Slotted & drilled rotors - worth it?

[ntsqd]

The reason that holes promote cracking is that the rotor expands when heated. That's obvious. What isn't so obvious is that the growth isn't even. Heat a disc of sheet metal and watch how it distorts from the differential growth.
That difference in expansion results in internal stresses in the rotor. Holes anywhere are evil, avoid them at all costs. Anywhere there is a discontunity in the metal or a change in cross section will cause an increase in stress.
Factor in that most rotors are made from cast iron. In comparison to other ferrous metals the group of metals known as cast iron are fairly brittle. So now there is non-uniform stress, and stress risers in a brittle material. That your A4 rotors haven't cracked is the exception rather than the rule. My guess is that if you had them Magnafluxed that there would be cracks, they're just not yet visible at ambient temperature.

A very odd and esoteric example of drilled rotors being desirable:
It was done for a very specific reason, that being to intentionally increase the temperature rise of the rotors by decreasing their mass. The nature of the course combined with the available pad compound for the application resulted in a pad that was slow to "come on" (meaning that it took too long to warm up to operating temp) and that there were long periods between brake uses. The pads & rotors had cooled considerably by the next use of the brakes.
By drilling the rotor they were able to increase the rate that the rotor and pads came up to temperature by enough to stop the driver from complaining about the lack of brakes. This is very much an exceptional use of drilled rotors.

 

[madizell]

Good point. What about carbon-kevlar? :-D

js.

I don't specifically know. Haven't ever used them or had a vehicle where the pads were both available and attractive. But I suspect that they are like metallic in that they are hard as the dickens and wear forever, which suggests that they too are intended for hard use and high speeds. Carbon is just about as hard a material as you can find, and kevlar is pretty stable. Can't imagine that kevlar by itself is much of a brake pad material, so I suspect it is a binder/backing for the carbon fiber or carbon content.

For slow going off road I would prefer a brake pad that sacrifices itself, which means that they will wear more quickly than hard pads. Unless the vehicle is also your daily driver, servicing the brakes regularly should not be a big deal. For a daily driver I would use OEM or equivalent, or if you are carrying lots of extra weight and thus putting your brakes to hard use, semi-metallic might be a fair choice. Ideally, you want to use a pad material that is balanced against the hardness and material of the rotors so that one does not inordinately eat the other in use. OEM pads are generally good about keeping this balance since the engineer that specified the materials had the best chance of knowing what he was working with.

There are, of course, exceptions to that rule. The disc brakes on my Mazda MX-6GT were terrific, and in 216,000 I went through something like 5 sets of pads but only one rotor. Wear on the rotor was always even and not severe, so it was mostly the pads that wore out. The Audi A6 is the other way around. The pads throw dust everywhere so they obviously wear out quickly, but they also take the rotor with them, so both rotor and pads are junk.

I would suggest observing your brakes for a while to get a sense of how they work with the pads you have, and if wear out and general brake performance are acceptable, stay with the pad type you have. Many of the high performance pads and rotors are intended for the rice rocket crowd.

 

[R_Lefebvre]

I don't understand why eveyone keeps saying drilled rotor is easier to crack? I've drilled rotor on my A4 for 5 years with 50k on them and they are not cracked. My A4 is my DD and I drive it like a normal commute car. I did see drilled rotor cracked BUT that is on a full race car with stripped inside and roll cage. I'm not sure if it'll help braking or not but I'm pretty sure they will not cracked if I put a set on my Tacoma.

Light street use they are probably unlikely to crack. I don't think they'd be on the market otherwise.

However, when used heavily, such as on a racetrack, or a heavily laden truck in the mountains... watch out.

 

[goodtimes]

Of the thousands and thousands of cross drilled rotors we produced, there was a very low percentage of them that came back as defects due to cracking. We are talking about a dozen rotors, maybe two dozen at the most, spread across 5 years. It isn't like they all cracked....the numbers are low, but worth mentioning. Also worth mentioning, I never heard of a catastrophic failure from one of the cross drilled rotors. Cracks, but nothing else.

