Watkins Glen is tough on brakes, especially when running back-to-back sessions.
Another S was @ the past weekend's Riesentoter PCA HPDE- he also cracked a front rotor.
This disc cracked well after the session, prob 30+min after, after some very light braking while coasting down the hill into town.
These were NAPA Premiums - got a day at Lime Rock running back-to-backs (8 sessions total), lots of auto-x, a day at NJMP (5 sessions), a weekend at WGI (16 sessions, all but the final had 20+min btw sessions).

This is a reminder to bring spares (and enough for others!) and the req'd tools.

Picture:

 

When you crack a rotor, can you feel it, hear it, etc.? What gave you the indication or thought that the rotor had cracked?
Just curious.

 

You can feel it.

 

Both. It wasn't quite the "ting" that I expected - but I heard something and then the pedal feel was obvious.

 

If you look near the camera flash reflections, you can see very fine cracks. They're all over the disc, and are the indicator that a crack will occur soon. Cracks are also more likely when there's lots of shades of grey on the disc, a sign of uneven heating or uneven brake pad material transfer.

 

yeah some little ones, but those generally aren't much to worry about.

 

It that heat bluing? (And yes bluing is a surface phenomena.) I don't care about fine cracks but that is really uneven wear, and a lot of transfer.

 

I put on some spent racing pads for the drive from WGI to NYC which I think had something to do with it.
Pedal finally felt good, after previously having a slight shudder under hard braking during the prior track days once the discs and pads got hot.

 

I have yet to crack a solid centric brand disc. I have been able to take them below the min thickness without an issue but once I get below it I chuck them.

They do get hot enough to go past the temperature range of the xp10s but the xp12s have been doing ok