Thanks guys, it seems the difference is sidewall rigidity. is this correct? I have 13,000 on the stock RP4's and they've been great. Best tires I've ever had on a motorcycle. I rode 100 highway miles Saturday and 50 backroads and the 4's are still working well but I'm leaning towards the 5's. Gman, I'm looking at the track days in Palmer, how was it?