To recap from another thread. With winter looming I fitted Michelin snow tyres just to the rear of the Focus to replace the shagged original tyres and am not having a happy driving experience. Even in the bone dry the back end feels like it's about to step out all the time, and often in fact does so if I push it a bit too hard.
I'm finally getting a handle on it all. It's partly about lower absolute grip from the snow tyres but more about cornering slip angle I believe. The snow tyres appear (feel) to be operating at a much higher slip angle than the front summer tyres causing oversteer which then leads to unbalance and in extremis to the back stepping out. In essence it's a similar thing to using radials on the front and crossplies on the rear which is stated to be a big no-no. I can now see why.
This explains why the feel improves when the rears are heavily over-inflated as this reduces slip angle but nothing like enough to fully mask the problem. Until I either get two more matching snow tyres for the front or some spare summer ones for the rear until winter really sets in I'll stick the existing snows on the front and the front summers on the rear. At least then all it should do is understeer like a pig but hopefully not visit any shrubberies backwards which it keeps threatening to do. Happily the dashboard "bendy thing sorter out" light comes on when this happens and tends to keep everything under control.
I can find very little online about the slip angle of snow tyres vs summer ones. Any links would be appreciated. It might also serve as a warning to anyone contemplating doing the same.