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:idisagree:Not 100% correct. You guys want the truth from an engineers point of view.

First and foremost, the purpose of the brake rotor is as a surface for the friction compound (brake pad to act upon) and as a heatsink. The brakes slow down/stop the car simply by converting kinetic energy into heat energy by means of friction. The reason rotors are big heavy things is because they need to be able to absorb the energy in the form of heat. The main reason for Big Brake Kits is not really to increase braking torque (how much force the brakes can retard wheel rotation with) but simply to install bigger rotors that can handle more heat. If you wanted to increase braking torque ONLY, you could do that simply by using higher friction coefficient (known as Mu) brake pads. On street tires, all that would do is make it easier to lock up your wheels when braking.

How well a brake system (not looking at the brake pads yet) handles heat is affected by how much heat the rotors can absorb and also how quickly they dissipate the heat they do absorb. The more mass, the more heat they can absorb, the better the airflow over and through the rotor, the more effectively the rotor can transfer that heat to the ambient air flowing over it.

Vented rotors make use of this by increasing the surface area and airflow and directionally vented rotors are even more effective. A vented or directionally vented rotor acts in a similar fashion to an impeller blade/wheel (like a turbo compressor or hairdryer). The spinning action forces air outwards from the center and it induces airflow through the rotor with cold air being sucked in at the center and the hot air being flung out the edges. Vented rotors dissipate heat much better than non-vented rotors and directionally vented rotors improve the effectiveness.

Crossdrilling on the other hand does NOT show appreciable cooling differences when in use. They will help cool the rotor qucker when the car is stopped and sitting but they actually REDUCE the heat capacity of the braking system when the car is being driven hard maing the brakes actually run a little hotter. So you're thinking, "Wait a minute,... say what? If increasing surface area means better heat dissipation, then more holes means more surface area, so it should help at least a bit right?" Wrong.

The crossdrilled holes do not act the same as the vented passages in the rotor. The venting is radial to the rotation and draws air through the vanes. the cross drilled holes are axial to rotation. The difference in airflow when you're blowing through a straw (the vented vanes) as opposed to blowing across the hole of the straw is huge. Now you're thinking, yeah, but they use devices like paint guns that work by drawing paint out of the bottle by blowing compressed air over the thin tube. Well, you're talking about Bernoulli's principle here.

Fast moving air has lower pressure than slow moving or stagnant air. The fast moving air over the tube is at lower pressure than the air in the spray gun bottle so the higher pressure pushes paint out and Voila, you're apinting your car/house/whatever. The principle doesn't work when the air is moving fast over both ends of the tube (in this case, the hole drilled through the rotor. There is no significant airflow through those holes when the rotor is spinning and any airflow is FAR less tha the airflow through the vanes in a vented rotor. Ah... now you see,... but wait, wasn't there a mention of higher temps due to cross-drilling? Even if the airflow is minimal in the drilled holes, why would it cause higher temps? Because it reduces the mass of the rotor, thus reducing it's capacity for absorbing heat.

What happens when you use your brakes hard? Well the brakes get hot obviously. But what would a brake rotor temperature chart look like? Well, upon first application of the brakes, the temp rise is very rapid and increase as long as the brakes are applied and the wheels are still turning (friction still producing more heat). then when you let off the brakes, the temps level out and start to drop. They drop faster if the car is stil moving since there's more air flowing over the rotors, but they don't completely cool off that rapidly. if you're driving hard and braking a lot, the rotors never cool off, but each subsequent braking event adds more heat to the rotor potentially before it can dissipate it. This is where rotor size and thermal capacity comes in.

if you plan on driving hard and doing a lot of hard braking, you need to make sure you have enough rotor mass to absorb all that heat because if you don't, you could exceed the thermal capacity of the rotors to act as heatsinks. When that happens, your rotors simply can't dissipate the heat as fast as you're putting them into the rotors and since any guven material of a set mass has a specific thermal capacity, once you reach this point, the rotor cannot efficiently absorb the heat anymore and the brake rotor and pad temperatures will rise very rapidly with each application of the brakes. The temps shoot up fast and high enough to easily exceed the MOT (maximum operating temp) of the brake pads and you get pad fade. You can also get things so hot that it boils the brake fluid in the calipers and this is also where it's possible to actually warp rotors. You can identify a rotor that has really been warped. It changes color. You'll see a bluish boundary somwhere between the rotor and hat as the heat at that point changes the molecular structure of the metal. The heat stress is visible.

