Why you don't drill rotors that weren't designed for drilling

[175K911]

I've always said that you should never drill a rotor that wasn't designed to be drilled. But violated my own rule and common sense. When I bought this car, the PO had installed a set of Zimmerman rotors that had been drilled (they did not come from our host.) Rather than take them off and turn them into boat anchors (I'd never sell them either knowing that at some point they'd fail), I thought I'd run them to see what they'd do.

At Mid-Ohio a couple weeks ago, heard a noise in the front end, and felt it in the steering wheel. My student in the car with me heard it too. Came in immediately. Didn't see anything obvious in the suspension, but since it was the last run session, I just loaded it on the trailer. Finally found time yesterday to dig into the front end, and found this on my right front rotor. Cracked clear through. Interesting how the crack not only found two of the holes to run through, but looks like it was trying to migrate to a third. Other rotor had 3 cracks in it too, from 3 of the outer holes to the outer edge.

Now what do I get this week so I can reassemble the car by Friday?

[dtw]

I black-flagged a guy for this once. Several cracks running from OD to ID of the rotor. The guy was furious. Some people just don't get it. Glad you do.

[DSouth]

From my somewhat inexperienced eyes, it looks to me like most all of the other drill holes have cracks starting from either sides of them. Maybe that's the way drilled rotors always look though.

[vreference]

No doubt they've been trying hard to get worse for a while. No, the cracks at each hole are not normal. Mine have been fine for street use; it'll be interesting to see what happens at the track.

[Bill Verburg]

Originally Posted by vreference

No doubt they've been trying hard to get worse for a while. No, the cracks at each hole are not normal. Mine have been fine for street use; it'll be interesting to see what happens at the track.

They're gonna crack, even oem's do that

The cooler you can keep them running, the longer they'll last

[Wil Ferch]

I've thought plenty about this over the years....

If you do a search here, you will find that there is still some debate and uncertainty as to the factory drilled rotors. Common assumption is that the factory rotors use cast-in-place holes..... and therefore and somehow are less prone to cracking. The search links will show there is some doubt about all this..the factory holes may either be drilled or cast.

It seems to me that the issue is microscopic stress risers. It therefore seems reasonable to postulate that the issue ( besides overall quality of the alloy used), is the surface finish of the holes. Even the factory manuals say that factory-produced holes may be the starting point of cracks ( much like these photos show), and go on to illustrate acceptable vs unacceptable levels of cracking for continued use.

So..the method of how the holes were produced may be a diversion from the real issue....surface finish within the hole surface.

Perhaps if after drilling, if the holes are burnished or polished, that this means there are fewer places where the crack can start? Could this be the real issue ??

[KTL]

Ed,

One of our locals (Jay Auskalnis here on Pelican) had that happen on one of his SC front rotors at Blackhawk a few years ago.

There's no reason to get drilled rotors, or really any other rotor brand on our cars. The factory Zimmerman rotors are absolutely worthy of street use, and easily withstand track (ab)use as well. All of the Porsche DE guys in our area use the factory rotors with no issues whatsoever. One guy thought the PowerSlot rotors were the hot setup because of their quality and slotting. When I told him they were simply rebranded OEM blanks, he decided to use the factory rotors upon needing to replace his worn PowerSlots. He was sold on them. He beats the proverbial crapola out of his car too.

Call up Chris Streit (cstreit here at Pelican) at Motodelta (www.motodelta.com) and I bet he can get you a set in a few days. I'd normally recommend Pelican, but you obviously need them quicker from somebody more local. Chris is the man.

[175K911]

Originally Posted by DSouth

From my somewhat inexperienced eyes, it looks to me like most all of the other drill holes have cracks starting from either sides of them. Maybe that's the way drilled rotors always look though.

Nope, your eyes are correct. Lots of other cracks there. The other rotor has 3 cracks on it, between outer holes and the edge, and all the way through the rotor. It's when the adjacent cracks begin to line up that things go bang.

[175K911]

Originally Posted by KTL

Ed,

One of our locals (Jay Auskalnis here on Pelican) had that happen on one of his SC front rotors at Blackhawk a few years ago.

There's no reason to get drilled rotors on our cars. The factory Zimmerman rotors are absolutely worthy of street use, and easily withstand track (ab)use as well. All of the Porsche DE guys in our area use the factory rotors with no issues whatsoever.

