So this weekend I decided to do one more autocross hurrah before my surgery. Nothing competitive so I went on my old RS3 v2's. Plus I shouldn't be hauling around wheels and tires anyway. My RS3's have some tread on them still, not much but enough to say they're not quite shot, at least not at a glance. Well, they most certainly are. Despite tread, they are INSANELY slick. So slick that it feels like you're riding on hockey pucks. They are so done but why? Well, heat. And here's where heat cycling comes into play. Heat cycling in a tire is important because the compound itself needs to essentially "chemically settle", thermally. Then it stays in heat cycle and can grip and shape as it starts to wear away. But too much heat too fast and they heat cycle out. So despite having tread left, my RS3's are absolutely done. There is also another way to tell that they're totally cooked: the way the tread looks. A heat cycled tire, especially a super high performance summer tire, tends to chunk a tiny bit and feel somewhat rough and look like settled cement, rough and wavy. My RS3's look like plastic. They're extremely smooth and feel like an all season touring tire now. Essentially there's been so much expansion and contraction from heat that they have vulcanized into something totally next level. Think of a dying star that has puffed out as it's expelling all of its fuel. The tires have lost their capability to grip and dig. Instead, they more so now just roll, if that makes sense.
So the takeaway here is that you can't always life check a tire based on tread. There is another component to tire wear and in fact, a potentially dangerous one too. I would not dare hit the track on these tires as they sit.