I also have four bikes - two road bikes and two mountain bikes -one with 26" tires and another with 29" tires. I get that message when I try to change the bike on an active ride - but only on certain activities. That is, I can change the bike if I am just on "cycle" but not when I am on "cycle for distance". Might be a bug. I like cycle for distance as it better calculates stop time - which is a huge negative for this app. I have found it easier to just change it after the ride has completed.