As to the lower temps, while there is certainly less mass, there is increased surface area exposed to air in a vented rotor. Yes, there is air flow through those holes (outside to inside, when a directional vaned, cross drilled rotor is installed on the proper side of a vehicle), and yes, testing has shown temperature drops of approximately 200*F. No, I was not there when the tests were done. Also, keep in mind that it was not a constant 200* drop...that was the peak temperature difference between drilled and non drilled rotors during test cycles. Will you see this same 200* temp drop in a street application? I don't know, I wasn't there to see the conditions that the 200* difference was measured at.

"cheese grater effect"? Maybe with some really cheap cross drilled rotors that don't have the holes chamfered...but all of them I have ever seen had a chamfer cut on each hole in the friction surface (pad side only). No cheese grater there. I had a set on a chevy truck I used to have (because I could), combined with good pads, I had great life out of them and no more dusting than with the OEM rotors.

Pad material is constantly improving. As mentioned, it may have already progressed far enough to eliminate the problem of outgassing. I am far removed from the industry at this point, so I don't know.

 

[R_Lefebvre]

I have seen one that completely came apart with predictable results but I can't find it anymore.

Interesting quotes:
http://www.marcusfitzhugh.com/CLK/DIY/brakecdrill.html

 

[RAM5500 CAMPERTHING]

I have run DBA series 4000 Cross Drilled and Slotted on 3 different rigs, and just installed a set on my 100 and have zero complaints.

Happy Customer! :smiley_drive:

 

[madizell]

I have seen one that completely came apart with predictable results but I can't find it anymore.

Interesting quotes:
http://www.marcusfitzhugh.com/CLK/DIY/brakecdrill.html

Is it just the photo, or is this disc blue? If it is as blue as it appears, it got incredibly hot somewhere just before it died. As I recall, steel blues around 600 to 650 degrees F. That's on the warm side for a non-racing application.

 

[ntsqd]

Wilwood's 3 main street pad compound Temperature vs. Cf curves:

Performance Friction states:
"All our pads are made to withstand the hottest temperatures, ranging between 572°F and 1472°F (300°C and 800°C)." As best as I can tell they are not talking about their racing pads since those are on a different page.

From those I'll venture that 500*f isn't unreasonable and it could go higher in extreme but non-racing circumstances.

 

[R_Lefebvre]

Yes, the rotor was used in a "racing" application. Well, a trackday, so not quite racing. Yes, it got quite hot and I don't think a truck rotor would get that hot in normal use. However, if your rotors aren't getting that hot, I'd question why you'd need drilled rotors in the first place?

As for the temperature curves, those temps listed are pad temps. The pads get hotter than the rotors since there's less surface area doing the work compared to a rotor.

FWIW, I run the Q pads in my Wilwood Superlites on my Focus on the street. That's a nice street pad. I run the E pad on trackdays. These brakes are well sized for a Focus so they don't get real hot, I can use the lowest level racing pad. The E had more high temperature ability than the Q.

However, the E isn't as good of a all-round pad as the graph would suggest as they squeal a lot, particularly when cold.

Anyway, I think my point is not that drilled rotors would suffer catastrophic failure in truck application, it's that it's a waste of money. They offer no benefit, cost a lot extra, and reduce the longevity of the rotor and pads.

I think if you need heavy duty brakes, you should buy heavy duty brakes. I "invested" in a set of Wilwoods 4 piston calipers and two piece rotors early on for my track car, and I'm glad I did. My initial $1300 investment is still going strong. I have something like 15 track days and 20,000 street miles on the same set of rotors and pads and I can't even measure the wear. Meanwhile, all my friends with similar cars are still monkeying around with expensive stock-like rotors and "racing pads" for the stock calipers. They'll burn through an expensive set of pads and rotors in 1 or 2 trackdays.

