hmmmm some interesting comments.
I never knew those bags have thermal pockets. I'll have to check mine.
dmas - you don't think the frames could freeze? not doubting you but that was a concern of mine....not necessarily freezing solid, but freezing enough to fracture say you hit one off the end of the racket
I would love to keep my rackets in the car, and that's the only reason I don't (that and theft I guess), because I live so far out and sometimes a game of tennis springs up while I'm at work, so if I keep my gear in the car.........
I think, over time, frequent freezing could cause weakening the structure of the frame (faster than hitting with it, and constant restringing would already weaken it). Other than that, I doubt freezing temperatures would be that bad for the modern synthetic frames. It would be very bad for the string, however.
And you would want to get the frame and string to a reasonable tempurature before you actually start playing. I don't mean go into a reasonable temp to play, but have the racquet & string soak up the warmth before you begin. Frozen string would have very little elasticity, and would feel very dead. I'd suggest waiting 20 minutes or so at room temp before hitting.
Personally, I don't keep my frames in the car during the winter time.