Hi Pete!
According to Sheldon (
http://sheldonbrown.com/harris/chainrings/74.html ), the Tiagra uses a 130mm middle and large BCD (bolt circle diameter) and a 74mm inner. The 74mm inner will surely fit a 26T and even a 24T if you prefer.
If you go that route, I would suggest adding an inner chain deflector, such as the 3rd Eye Chain Watcher (
http://www.3rd-eye.com/%2801%29.htm ) or N-Gear Jump Stop (my preference):
http://n-gear.com/ This will allow you to adjust the front derailleur for more aggressive and positive shifting to the small inner 'ring without fear of dumping the chain onto the BB shell.
it is doable, but you'll have a large gap and poor, sluggish shifting trying to make the jump from the 26T inner to the 39T middle. Best to keep crossover shifts to 10-12T differences between adjacent 'rings. Others have gone before you with a measure of success and caveats. See:
http://www.bikeforums.net/showthread.php/397053-Smaller-than-30t-on-Ultegra-triple <-- Same BCD as your Tiagra.
[EDIT: Ian posted some very good advice while I was typing: You may well need to replace your existing front der if you fit the smaller 'ring to your existing setup. Another good reason to seriously consider the Deore setup below]
You'll have much better shifting results with a Deore 48-36-26. It is a good value in cranksets and comes with steel middle and inner rings, which add a bit of weight but also adds longevity for touring/high mileage use. You can substitute alu rings at extra cost, or go up a notch in Shimano's crank line and get all-alu rings as standard. Do be aware, these cranksets will have external-bearing BBs, which can prove shorter-lived and problematic compared to traditional square-taper BBs (poorer weather sealing and smaller/fewer bearings are the culprits). That said, I decided to go with a Deore HollowTech II on my Nomad as I did on the Sherpa, and have been pleased with it to date. I have a Phil Wood external BB in reserve for when the Shimano unit eventually goes bad. These cranks and external BBs are very stiff and have the added convenience of being serviceable with only a 5mm allen key. On the other hand, a crank arm-mounted extractor will do the same for removing conventional crankarms at a very modest cost. For a full discussion, see:
http://www.thorncycles.co.uk/forums/index.php?topic=4344.0If you're looking to do this cheaply, remember -- new or nearly new cranksets often show up on eBay for very reasonable prices. If you went with a 110/74 BCD crankset, you'll have a good selection of 'rings to choose from and good weather sealing/long life for the BB. It is proven technology, but not the direction the industry seems to be heading.
Hope this helps.
Best,
Dan.
