Tread depth is an interesting part of it as said. How the tyre behaves as they wear over the last part of it's life is equally important as the start, and IMO is why nobody should form an opinion of a tyre until they are worn out.
My theory as to why, trailing arm suspension on the back works quite differently to mcphearson strut suspension on the front, camber seems to deflect a lot more on the front than the back because of these two designs. Add in the job of steering on the front only, and power driving the rear only, all conspire to wear the tyres in very different ways. Fronts loose the edges first and give a rounder profile, where as rears wear the middles flat as if over inflated giving the exact opposite profile.
Why is this relevant to tyre choice? Not all tyres behave the same under such wear, some handle it better than others possibly due to construction. And has been a major lesson learned in my much bemoaned Falken disaster. When new they where ok in the stability dept, not good, but merely ok! Still thought the wishbone bushes where buggered tbh but they point is they are a less stable tyre generally.
So add in the tyre wear characteristics of the car previously mentioned, in my case Elite suspension which is soft which will allow camber angles to deflect even more exaggerating that wear difference front to back, and an enthusiastic driving style pushing the suspension movement and hence camber deflection further still, and the end result for a tyre can mean binning them with 3 or 4 mill of tread left as the car simply will not go in a straight line.
Don't get me wrong falkens are not the only tyres to reach their level of poor handling(comparatively speaking of course), ask TB after his " something's broken, as if disconnected" concerns before changing tyres and the problem vanished. The point is at what stage do they reach this unusable level and they just have to be changed because they bother drives so badly?
From my experience, I would suggest, once again, the tyre wear and resulting handling degradation to be totally unacceptable on Falkens ESP the 912... But then I really should say similar of the suspension as well, and admit, bought the wrong tyre.
