Pros and Cons of Drilled/Slotted rotors

[hotGTO]

I'm not. That's the only part of your post that didn't annoy me.



I hate to break it to you, but that's an opinion.



As I have said time and time again in this thread, the physics are the proof. Nobody has done a proper scientific test between the two, so the laws of physics are the only proof we have which are good enough for me. That is the reason I attacked you. The laws of physics aren't good enough for you. When a gas expands it cools, that's simple physics, but you won't believe it unless I show you some proof. I've even showed that the people that engineer these brakes don't recommend them either, but what do they know?

It's pretty absurd.



1. Well, stock to CD, that's pretty general. If you wanted to do a real scientific test between the two here's what you would have to do.

• You need to start with a control.
• Flush the brake fluid.
• Change the brake pads.
• Replace the rotors with OEM sized blanks.
• Run your stopping tests and checking temperatures.
• Once you've completed those start over.
• Flush the brake fluid again with the exact same fluid.
• Change the pads again with the exact same pads.
• Change the rotors the the CD. They need to be the EXACT same rotors, just cross-drilled. Same size, same metallurgical properties, everything. Just the addition of cross-drilling.
• Run the tests again.

If you move UP in size or get better quality rotors that are CD, yes they will probably stop better and resist fade better than stock rotors. That's why you need to test with the EXACT same rotors.

2. This is getting ridiculous. You need to stop using the word stock, they're called blanks. We've told you that racing teams developed them to fix a problem a long time ago that no longer exists. It isn't a problem that exists anymore for the street or the track. They didn't FIND or DEVELOP a better or newer technology. They simply went back to using blanks.

3. We have told you that out gassing was a problem created by brake pads back then. The holes gave the gas somewhere to go. The build up of gas between the rotors and the pads caused VERY poor braking performance. That is why they developed them. PADS DON'T OUT GAS ANYMORE, THEY HAVEN'T FOR DECADES. That is why racing teams went back to using blanks. How many time do we have to say it?

I'm lamb basting you because these are some of the most ignorant posts I've seen. The information is there, but you refuse to believe the laws of physics, brake engineers, racing engineers. You want a test that shows what is proven science. That's fine, I would love one to. It just irritates me that the other ting hold no merit to you even though most of the rest of the population understands these concepts. Add that fact that you keep refuting a point when the information is right there meaning I have to repeat myself. I mostly referring to the fact that racing doesn't use them anymore.

I'm done with this thread. I know that cross-drilling a rotor reduces it's contact patch with the pad, reduces it efficiency, and reduces it's strength. The laws of physics taught me that those three things means it's a worse rotor that the exact same rotor that's a blank and that it won't stop as well.

I'm sure that will never be enough for you and I doubt anyone is really going to do a proper scientific test of this.

/rant

You can NOT read! I wrote: "When have I said that""I believe drilled rotors are better than stock rotors" ? With a QUESTION MARK!

Edit: It looks like real testing of drilled brake rotors as compared to stock rotors blows your "everyone knows physics" out of the water!

[DV8]

Ain't they pretty? slotted and dimpled......FTW

[i3igpete]

I'm sure that will never be enough for you and I doubt anyone is really going to do a proper scientific test of this.
/rant

Please see my posted link and synopsis. I was surprised by the results, as I'm sure the engineers were. However, the durability problems should not be a surprise at all.

[GordonRamsey]

Please see my posted link and synopsis. I was surprised by the results, as I'm sure the engineers were. However, the durability problems should not be a surprise at all.

I was in a base model GT and it had stock size drilled rotors and upgraded pads, and it just about through my face off. but then again, I wonder how long it would take to crack them.

[hotGTO]

Are the high quality rotors really failing under normal street use? They might stand a good chance of surviving normal street usage since they are included as a factory option on some models. Manufacturers don't like to pay high dollar settlements so they don't usually sell parts that are known for failure as a factory installed option... And they might be listed as only an appearance upgrade because they are only for the street and not mistaken for a race application to be beaten until failure...

