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Drilled, slotted brake - break down please

5K views 56 replies 21 participants last post by  j 
#1 ·
Dubbers
Can someone please breakdown (besides how they look;)) what difference drilled, slotted and drilled/slotted rotors is? Is one better for regular driving as opposed to driving hard, yet not racing?
Cheers
 
#3 ·
Drilled/slotted rotors are great for competition applications where you need all the cooling you can get. For the road and for long life applications they may not be such a great idea because

a) They can wear through pads at a much higher rate

b) Over time (and especially with cheap disks) they can crack. Drilling disks is something that should be done with great care and by someone who really knows what they are doing.

c) A mistake that's often made with road cars is to put the most cooling efficient disks and hardest pads possible on them - cos their race parts innit?? The problem is that brakes actually need some temperature in them to work at optimum so over-cooled brakes and very hard pads are not necessarily the best idea for road use at all.

Unless you drive like an absolute lunatic ALL the time the standard B5/B5.5 rotors are fine together with maybe some slightly harder than standard pads.

Correct maintenance of your brakes (i.e. good quality fluid and regular fluid changes) is very important for you to get the most out of them as well.

I.
 
#4 ·
There's no such thing as overcooled brakes. And the drilling does absolutely nothing for cooling. You actually increase the overall temperature by removing rotor mass.
 
#6 ·
stealthx32 is exactly right. Cross-drilled rotors do nothing in the way of cooling. In addition, cross-drilled rotors are not recommended for and in numerous instances prohibited from track use due to their inherent issues. The mis-information around these rotors is largely due to marketing efforts and often forwarded by those that haven't looked into or don't understand the issue.

I tried to bring up the issue, beginning in the lower third, of this recent thread (I also included a few links):
http://www.passatworld.com/forums/showthread.php?t=172644
 
#7 ·
Interesting... I have had a set of Zimmerman cross-drilled on (with Mintek Red pads) since 55000 miles (just crossed 107,000) with no problems. In fact, today during removal of my coilovers (RIP B5), I took a good look at all the rotors and pads and seriously considered taking them off and putting the stock ones back on- had I had more time, I would have.

I certainly cannot imagine my pads were wearing prematurely due to the drilled rotors- I still had nearly 1/4 inch on all sides (very even wear). Granted most of my miles are highway (not much braking- mostly constant freeway speed for long trips), but I have been nothing but happy with them.

As for removing mass and causing the brakes to heat up, I seriously doubt the amount of mass being removed significant enough to measurably reduce the amount of heat the brakes can contain... whats more, most any brake will "heat-soak" in a few serious stops anyway. They need to dump that heat to the environment and increasing the surface area is a good way to increase this capability. However, I doubt the surface area increase is enough to change the dump rate significantly, either...

An example: heat sinks for computer chips are not getting bigger to contain the heat- they are increasing their surface area to dump that heat into the air more effectively.

but, hell... they sure look cool!
 
#10 · (Edited)
j said:
As for removing mass and causing the brakes to heat up, I seriously doubt the amount of mass being removed significant enough to measurably reduce the amount of heat the brakes can contain... whats more, most any brake will "heat-soak" in a few serious stops anyway. They need to dump that heat to the environment and increasing the surface area is a good way to increase this capability. However, I doubt the surface area increase is enough to change the dump rate significantly, either...

An example: heat sinks for computer chips are not getting bigger to contain the heat- they are increasing their surface area to dump that heat into the air more effectively.
Rotors need to do two things well. First, they must disperse heat. Second, they must dissipate that heat. In addition, they must have the appropriate amount of surface area.

The mass of the rotor assists with heat dispersal. It takes much more energy to heat a large item than it does to heat a small item. As brake force is applied and things heat up, the heat is able to be distributed throughout that entire mass. If you remove some of that material, which is the case when rotors are cross-drilled, you not only have removed material that assists with dispersal (as stealtx32 also mentioned, this may actually result in increased temp) but you've also reduced your swept area (cross-drilled rotors typically have ~85% or so of the surface area that a solid/blank rotor will have – 15% less). So, decreased mass results in a lesser ability to distribute heat energy thoughout the rotor, thus making it less "efficient" at dissipating heat, which in turn results in increased rotor/pad temperatures and reduced braking performance. In addition, there is an additional point of heat stress, which of course means it’s a risk to structural integrity. If you look at used cross-drilled rotors from a race (this is becoming rare...not to mention they may use different materials) you’ll notice heat cracks originating from the holes (you’ll also notice they throw these away after just about every race...how practical is that?).