If you are *really* having a problem with braking, I'd look into upgrading to a 4 piston caliper before spending extra on drilled rotors and fancy pads.

 

[ntsqd]

I would expect that the rotor surface, at the point of contact with the pad, is very close to the pad temp. Depending on the length of the brake application the rotor as a whole may or may not get up to those temps.

FWIW I had the squealing with the 'E' pads too. In a GM D52 caliper. The 'D's were so much better except for their lifespan and their dust......
oh, and for their killing the rotor when the loose nut behind the steering wheel didn't catch that they were done......

 

[R_Lefebvre]

What's a D? I don't see them listed.

I have a friend with a similar car, turbo Focus with the same brakes, and he just leaves the Q's in on track and doesn't have a problem with them.

 

[ntsqd]

They're NLA.

 

[madizell]

Wilwood's 3 main street pad compound Temperature vs. Cf curves:

Performance Friction states:
"All our pads are made to withstand the hottest temperatures, ranging between 572°F and 1472°F (300°C and 800°C)." As best as I can tell they are not talking about their racing pads since those are on a different page.

From those I'll venture that 500*f isn't unreasonable and it could go higher in extreme but non-racing circumstances.

The chart describes relationship between temperature of the pad and coefficient of friction. The tag line suggests that the pads will survive temperatures within the stated range. For at least two of the pads, cf peaks at 500 to 600 and then falls off.

That is not to say that they regularly operate at these temperatures, or that they should. I can see it on a track car because one is always pushing the limit. Even on the highway, I would not expect to see such temperatures because a panic stop from 80mph would only last a few seconds and would not be repeated 15 times in a few minutes, so the rotors would soak up heat, then have lots of time to throw off that heat. To get the entire rotor to 600 degrees takes a bit of doing.

And I agree that for expedition and general off road use, cross drilling rotors doesn't add a real benefit. Drilled brake drums -- now that would be a different thing, and there is probably a real benefit to the attempt. Whether there would be an issue with cracking in drums, I don't know. None of the articles I have seen mention it. Has anyone here tried it?

 

[ntsqd]

The ranges shown in that graph are exactly that, the operating ranges of the various pads listed. Going beyond those upper limit temps would be trouble, but staying within those ranges is not. One of the criteria in testing new pad compounds was maximum temp for a set minimum life span. Those graphs stop at that upper limit temp.

I've read of drilled drums. Same cracking issue coupled with a drum's natural tendency towards bell-mouthing results in greater stress in the drum. I've also worked on a vintage road racer that had them, but the car was no longer driven in anger so the issue never came up.
I think a better method are diagonally slotted shoes. Use the rotation to "screw" the debris out of the brake.

 

[goodtimes]

FWIW, I run the Q pads in my Wilwood Superlites on my Focus on the street. That's a nice street pad. I run the E pad on trackdays. These brakes are well sized for a Focus so they don't get real hot, I can use the lowest level racing pad. The E had more high temperature ability than the Q.

Just curious, do you have any problems switching between pads for track days, other than needing to bed them in each time? I have heard some people claim they have problems if they don't turn the rotors (resurface them) when they change...and others say they have no problems. I don't spend time on the track, so I don't change pads until they are worn out.

I think if you need heavy duty brakes, you should buy heavy duty brakes. I "invested" in a set of Wilwoods 4 piston calipers and two piece rotors early on for my track car, and I'm glad I did. My initial $1300 investment is still going strong. I have something like 15 track days and 20,000 street miles on the same set of rotors and pads and I can't even measure the wear. Meanwhile, all my friends with similar cars are still monkeying around with expensive stock-like rotors and "racing pads" for the stock calipers. They'll burn through an expensive set of pads and rotors in 1 or 2 trackdays.

Yup....there is a reason for the larger rotors, stiffer calipers and increased clamping pressure.

How does that saying go..."buy the best, cry once".