This.
There are 13-28 freewheels out there. I have a couple. They'll index.
Originally Posted by
rccardr
If you have 105 or 600 on your Ironman, it should index shift a 28 cog without any trouble. Just screw in the B screw and pull the rear wheel as far back in the dropout as you can.
You can buy a 38 tooth inner ring, eBay's the place. That's as small as you can go without replacing the crankset with one that has a 110 BCD.