Vented rotors (pretty much all rotors today are vented) were designed to assist with the heat dissipation. They assist in the removal of heat from the metal and the pad while maintaining the surface area. Cross-drilled rotors have holes drilled into each face of the disc, either right through, or into the vents (in most cases manufacturers are just drilling into a blank). This does nothing to increase surface area for cooling. I like the analogy from one of the links I post earlier – which does a better job of handling heat, a cast-iron kettle or a pizza pan with holes in it? The original idea behind cross-drilled rotors was to allow gas build up from the pads somewhere to go in order to prevent brake fade (a much larger issues several decades ago), but this comes at the cost of surface area for the pads and thus less braking force. If manufacturers claims are true, then given the modern compounds used in brake pads, there is no need to cross drill rotors.

As for the swept area, the more friction that can be generated between the rotors surface and the brake pad, and greater energy transfer (stopping power) can be obtained. A smooth surface will help insure that more of the pad surface area is in contact with the rotor.

The only way a cross-drilled rotor will make a difference in overall braking is if you increase swept area and/or increase the mass to make up for the material that was removed. Again, you need to account for heat dissipation and maintain braking surface area.

You are right on this point though, cross-drilled rotors certainly look nice…
 
#11 ·
J-rad, Im not arguing your logic... only how noticable the results will be. Removing the mass by drilling will CERTAINLY not have an appreciable change in mass, AND the rotor, drilled or not will "fill" to its heat capacity in only a few hard stops, anyway; effectively heat soaking. I also agree that I am decreasing my swept area by drilling or slotting, but I counter that with allowing the gassing someplace to expand- both are probably negligable anyway.

Further, I agree that drilling increases the surface area insignificantly, so I am not claiming any gain.

However, once the brakes reach maximum temperature, there is only one place to dump the energy- the atmosphere. Can we agree that ALL brakes will reach some maximum temperature? Once this occurs, the energy has nowhere to go but dump to the atmosphere. To increase this dump you can increase the surface area.

The pizza pan/cast iron kettle analagy seems a bit exaggerated; though your point is taken. I accept the pizza pan will heat up much quicker, effectively maxing its capacity faster than the cast iron kettle, but which is more effective at dumping the heat to the atmosphere?

Here's how I see it:

The heat from friction will travel throughout the rotor (and any attached mass) VERY quickly through conduction. The bottleneck is certainly during the release of that heat through convection and radiation. An effective way to increase this is increasing the surface area.

Also, most "big-brake" kits are pretty happy to disclose the relative light weight of their rotors- case in point:

http://www.ecstuning.com/stage/edpd...agen&model=Passat B5&submodel=FWD&engine=1.8T
 
#12 ·
The ATE slotted rotors seem to work well with Hawk pads, but are recommended against by people I know who have them unless you are doing some serious driving. Slotted rotors just eat up pads faster (good for intense braking, bad for longevity or normal driving). Stick with plain-face unless you plan on hitting the auto-x circuit all the time when you move here ;)
 
#13 · (Edited)
j:
You're missing a couple very important points. One, the structural integrity of a cross-drilled rotor is significantly less than that of a solid rotor, for the reasons I mentioned above. As an example, you've mentioned heat soak after repeated braking a number of times. Given this added stress, which rotor is more structurally sound? When the cross-drilled rotor fails how will that impact braking performance? That is why many race circuits prohibit them. Second, there is a decrease in performance due to a lesser ability to effectively disperse and dissipate heat. Simple as that. The impact of that decrease is dependant on a number of factors. However, in a cold braking situation, which accounts for most daily street driving and heating is not a factor, the impact to stopping power is noticable simply due to the decreased surface area.