[bluemax_1]

LOL, Pete beat me to the punch as I hadn't had time to really post yet. I posted the results of that SAE paper on the 'other' forum back in '07 but I guess not too many folks read it, but Pete summed it up perfectly:

- Crossdrilled rotors actually DO reduce brake temps AND increase brake torque.
- It's not a good idea to use them on track because the holes create stress risers that cause the cracking in the pics Pete posted
- I personally wouldn't use them on any car driven hard even if not taken to the track (etc. mountain runs).
- Crossdrilled are best for street driven cars that occasionally see a little 'spirited' driving

After reading the interesting results of the GM study, I was curious so I did a little more research. One of the key impressions is that the holes didn't make that much difference in radiant area and this is true, but as the compiler of the SAE paper mentions, it's the convective cooling benefits that come into play.

In the old days, when they first started crossdrilling rotors due to pads outgassing, there were a lot of non-vaned drilled rotors. The holes were mainly for the outgassing and didn't affect convective (or radiant) cooling much because the holes were straight through the flat plane of the rotor, and as folks can easily see, the holes are perpendicular to the direction of travel (as the rotor is spinning) and there's not much airflow through them at all. With vaned rotors though, there is a lot of airflow through the vanes because they act in a manner similar to the impellers in turbos and hairdryers. They suck air in at the center/hub and the centrifugal force forces the air outwards (alright, no smartass comments about centripetal vs non-existent centrifugal forces). That means a lot of airflow through the vanes, and because of the big differential between airflow on the inside of the holes (in the vanes) vs the outside faces of the rotors, there actually is a substantial amount of airflow through those crossdrilled holes causing the resultant improvement in cooling.

They also increase brake torque through mechanical means. Although brake pads feel hard to the touch, at the pressures (and temperatures) seen under braking, they are actually malleable. They conform to the surface of the rotor. The holes from drilling (or dimples) and to some extent slots, cause indentations in the surface plane of the rotor and the malleable surface of the pads slightly bows into those depressions. The trailing edge of the depressions increases 'bite' into the pads thereby increasing brake torque. That is also why they wear pads faster. It really does act something like a cheese grater. The hotter your pads get, the more the surface will be affected, but the continual wiping of the surface prevents the pads from glazing over which is the main reason some race teams use slotted rotors (while others stick to blanks). Although modern pads don't outgas the way old pads used to, they can still glaze over.

The reason normally slotted rotors don't cause as significant an increase in brake torque as crossdrilled rotors do is due to the fact that drilled rotors are generally drilled with a pattern that has enough holes that at any given moment, there are numerous edges under the pads to create that increased bite. If you look at the regular slotting pattern in most slotted rotors (usually 6-8 long slots) at any given time, there is very little 'edge' under the pad to create the added bite. This is why some race teams (and brake companies like Baer and AP Racing for instance) have developed different slotting patterns like the numerous short slots.

The newer different slot designs (some of which cost more to machine) use many short slots. One of the designs use 3 'rings' of staggered short slots. Each slot is slightly longer than 1/3 the length of a normal 'long' slot. They are staggered in a way that ensures that the edges of the 3 rings of slots very slightly overlap so the entire swept area of the slots is similar to the long slot swept area. At any given moment though, there are many more edges under the pad surface to provide the increased bite, but if the slots are properly machined (in particular, the ends of the slots must be properly radiused), they do not create the stress risers that cause the cracking that drilled rotors do under racing or severe use conditions.

The other odd slotting design that works even better than the '3 rings' slotting is J-hook slotting (a design, I believe developed by AP Racing). The slots are machined to look sort of like the letter 'J'. The J-hook slots increase pad bite as much as (or sometimes more than) crossdrilled rotors (because they have as much or sometimes more total 'edge' under the pad surface for initial bite), but don't cause the cracking under racing.

This is an example of what J-hooks look like
http://www.stillen.com/product.asp?i...SAN&model=GT-R

The only place that I currently know of that 3/S owners 'might' be able to get J-hook pattern rotors for stock 3/S brakes is KVR. Now I won't lie, KVR rotors are NOT cheap, but I've used them in the past (not J-hooks though ) and they are, IMHO, worth the money. In fact, after moving to 355 Stoptechs in front, I STILL use slotted KVR's in the rear and those puppies are holding up perfectly fine (got them cadmium plated so the centers wouldn't rust).

If Stoptech ever gets around to adopting the J-hook pattern, I'll probably get a track pad set of rotors.

BTW, my slotted Stoptech fronts and slotted KVR rears show ZERO signs of stress cracking even after being run on the track numerous times.


