I had the same problem on my Corsa with the Euromesh Seat. I installed a ventiseat pad and the problem still ocurred. I tinkered around with the seat recline and I still had a sore butt and my middle back hurt starting around 4 to 6miles out. I tinkered around with the bike for the last year to no avail. It hurt so much and was so uncomfortable that I went back to my DF bikes. My wife wants me to keep the Corsa but I am looking at selling the bike. I found that I am much happier on my DF bikes and how much I missed riding them.