An interesting little issue I found by accident - I was just strapping the car to the trailer for the Cadwell Park trackday and happened to spot this little charmer as I glanced through the wheel spokes. Stroke of good fortune (bad news, good news).
All the way through to the outer (peripheral) edge. An AP Racing disc having covered around 8,000 miles. Note the interesting wear/contact markings - as though the fractured area had been flexing away from the pads.
Worth checking your cracks from time to time!
I don't like the look of that, not one little bit, James.
Democratic dissent is not disloyalty, it is a positive civic duty
Nasty! A timely prompt to check my own discs.
I'd be interested to hear AP's comments.
With the benefit of whatever the auditory equivalent of hindsight is... any unusual sounds?
A couple of risk areas that could causing cracking like that are hitting standing water with hot discs or not being able to cool the discs evenly, such as having to come off track with hot brakes and no cool-down lap.
Thermal stress from water certainly but standing with hot discs is more likely to cause variation than cracking, we would regularly see this form of radial cracking and micro cracking on rally car discs after being abused around a forest often spending most of the stage cheery red..... you must have been pressing on at Cadwell James....
I think the odd pattern that James B is referring to is a result of the now open ends of the disc moving in a manner not dissimilar to a bimetailc strip
For clarity, I found the crack when loading up to go to Cadwell, so it is not as a result of any heroics I might have attempted there! And, as a general rule, I am always careful (very careful, in fact) to drive cooling-down laps and to allow for time to cool, etc.
Wow. Serious failure !
Sadly it's not unusual with those vented discs. I'm on my second set (or is it my third?) with spares sitting in the cupboard. With hindsight it would be better if I'd fitted bells and replaceable disks. Initial cost is higher but disc replacement is (or was) lower.
And they look nicer!
Good spot James
Not to soapbox you, but imo rotor and pad inspection should be done before every trackday, and sometimes even multiple times during a trackday, depending on the car and condition of said consumables in the morning.
Glad you caught this before something bad happened.