Max

P.S. Forgot to add, aside from cracking issues from serious, severe track use on 'holey' rotors (since it doesn't matter that much whether the holes are drilled or cast), on rotors of a given size, their mass is STILL the mass of the heatsink, and as such, removing the amount of material from lots of holes is still removing mass (and thus thermal capacity) from the heatsink. Thus far, there have been no conclusive tests that have demonstrated whether the improved cooling from the holes offsets the reduced thermal capacity under track conditions (keep in mind that we're talking about rotors of the same dimensions. You can of course, use bigger holey rotors to maintain the mass of the heatsink). IIRC, the SAE paper actually used slightly different diameters of rotors in their tests (can't recall why. Might have been due to availability), but that still doesn't change the fact that they will crack eventually under severe use.

P.P.S. If slotted rotors are chosen, AVOID end-slotted rotors (where the slots reach the ends). Although some members have posted pics of rotors that crack with slots that don't reach the ends (the slot ends were probably not radiused properly), end-slotted rotors WILL crack if used hard.

P.P.P.S. As with motor oil, if you drive like a grandmother in a Buick, use whatever you like. If you don't push the car hard, it won't make much of a difference.

[Jimvr4]

Please see my posted link and synopsis. I was surprised by the results, as I'm sure the engineers were. However, the durability problems should not be a surprise at all.

LOL, Pete beat me to the punch as I hadn't had time to really post yet. I posted the results of that SAE paper on the 'other' forum back in '07 but I guess not too many folks read it, but Pete summed it up perfectly:


Max

Been waiting for the voices of reason on this thread. Thanks to both of you for posting

I used to track my 91 VR4 years ago. I started with stock brakes which faded badly after only a couple of laps. I boiled the fluid and glazed the rotors. Next trip to the track I had upgraded fluid, carbon kevlar pads, and drilled rotors. The new setup was capable enough for the cornering speeds I was comfortable with. When I got my Spyder I experienced brake problems again with stock rotors and compounded by the increased weight. I switched to slotted / dimpled rotors 7 years ago and they're still holding up. Of course I haven't tracked the Spyder so it doesn't see the level of temperatures that would put the rotors at risk. No cracks or visible stresses after 7 years (36k miles)

[Polygon]

You can NOT read! I wrote: "When have I said that""I believe drilled rotors are better than stock rotors" ? With a QUESTION MARK!

Edit: It looks like real testing of drilled brake rotors as compared to stock rotors blows your "everyone knows physics" out of the water!

Please see my posted link and synopsis. I was surprised by the results, as I'm sure the engineers were. However, the durability problems should not be a surprise at all.

I have to question that. I kind of have to take his word for it. He doesn't show his testing parameters. There's no talk of a control. What rotors were used, were they identical save for the cross-drilling? Did he use a fresh set of brake pads each time and were they the same. Was this on the same car, or perhaps testing rig? Did he also change the fluid after each test? Like I said, I still have yet to see a PROPER scientific test that proves physics wrong. So, no, that doesn't blow physics out of the water.

Are the high quality rotors really failing under normal street use? They might stand a good chance of surviving normal street usage since they are included as a factory option on some models. Manufacturers don't like to pay high dollar settlements so they don't usually sell parts that are known for failure as a factory installed option... And they might be listed as only an appearance upgrade because they are only for the street and not mistaken for a race application to be beaten until failure...

No, those failures should not be an issue with street driving. At least they shouldn't as long as people don't drive like an asshole everywhere. Small cracks might start show up but I highly doubt you'd get actual failure from street driving on GOOD CD rotors. The cheap eBay crap, well, those could do anything but simply because they're cheap.

[mb7050]

good posts .

real brakes / discs

1st


J-hook pattern rotors been used in rally cars for years its proven to be the best desing so far .

[bluemax_1]

Polygon,

yes, it hurts to have your theories smack you in the face (I'm sorry, even when I didn't have time to post yet, there was a bit of perverse pleasure in watching the arguments and noting the holes folks were digging for themselves). But SERIOUSLY? If you want to look at the SCIENTIFIC test results, why don't you pay for the SAE paper yourself like Pete and I obviously did. It's not as if it's THAT astronomically expensive (something like $14.95?).

I only posted my findings from it on the 'other' forum because I didn't want to reproduce a copyrighted article.


Max