As Tentcamper mentioned, when I changed to 15" tires, the spare would not fit into the carrier. Not even close - not even with a pry bar to lift it. There was a lot of discussion in the thread he referenced
http://www.trailmanorowners.com/foru...ead.php?t=5828
including a good solution by Eric Larson which seems to have disappeared. At any rate, the rack extension should be enough to lift the tire snug against the underside of the TM, but not so tight that you can't get it in.
A simpler solution - one that I used in the short term until I modified the rack - is simply to keep the best of your 14-inch tires as a spare. Obviously it will fit in the rack, and for a relatively short journey to a tire store, it will work fine even though the tire on the other side is a 15-inch unit. You would have to transport the blown 15" tire somewhere else, but that is OK for a short journey. I carried a giant garbage bag which I could slip over the blown tire to keep everything else clean.
Bill