Front Rotor Cracks

[StigsBaldBrother]

Anyone crack a rotor yet? Rotors only have 7 track days on them, granted 4 of those days are at Sebring. Running G-loc R-12 pads.

I've never liked cross-drilled rotors, but then again, I've cracked vented rotors too. I was hoping to get some more life out of them, but 80% of the holes have similar cracks.

Is that just normal wear? Running too hot?

 

[VoodooBoss]

I'm curious what others think but I don't think those are cracked (meaning non usable). More heat check stress cracks. When the cracks go out to the edges they are done. The photo below is mine after five track days all using Pagid RS29 pads. Mine aren't as bad as yours but look at the top and bottom holes in my photo and there are small stress cracks.



They could be worse.

 

[mattlqx]

Mine look pretty perfect after 5 track days and 6000 miles on stock pads. I've got a surplus of OE pads, so I'll be the long term Guinea pig on those I suppose. Obviously, those of you with more aggressive pads will run through rotors quicker. One of the reasons I've stuck with stockers.

 

[302 Hi Pro]

If you run a drilled floating rotor ring with proper cooling ducts, would it be helpful at all to run a few cool down laps at reduced speed using only light braking to bring the rotor temps down before going into the paddock?

In other words to keep from going from rotors that are glowing from heat to straight into the paddock? Also, is there a temper procedure for new drilled rotors aside from the new brake pad bed in process?

I'm not sure, just asking if this might help to minimize cracking.

Our Racing Members here on TMO can best answer your questions about this.
302 Hi Pro

 

[Flyboygsxr]

Mine look pretty perfect after 5 track days and 6000 miles on stock pads. I've got a surplus of OE pads, so I'll be the long term Guinea pig on those I suppose. Obviously, those of you with more aggressive pads will run through rotors quicker. One of the reasons I've stuck with stockers.

I'm right there with you. The stock pads are a great design. No noise on the street and no fade on the track. I realize if I went with a stickier tire like the Cup 2's that a pad upgrade might be worth it. For now I'm just enjoying the car exactly how it is. I'd replace those rotors though, 7 track days on an aggressive pad is pretty good. On my Boss 4 track days on DTC 60 pads and the rotors were toast.

 

[Grant 302]

Anyone crack a rotor yet? Rotors only have 7 track days on them, granted 4 of those days are at Sebring. Running G-loc R-12 pads.

I've never liked cross-drilled rotors, but then again, I've cracked vented rotors too. I was hoping to get some more life out of them, but 80% of the holes have similar cracks.

Is that just normal wear? Running too hot?

Looks like normal heat checking like Rick said. I'd run that until the cracks propagate toward the inner or outer edges. Inspect after every session.

Seems like full ducting and vented rotors would still be a better solution than the OEM setup.

 

[bpracer]

I've never liked drilled rotors for that reason. I've run worse heat checking, but not with a crack that large off the drilled hole. It is harder for the crack to propagate on a curved vane rotor.

When I put the GTPP brake defectors on my car it sure didn't look like you could take the stock air ducts and add a hose. The tire looked too close.

Sent from my SAMSUNG-SM-G900A using Tapatalk

 

[StigsBaldBrother]

I wore through the entire set of stock pads in the first 2 days I tracked the car at Sebring. They didn't fade, but they didn't bite as hard as the race pads, obviously.

I guess the race pads are still the cheaper way to go. (stock pads ~$93 per day, only ~$57 per day for the rotors ($400 per pair over 7 days).

 

[ls110]

Anyone crack a rotor yet? Rotors only have 7 track days on them, granted 4 of those days are at Sebring. Running G-loc R-12 pads.

I've never liked cross-drilled rotors, but then again, I've cracked vented rotors too. I was hoping to get some more life out of them, but 80% of the holes have similar cracks.

Is that just normal wear? Running too hot?

I agree with Rick and Grant. Not until one of the cracks reaches the inner or outer edges is necessary to replace. The important piece is to inspect the rotors after each track session to ensure no cracks have reached an edge.

 

[superhero]

I have a question about the cracks. Why do all the cracks run from inner to outer on the rotor. Not one crack running with the direction of the rotor.

 

[OPMustang Tim]

So they do crack...I wonder if the Girodisc would last longer.

 

[Grant 302]

I have a question about the cracks. Why do all the cracks run from inner to outer on the rotor. Not one crack running with the direction of the rotor.

