Join Date: Dec 2004
Location: Middle of da Mitten
Bikes: Trek 7500, RANS V-Rex, Optima Baron, Velokraft NoCom, M-5 Carbon Highracer, homebuilt recumbent
Twenty-eight teeth from granny to big chainring is a huge range, possibly more than the derailleur can handle. Take the chain off and try to shift the derailleur up to the 3rd ring. It should center laterally over the ring and should have about a mm of clearance between the tops of the teeth to the derailleur cage. Too much gap and it will shift the chain off the outside, too little and the cage will hit the chainring when you try to shift. You may have to loosen the clamp at the shift tube and slide the derailleur up or down the tube to get it positioned correctly. (Also make sure the cage is straight along the chainrings.)
If that's OK and after reinstalling the chain, it doesn't shift, you might need a larger *middle* ring. Derailleurs can only shift so many teeth at once, and 18 is likely too many. Get a 46T ring, tha'll only require a shift of 14 teeth up and 12 teeth down, which will be much easier.
Before taking that step, though, shift to the granny ring and verify that the chain doesn't drag on the bottom of the derailleur cage. If it does, you'll either need a larger granny or a smaller big ring. Adjust the size of the middle ring as needed so that it's halfway between the largest and smallest.