Originally Posted by valygrl
Did you have do change out the bottom bracket or derailleur? anything else? I've toyed with putting mtb cranks on my 520, but haven't b/c I didn't know how much of a can of worms I was opening.
To change the 105 (or Ultegra) crankset with an XT crankset (and I think LX too), you need to change the bottom bracket. Not sure exactly which one or if it still is the case, because I bought mine in 2000.
As for the derailleur, the 105 and Ultegra derailleurs are optimised for standard "road" chainrings (i.e. 52-40-30) and should work without problem if you go down to 50 or 48 for the large ring. But all this is official text that
might be true for STI. Yet many people have successfully adjusted STI shifters with moutain gears, and the 105 or Ultegra derailleurs work without problems if you use bar-end or Ergo shifters. And BTW, I have bar-end shifters with 105 front derailleur and 44-34-22 rings on my tourer, and bar-end shifters with Ultegra front derailleur and 48-38-24 rings on my tandem. Both works flawlessly.
And Anna, since you already have the bicycle, changing the crankset is probably out of the question. But there are cheaper options.
1. Keep the large rings as is, and change the granny for a 26. You could also optimise gearing (less highs, closer ratios where you need them) by installing a Cyclotouriste 14 cassette. You can order it from Sheldon Brown or a few other shops, but you can make your own by using your existing cassette (11-32) and one of the cheap 12-25. Details on how to do it at
http://sheldonbrown.com/k7.html
2. For even lower gears, change all rings for 48-38-24 (which are the lowest rings that fit). For optimal results, lower the front derailleur by 4-5 mm.