Originally Posted by
europa
You know what I'd do
WNCGoater? Just take your bike (or crank) to a decent bike shop and ask them to supply the right chainring, either 24 or 26 (I reckon 26 will be small enough). It's probable they've got access to parts you don't and will be able to measure the right stuff to get the right answer.
Good luck with that, but I would bet you that no such parts exist. What model of crank set would it be made for?
For the past couple of years, ways to arrive at lower gearing has been something of an obsession of mine, as I was contemplating getting a 700C or 650B bike in addition to my 20" Bike Friday and didn't want to lose the low gears enabled by the smaller wheels.
5 bolt has been the established standard for 74 BCD with Shimano (e.g. Tiagra up to 4503, 105 up to 5703, Ultegra up to 6603), FSA, Sugino, etc. Virtually all road triple cranks use either a 5-bolt 74 BCD inner or a tripleizer 110 BCD middle ring. Dura Ace 7803, Ultegra 6703 and Tiagra 4603 Shimano used a 5-bolt 92 BCD inner on a 110 BCD middle tripleizer. Neither of those could use after-market rings smaller than 30T.
Since Tiagra 4703 would basically be the only crankset a 4-bolt 74 BCD inner would fit and that's only been around for like the last two years, there would only be a tiny market for a 4-bolt 74 BCD sub-30T ring, if one existed.
So I think that rules out lower gearing via a smaller inner on any Tiagra triple after 4503. You either need a larger cassette or swap the crank set.