Reading the comments is quite interesting. I do not understand some of it, but it appears I was probably close to my comfort limit when the speed increase occurred. Add to the speed increase is the fact that the speed did spread the formation somewhat which caused a loss of protection from the wind and a loss of the draft effect. As I said earlier, there were 6 more riders off the back with me, so the formation did break apart. I know the physical science of this situation explains it, but I still find it amazing how 'absolute' the cut-off was for me in that 2 MPH differce. I felt like I could ride the entire 40 mile route we were on at 22+ MPH, but buy moving up just 2 MPH I was completely winded in a little over a mile. Although I have tried any number of sports, I find cycling baffling at times, but an amazing sport.