotely like tracking a car. It's hardly like that situation is going to push the limits of a braking system, and that holes are going to cause problems.

If you're worried about different parts of the surface of the rotor heating up differently, leading to high and low spots, you must hate vaned rotors as well. After tracking stock or stock replacement rotors, I can see exactly where the internal vanes are due to heat discoloration on the surface.

Don't necessarily agree with your proposition that drilled/slotted rotors warm up faster, no reason that should be as far as I can see.

Thanks anyway, but your hypothesizing about the possible positives and negatives of drilled rotors don't to me sound like the pontifications of a "brake expert".