The point is, that contrary to the popular belief that many have, cross-drilled rotors are not a performance enhancement. Cross-drilled rotors are much more the result of effect marketing campaigns. They're a cosmetic enhancement, great for SEMA and cars that aren't driven real hard (you'll notice the fine print on many of these indicate they are for street use only). You can try to argue how noticable the difference is all you want but the fact is that it does not improve performance but rather decreases performance. Given that, I'm not sure why anyone that is truly interested in improving their cars performance would make this trade off.

P.S. You mentioned big brake kits, how much larger in diameter do you have to go to replace the surface area that you removed through cross-drilling so that the brakes have the same stopping power? Now, of course, you've got to increase your caliper size. What about your master cylinder? See all these add up. Not to mention, if that same larger rotor was solid, braking power would be even greater.
 
#14 ·
For the curious:





And there's a picture of a Zimmerman that exploded somewhere on VWVortex...
 
#16 ·
I ran the Passat fine @ Lime Rock, and NHIS with stock brakes, race pads, and Valvoline SynPower Synthetic DOT4 ($5/quart @ Autozone/Pep Boys/Kragen, etc). Many people venture on the track with street pads and while they experience some fade, its not life threatening to the point of total failure (unless you're stupid enough to keep pushing once they're overheated). BBKs are useful when you are actually exceeding the capacity of your braking system with too much heat. The premise being that you have enough tire grip to produce that level of braking.

Unless of course you're out to win a car show or something...then you have to justify the cost in your own head. ;)
 
#19 ·
yikes there's lots of hype and misinformation in this thread...

when i do my brake upgrade, im getting slotted rotors for their pad cleaning ability.

cross-drilled are just a liability with all those stress concentrations.

in reference to the pictures above, its obvious to see the second pic is off a porsche, and who knows what braking was done to start the cracks (track driving?) the pics in themselves prove nothing, but they are good at showing how fatique cracks begin at stress concentrations. :)
 
#20 ·
modifiedA4 said:
yikes there's lots of hype and misinformation in this thread...

when i do my brake upgrade, im getting slotted rotors for their pad cleaning ability.

cross-drilled are just a liability with all those stress concentrations.

in reference to the pictures above, its obvious to see the second pic is off a porsche, and who knows what braking was done to start the cracks (track driving?) the pics in themselves prove nothing, but they are good at showing how fatique cracks begin at stress concentrations. :)

not necessarily. It could be any car with a brembo caliper on it . . .
 
#21 ·
stealthx32 said:
I ran the Passat fine @ Lime Rock, and NHIS with stock brakes, race pads, and Valvoline SynPower Synthetic DOT4 ($5/quart @ Autozone/Pep Boys/Kragen, etc). Many people venture on the track with street pads and while they experience some fade, its not life threatening to the point of total failure (unless you're stupid enough to keep pushing once they're overheated). BBKs are useful when you are actually exceeding the capacity of your braking system with too much heat. The premise being that you have enough tire grip to produce that level of braking.

Unless of course you're out to win a car show or something...then you have to justify the cost in your own head. ;)
good to know that. I thought upping the rotor size means more stopping power automatically. now i know better.
 
#25 ·
Pad selection for the type of driving is the most important. The operating temperatures of your pads will determine the ability to resist fade. Brake fluid that is rated to match the operating temps of your pads is also a critical component. Big brake kits will not always improve stop distances but will help remove heat from the braking components.

Upgrading pads can improve braking grip by as much as 50% while improving fade resistance as well. For example, 40% grippier pads is like running stock pads and calipers on a 14.5-15" rotor!.

Slotting will help for wet braking by reducing the amount of time (ms) it takes for the gasses to escape from cleaning the rotor surface of water.

Taken from the Tech forum on Vortex:
9.4" Kelsey Hayes vented fronts/rear drums or rear discs, good up to 2400 pounds/125-130mph.

- 10.1" fronts/rear discs, good up to 2600 pounds/135-140mph

- 11.x" fronts/rear discs, good up to 3000 pounds/150mph.

- 12.3" fronts/rear discs, good up to 3200 pounds/160mph.

Lastly, tire compound and weight will impact your stopping distance.
 
#26 ·
I read that the C6 Corvette has cross-drilled rotors only because (uninformed) customers think they look better. The actual Corvette engineers said that solid would've worked just as well.
 
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