Originally Posted by
fuzz2050
However, if you really disobey manufacturer recommendations and use a wide tire on a narrow rim with a narrow tube, you do get more flats (ask me how I know). The tube around the stem seems to stretch out and wear thin, and I got a string of holes right around the reinforced grommet. Switching to a tube that was closer to the right size helped out a bunch with that particular problem.
That is some of the best advice I have seen in this thread. The weak point when using undersized tires is the valve area. Just pump a tube up with no tire and you'll see how it expands. We are not talking thin like a balloon here, but tubes do stretch a fair amount. If you are going to have a problem going undersized, it will be in the area of the valve.