In general, any rotor is designed to prevent tangential over radial cracks. Tangential cracking would be more potentially catastrophic. Several elements of all rotors are used to prevent them. The GT350 rotors include one that most others don't with the radial pin connection. 1-piece rotors use the hat shape and grooving to relieve such stresses. Within the rotor, vented or solid there is enough thermal bridging to prevent differential cooling. That way the entire rotor area expands and contracts radially relatively unrestrained. In vented rotors the direction of the vanes also generally prevent tangential cracks. Tangential forces between the pad and rotor also promote radial over tangential cracks.

There are other elements, but I hope that answers your question.

 

[k98dave]

Any rotor that has been drilled and NOT re aged (i.e.heat treated) will be prone to such cracking if the material stress induced by the drilling is not relieved. If you note the cracks always originate at the holes. Another issue is both the size of the hole and most importantly the alloy content of the rotor. Porsche uses a much higher carbon& graphite & other compound content material plus the holes are about half the dia. of typical aftermarket rotors. All rotors will develop surface cracks (heat checking) under heavy use, adding holes just set up additional failure points especially when the are approaching min thickness. Slotted or plain rotors is the way to go overall.

 

[Flyboygsxr]

I've got a friend with a Carrera 4s and his rotors do the same thing. If you track your car the rotors should be replaced as necessary as a normal wear and tear item like pads and tires.

 

[k98dave]

I've got a friend with a Carrera 4s and his rotors do the same thing. If you track your car the rotors should be replaced as necessary as a normal wear and tear item like pads and tires.

That's right and to my point even the best material rotors will do this eventually as its a product of thermal cycling. But no point in setting up additional failure points that drilled holes provide.

 

[Grant 302]

Any rotor that has been drilled and NOT re aged (i.e.heat treated) will be prone to such cracking if the material stress induced by the drilling is not relieved. If you note the cracks always originate at the holes. Another issue is both the size of the hole and most importantly the alloy content of the rotor. Porsche uses a much higher carbon& graphite & other compound content material plus the holes are about half the dia. of typical aftermarket rotors. All rotors will develop surface cracks (heat checking) under heavy use, adding holes just set up additional failure points especially when the are approaching min thickness. Slotted or plain rotors is the way to go overall.

The issue of drilled rotors being prone to cracking is very well known here. Your explanation above is correct, but empirically, heat or cryo treated drilled rotors will still crack under track use. The physical hole is a 'stress riser' regardless of the metallurgy. Simply put, having a hole in the rotor causes stress/strain to concentrate at the edges of the holes. The GT350 and other modern drilled rotors also use extra structural ribbing to prevent such concentration from becoming an issue.

Some of the extra ribbing can be seen in the cut away FP had on display:

Slotted or plain rotors is the way to go overall.

While I agree in general and often preach the same, I would trade my stock Boss or Brembo GT ducted/slotted setups for the OEM GT350 brakes in a heartbeat...even the rotors in question from the OP.

 

[Flyboygsxr]

The brakes are amazing on the 350. I know all the hype was about the FPC Voodoo motor but the brakes are a work of art. First car I've owned that I doesn't need dedicated track pads.

 

[VoodooBoss]

The brakes are amazing on the 350. I know all the hype was about the FPC Voodoo motor but the brakes are a work of art. First car I've owned that I doesn't need dedicated track pads.

And they're even more amazing with some race pads. ;D

 

[StigsBaldBrother]

The issue of drilled rotors being prone to cracking is very well known here. Your explanation above is correct, but empirically, heat or cryo treated drilled rotors will still crack under track use. The physical hole is a 'stress riser' regardless of the metallurgy. Simply put, having a hole in the rotor causes stress/strain to concentrate at the edges of the holes. The GT350 and other modern drilled rotors also use extra structural ribbing to prevent such concentration from becoming an issue.

Some of the extra ribbing can be seen in the cut away FP had on display:


While I agree in general and often preach the same, I would trade my stock Boss or Brembo GT ducted/slotted setups for the OEM GT350 brakes in a heartbeat...even the rotors in question from the OP.

That extra ribbing is very interesting indeed. All of the cracks originate in the center 2-3 rows of holes. None of them are cracked near the inside or outside edge.



Don't get me wrong, the brakes on this car are amazing. I was just hoping to get a little more track time out of the rotors before replacing. The Boss 302 brakes I had on my 03 Cobra lasted about 12-14 track days and were half the price.

I wonder if carbon-ceramics would've been cheaper per track day... ;D

 

[Black Boss]

This is normal wear for these rotors under very hard track use. I ran mine further, and only replaced if I could get a finger nail in a crack of if any crack propagated half way to the edge or if any large crack is within 1/2" of either edge. The s is obviously a rule of thumb -- use at your discretion.

The OEM rotors are dead cheap. The other option is Girodisc slotted 2 piece rotors.