Cracks in drilled rotors-when to replace rotor?

[sandiego]

I have small cracks after 3 DEs on new front rotors with Pagid yellows. After 5 DEs they still are not continuous from one hole to another.
My questions:
- Is it inevitable that the cracks will eventually go from one hole to another?
- Are we talking just another DE or two?
- When the cracks do touch, what is the failure mode? Will a chunk fall out of the rotor? Will I feel it in the brake pedal? Is a small chunk dangerous?
Car is '84 turbo look, 2800 lbs. I had no cracking problem with Cool Carbon pads except they don't stop as well and wear out after 3 DEs.

[Sooner or later]

Have a picture?

[bkreigsr]

You're thought process is correct. Replace when the cracks connect to a hole or another crack.

Yes, they will migrate to a hole - it's just a matter of time and severity of usage.

Yes, you will know it when a piece of the rotor becomes dislodged.

Yes, even a small chunk is very dangerous and destructive to your car and those around you.

There is no warning other than the visual cracks.


There is a specific break-in procedure for new rotors and pads - I'm guessing you did not follow it.


Bill K

[Canada Kev]

Aren't almost all drilled rotors subject to cracking, depending on how the holes are formed, whether cast-in or drilled afterward?

[Flojo]

I installed my Zimmermans about May 2015...., no cracks. Stock calipers. 1st season with EBC blackstuff pads, now with Ate pads.
Guess I'm Not the type of hard braker...

[HaroldMHedge]

Thanks to Bill Verburg we have this factory information.



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

GT3 front brake rotors

Harold
87 Carrera Coupe

[Walt Fricke]

It is the cracks which reach the edge which tell you it is time to replace. The next phase is for edge cracks to create an offset, which you can feel through the pedal.

For us track and AX drivers, all that tire changing means we have plenty of opportunity to see how cracks are progressing. Harder if car is just street driven, but then cracks ought not to propagate as quickly, because street driving ought not to stress brakes all that much.

Always a good idea to have a new set of rotors on the shelf so, once you decide it is time, you can change without much down time.

[tdw28210]

seems like I see a lot more slotted rather than drilled rotors on track-going sports cars these days. wonder if it is because of this issue?

[Walt Fricke]

Drilled rotors have a bad reputation in the racing world. Plain or slotted is the way to go.

[bkreigsr]

...unless you're also looking to reduce unsprung weight....

[sandiego]

Good knowledgable info. Thanks.

Host does not sell solid or slotted rotors for my '84 Turbo Look ( with turbo rotors). Who sells them?

[DaddyGlenn]

Originally Posted by Walt Fricke

Drilled rotors have a bad reputation in the racing world. Plain or slotted is the way to go.

I take it, this is a deserved reputation?

Do drilled rotors not offer a cooling advantage over solid? Or is it just not enough to offset the risk of catastrophic disc failure?

Do endurance racing teams, break in several sets of discs and pads before a long race?

Inquiring minds want to know.

[T77911S]

i would just put zimmermans back on, with holes.
if you want something else there was a site that just sold rotors. don't know the quality of them.
had a set on a jeep grand. they stopped it really well but it seemed like they wore out fast.

consider it a racing expense.


I am curious if anyone has had a rotor failure...from a good quality rotor.

if a piece came off it would hit the rim, no?

[DaddyGlenn]

Found video of catastrophic brake disc failure.

[tdw28210]

Originally Posted by sandiego

Good knowledgable info. Thanks.

Host does not sell solid or slotted rotors for my '84 Turbo Look ( with turbo rotors). Who sells them?

Do a google search. The Geek and Rock sites both appear to show plain later turbo rotors for sale from different manufacturers.

[sandiego]

Most important to me is not having a catastrophic failure.
On the other hand changing drilled Zimmerman rotors after 3-5 track days is not a desirable solution.
I would never notice the difference in unsprung weight between a drilled and slotted rotor, or for that matter with a solid rotor.
I'm willing to try a slotted rotor if I could find a source. Anyone ?

Or I may have to switch from the Pagid yellows to something more rotor friendly at the expense of stopping power.

[Steve@Rennsport]

Originally Posted by sandiego

Most important to me is not having a catastrophic failure.
On the other hand changing drilled Zimmerman rotors after 3-5 track days is not a desirable solution.
I would never notice the difference in unsprung weight between a drilled and slotted rotor, or for that matter with a solid rotor.
I'm willing to try a slotted rotor if I could find a source. Anyone ?

Or I may have to switch from the Pagid yellows to something more rotor friendly at the expense of stopping power.

You do NOT need to change pads; those Pagid Yellow (RS-29) are quite good.

Preventing excessive rotor cracking is a matter of heat management. Note the word 'excessive',......a GOOD QUALITY drilled rotor may display small cracks that do not migrate from the holes and will wear out before they connect between them. Cheap ones do not have the same metallurgy and can initiate cracking more easily and migrate to the outer edges, requiring replacement.

The key is to have effective front brake cooling and proper brake bias so the thermal loads are more evenly distributed, front to rear. You can see the latter by watching the ratio of front & rear pad wear.

Stock 930 brakes are heavily front biased and for that reason, we use & recommend using Pagid RS-14 Black pads in the rear to help balance the braking, front to rear. Works VERY well.

There are real racing rotors (2-piece) available, however these are quite expensive and generally only worth the investment for competition cars. I'd recommend changing rear pads before spending the big money for better rotors.

Hope this helps,

[sandiego]

Thanks. I'm already using Pagid yellow front and black rear. No cracks in the rear and cracks in the front only on the outside. Maybe I need to duct some air in there with a cool brake kit if one exists for the turbo brakes.

[Steve@Rennsport]

Originally Posted by sandiego

Thanks. I'm already using Pagid yellow front and black rear. No cracks in the rear and cracks in the front only on the outside. Maybe I need to duct some air in there with a cool brake kit if one exists for the turbo brakes.

You REALLY need a front brake cooling kit if you are tracking the car!!!

[Walt Fricke]

You need to block off the openings in the hubs so air sent into the center of the rotor has to exit radially between the rotor faces (or out your holes). You can make these from thin aluminum and tin snips or some kind of hole cutter. I use a leather punch to make the holes for the 13mm bolt heads - you don't want to sandwich aluminum between the rotor/hub fasteners and the rotor - too likely to squish. You just want this block-off piece to lay flatter than it would if it sat over the bolt heads. You can add high temp silicone if you want for better gap filling, but not critical.
To hold this piece in place just drill through it and the rotor and use pop rivets - the smallest diameter you have.

To get air directed to the center of the rotor, you might consider a sort of diverter piece from the 993 RSR or the like, or a Cup car. You can fabricate mounts on the A arm triangulation rod to hold this. It would be good if you can use some openings in the front valance - like where fog lights are? - to let more air through, and maybe attach some ducting which runs far enough back to point at the diverter.

Maybe someone with a setup like this will post a picture. Me, I've got a 25 year old Holbert piece which covers the center opening of the rotor and has a place to attach a 3" hose. It works, but calls for constant maintenance as the sway bar (through the chassis type, if you have the stock sway you have more room between chassis and strut) tends to squish it. I've also added stops to limit how far I can turn the wheels (might not be practical for parallel parking a street driven car), and I rivet a piece of aluminum over the ducting where the tire can rub against it, which increases longevity. The 993 type deflector avoids all this, and you don't have to fabricate or purchase the external center blockoff and hose attachment piece.

If the Holbert kit is still sold, you can just use that, but if you are like most of us you will fairly quickly damage the scoops which hang down below the A arm. But if your car is at US ride height and you avoid off track excursions that might be the simplest start.