Cross-drilling = less mass, = less capacity to absorb heat before this point is reached. when you're running close to the edge of the thermal capacity, the brake system will be consistently a little hotter because it's easier to push the system with less mass and thermal capacity over the edge.

There's also the problem that cross-drilling causes stress risers on the rotors. coupled with the thermal expansion that occurs during braking and repeated hard use, the holes make it much easier to develop deep stress cracks in the rotors. There's a reason NO race team that uses iron rotors has cross-drilled rotors. They use either plain or slotted rotors. The cross-drilling simply reduces the thermal capacity AND weakens the rotor courting the possibility of failure from stress cracks actually leading to a fragged rotor.

 

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My brakes on my car are not that great , and my car only has 8000 km , I think it's to do with the 16 inch rim and big side wall just havering 18 inch low profiles would improve my braking dramatically , but my ride would be compromised and suspension components would wear quicker ,
 

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Just my $0.02, ceramic pads on a car that wont see north of 160mph is a complete waste of your money, and I'm here to explain why.

Ceramic pads have less bite/braking power as non ceramic, which means that you will actually lose braking power compared to a decent quality non ceramic. The main function of ceramic pads is that they manage heat MUCH better than non-ceramic, which is why you see them on all the crazy top of the line sports cars, because when you're braking from high speeds you need brakes that wont fade.

Now, as for what pads you should buy, I HIGHLY recommend Hawk HP or HP Plus pads if you really use your brakes.
Just to clear up a bit about ceramics, its not that they "handle" heat better they dont bite the rotor untill a certain temperature. they more so silp on the face and dont do anything. thats why non ceramics are better for stopping unless like you said youre slowing down from a high speed.
also there is no brake fade with disc brakes. thats a drum brake thing. as a drum heats up its fades away from the brake lining and then loses braking force. a disc as it heats up expands and pushes into the pads actually increasing the brake force.
 

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there is no brake fade with disc brakes. thats a drum brake thing. as a drum heats up its fades away from the brake lining and then loses braking force. a disc as it heats up expands and pushes into the pads actually increasing the brake force.
What he said ! I'm sure the fade you will get will be the brake fluid boiling or the pads melting ( breaking down )
 

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So I take it dimpled, and drilled are no different as far as causing stress cracks?

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Hey who is the engineer you're quoting? I can only find it as some Max at 3si.org (3000gt forum). Would be nice to finish that read.

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Discussion Starter · #29 ·
Hey who is the engineer you're quoting? I can only find it as some Max at 3si.org (3000gt forum). Would be nice to finish that read.

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my friend is an engineer for GE but he used to work for a company that developed amongst other things brake materials. That meant countless hours of testing or driving around in a brand new Tahoe and slamming on the brakes from 75mph.

His explanation was even more in depth which why I only posted a piece from another site. Sorry I meant to include the link - Cross-Drilled Rotors Myth - Xtreme Import Performance
 

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If you go up a page from extreme import, it states that write up was taken from 3si.org. so the extreme link is not where it originated. I'm curious to see where that write up came from.

-This is something important that everyone should know and understand.
It is a post made by a very knowledgeable user on 3si.org about
cross-drilled rotors
Cross-Drilled Rotors Myth
Knowledgeable user does not necessarily equal engineer.
Now I appreciate this write up but so far it is just a well written piece with little documentation. I can appreciate you have a friend who works in the field but I find opinions on brakes vary to greatly to be satisfied by one well written article that has only circulated the net with no substantiation.

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Discussion Starter · #31 ·
Not opinion this is fact. My friend verified and agreed with everything stated - I actually confirmed with him.
 

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With lack of substantiation though. You're just telling me a buddy of yours(unnamed, no credentials) states this to be fact. Sorry but however a great read it's nothing more then that without documentation.

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I've been reading nothing, but good reviews on the Disc Italia slotted rotors, and pads. I'm giving them a shot on my Wife's Cruze next time minus the ceramic pads. Nothing else has worked so why not give them a shot. I've had great luck with slotted before.

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Discussion Starter · #35 ·
Read that and you'll see how bad drilled rotors are for our cruze (any daily driver). Slotted rotors are gaining popularity but I don't trust the integrity of these rotors and not too mention the add cost/benefit is just not there.
 