Call up Chris Streit (cstreit here at Pelican) at Motodelta (www.motodelta.com) and I bet he can get you a set in a few days. I'd normally recommend Pelican, but you obviously need them quicker from somebody more local. Chris is the man.

Thanks Kevin. I've always been happy with stock Zimmermans, just thought I'd see what these drilled rotors could do. Obviously I've been keeping an eye on them anticipating some kind of failure.

Trying to get the car back together for an advanced drivers day at Autobahn Monday the 28th. With only maybe 25 people coming, and having the full course, should be lots of great seat time. Care to join us?

[RWebb]

makes sense, Wil - I assume the cost of treating each hole is too high

I am still waiting for ceramic rotors for my car....

[304065]

I use the factory vented rotors, work great.

I can see lightening the rotors if you get paid to win races. Barring that . . .

[KTL]

Originally Posted by 175K911

Trying to get the car back together for an advanced drivers day at Autobahn Monday the 28th. With only maybe 25 people coming, and having the full course, should be lots of great seat time. Care to join us?

Thanks for the invite. Just had a 2 day event Sat and Sun on the South course with Chris and company at the NASA event. Good times!

Unfortunately as you can see, my car is running a bit hot and something is up with my shifter. Shifter is all floppy and I can't get reverse or fifth. Happened on the drive home from the track yesterday. Have open up the shifter and see what fell apart in there.......

Plus i've got a church volunteer event I have to help out with that day. We'll run into each other one of these days this season!

[bonkers]

That looks like a LOT of holes - too many - drilled without any thought to the structural integrity of the disk. Notice the way the cracks are all aligned from the center to the outside of the disk?

[bkreigsr]

175K911
curious as to what calipers/pads caused this.
that amount of heat did not come from the stock set-up did it?
also, those Zimmerman's have about 3 times more holes than my drilled Zimmerman's. what's up with that?
thanks

Bill K

[gtc]

Good drilled rotors usually come with the holes chamfered to reduce stress risers around the holes and slow crack propagation. Heat treatment would also help eliminate any stresses created by a drilling process.
Once the rotors are worn down past the chamfer, it's pretty much a given that you'll have cracks. How close were these to the wear limit?

[burgermeister]

Could also be that, during braking, the holes heat up evenly with the rest of the rotor. Then, the 90 degree sharp edge of the hole cools off really quickly (lots of cooling air, little material relative to surface area) while the surrounding rotor stays hot. Thus, the edge of the hole would shrink relative to the surrounding rotor material, which would want to keep the hole size larger as it is hotter. This might cause cracks to form. That, and a sharp 90 degree edge is usually a good crack initiator to begin with.

No theory as to why they are all radial though. Maybe the shear loads imparted by the brake pad?

[911mot]

Those discs are way past there use by point. Im not surprised it cracked. That other disc has seen some action too. Nice colours

[175K911]

Bonkers- yea, way too many holes! Going back through the PO's receipts, looks like he got them from Performance Products.

bkreigsr- Stock '86 calipers, Hawk H-10 pads.

911mot- Can't speak to how the PO drove it on the street, but for me it's purely a track car. But PO didn't put more than a couple thousand street miles on them based on his receipts. I have maybe 100 street miles on them, and 12 track days (Instructor so I'm not babying them) or maybe 55-60 heat cycles.

Frankly I'm looking forward to putting basic Zimmermans back on and suspect that I'll have way better braking bite now that I'll have full rotor surface area back for a friction surface. That may be why I never had a heat issue- there wasn't enough surface area to generate enough heat.

[barney911rs]

It all comes down to weight of the car and the heat generated. Most open wheel cars use drilled rotors, but they get more air to cool the disks, and the cars are very light compared to a production based car. I've always used slotted Zim rotors on my car. Stripped out 73 tub, so it's pretty light. Never had a cracking problem. (knock knock knock)

[Bill Verburg]

Even totally stock rotors(no drilled can crack like that) it all has to do w/ heat. Keep the temps down and they last longer, run them hotter they suufer from reduced life expectancy.

here's a stocker undrilled, cracked


Here's a cracked slotted rotor


here's the first sign of trouble


The blue spots are localized coinversion of gray cast iron into cementite at high temperature, theses rotors are toast too

the rotor holes should be determined by the internal structure of the rotor,
SC rotor have radial vanes and if they had foles the holes would be radially aligned

Modern rotors have curved vanes which gives rise to the curved hole pattern

Worn out modern rotor, how can someone be so oblivious as to use a rotor to this point?