downsizing tires on an old mtn bike
 

I'm going to be using an old Specialized Hard Rock to start out in cyclocross for at least a couple of races.:) I'm planning on switching out the old 26 X 1.95 tires with 26 x 1 3/8. The rim's internal width is about 25mm.

I've looked at conversion and tire/rim size threads here as well as good old Sheldon's page on tire/rim size. It looks like I'd be well outside the ideal range on Georg Boeger's chart, so just to err on the side of caution before I buy new tires (since I'm so low budget as it is): Has anyone done such a switch down in size and had it work ok?

They'll be just fine.

I think you are misreading that chart, btw. 1 3/8" is off the right of the table.

Thanks Flargle,
In searching threads, it looks like you've been a good resource for folks here.

I found some used (affordable) 1.5s for my mtn bike and am trying to figure just one more thing out:

At what point could UCI rules even affect me? I see in races for elites, UCI doesn't allow over 35mm tires. Unless I turn into someone else in the near future, I won't be racing elite. So will using 1.5s limit me in any way in terms of entering cat 4 races? I just want to make sure there are no cat 4 races I'd miss being able to do because of tire size. I looked over the registration info. for a bunch of races and couldn't clearly figure this out. Thanks again.

I'd triple check those measurements. 26 X 1.95 (decimal) and 26 X 1 3/8 (fractional) generally refer to different bead seat diameters (559 vs. 584 or 590).

http://sheldonbrown.com/tire-sizing.html

Yeah--I figured out that the x1 3/8 were the wrong size before I bought them--as you mentioned, the BSD for fractional 26" tires is different from decimal 26". I needed the 559s; I'm gonna go with some 26x1.5 Continental Cross Country, which looks like a safe bet since I see lots of people here have liked them.

After a race or two, I'll figure out what else I might want to get. Just saw a Bianchi San Jose listed on Craigslist here in MA--SS cyclocross looks like it could be a lot of fun and the bike doesn't look half bad either.