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Great read GN, thanks for that. **** had me smiling and highlighting the whole time until the bottom of page 24 where he states NO GOOD for Daily driver. .... **** lol.

However I would like to add the following table from his summary that supports Slotted rotors.

Text Font Line Number


However I am not convinced a truly attentive driver such as some of us, would be in a higher risk category due to the fact that we pay attention to our vehicles we look for things that are wrong and out of place. Cracked rotors to myself and others would stand out.

His biggest reason for them being bad for daily or spirited driving is due to lack of inspection and attention to the rotors. He also notes no added benefits in daily driving seeing as we won't see the braking situations like racing.(Not your once a month track day) but in most of the studies he provided braking was better, cooling was better and performance was better on the cross drilled and slotted rotors.

Also of note is as rotor technology goes away from Iron and towards carbon ceramics and other materials the worry of rotors cracking and failing due to the drilled and slotted will be gone.

So I take away from this, if you are going to run Drilled and Slotted, inspect your rotors if you do any serious heavy braking, such as coming down a mountain driving like, well I do.

Thank you again GN for posting that. I advise all to really read that investigation.

Zach
 

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rotor material is very important and each has their place. I work for bendix and for those who dont know we manfacture an air disc brake. our rotors are about 17inches in diameter 1.75" thick. Depending on setup they will last a over the road truck about 1 million miles and we do condone turning the rotors becasue it takes away material which is out heat sink. the rotors cant absord anymore heat and then the brake becomes in-efficient. but drilled and slotted rotors do have an advantage becasue they can dissipate heat quicker becasue they have more surface area but less mass so they cant absord as much heat. I personally like the looks of drilled and slotted rotors and will prolly put a set on my car down the road
 

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Discussion Starter · #38 ·
However I would like to add the following table from his summary that supports Slotted rotors.

Like I said, these are becoming increasing popular which makes me nervous. If the slots are not machined correctly or if the rotor material is ****, you're going to have problems. The other issue that I have is that typically slotted rotors cost a more and for us Cruzers, I just can't justify that. Lets look at this a little more:

DISCITALIA SLOTTED - $235/pair
EBC USR SLOTTED - $205/pair
POWERDISC SLOTTED - $190/pair
EBC® - RK Series Premium OE Replacement Rotor - $130/pair

Centric® - Premium Brake Rotor
- $61/pair (my current choice)

Just looking at the above are the cheapest slotted rotors (that I would recommend - I realize you could eBay a set from china) worth that much more than plain rotors?
 

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If the slots are not machined correctly or if the rotor material is ****, you're going to have problems.
This is assuming that there is no QC\QA. I have a little faith in the better brands out there, like stoptech, Adams Rotors, and EBC, Hawks etch... to market quality parts.

What your state above can and does apply to even regular blank rotors, pads, plugs etc...
You just cannot bias your rotors due to that if your dont bias everything else you buy.

Those are actually pretty fair prices you listed.

I bring with me to Cruzetalk a Motto from the Audi forum I am on.

""""Buy Nice or Buy Twice"

keep that in mind on any purchase.
 

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I've done a couple hundred hours in a street prepared Miata (track class for timed events). When I first set up the car, I did slotted rotors and EBC green pads. I learned a painful lesson with those brakes. First, the slots tore all the material off the pads, and second, I exceeded the operating temperature of the pads.

To get nice and consistent lap times on a 30 minute run, I went to slightly larger discs, with no slots, and Cobalt pads. They were rated 100 to 1100F. While not ideal for driving when it's cold out, just using the brakes once would get it into it's temperature range.

Having read what Giant posted, I agree that your are best off with a standard disc and high temperature pads. You want to match your pads to your braking duties. For instance, 30 minutes on 11" discs on a 2200lb car with 1100 degree pads was pretty good. This setup could easily handle 30 minutes of hot lapping. But I wouldn't want to double that time.

Think about what you are doing with your brakes!

If you are still on stock brake fluid, then you don't need better pads or bigger rotors. The longest and hardest I ever used brakes on the street was for 10 minutes on a mountain road in the middle of the night. And even that was only 50% of what a HPDE or Timed Event would need.

I have never had my brakes get hot on the street, mountain driving or not. You want to see hot brakes? Take note the next time you follow a bedding procedure. THOSE are hot brakes! And I am a far cry from that in any street situation.

And because of that, I'll be putting slotted/cross drilled rotors on my car. Because I don't heat them up enough to even get remotely close to their limit.
 
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