Originally Posted by
unworthy1
allow me to piggy-back on this thread: I'm trying to help a friend with a similar bike, a 1985 Trek 620. It has the same gearing 50/45/28 triple with a 5-speed Helicomatic in the back with 28t cog. We have determined that the Huret Ecopar is toast and have swapped in an early Shimano Deerhead RD but it's not happy shifting this modest range, either...you'd think it would be a walk in the park for it. The original Huret FD does an OK job shifting the big jump from 28 to 45, but it wouldn't hurt to find something even better: any specific suggestions for a "three dimensional" FD that would do the job? I'm going to try another approach and check out the RD hanger for straightness any maybe swap in a newer chain, tho the old one still shows no real stretch...It has new cables and housing...
I think the original deerhead XT r/der. came in a few different cage lengths, so you may have the shorter one, though I'd be suprised if it couldn't shift up to a 28t. Even my Shimano Sante rear on my Davidson (which has a very short cage) shifts up to a 26t no problems at all. Are you sure there is nothing wrong with it? Is it setup correctly, with the low screw turned out enough to allow the jump? Does the shifter have enough travel? Is there a lot of slop at the pivots?
As for the front derailleur, there are many that will work. I have an older XTR on my Miyata and it works very well, and that's with a 48-40-26 setup. Any decent late-80/early '90s Shimano (DX, LX, XT, XTR) etc.) or Suntour (XC, XC ltd. etc.) derailleur is my recommendation. The main issues you may run across when changing to a newer MTB-type derailleur are:
1) The bottom of the derailleur cages, where they join with a spacer and bolt or screw, may be very close to hitting the rear shifter cable when the derailleur is set at it's optimum level (i.e. - 1-3mm above the largest ring). However, this is typically only an issue for bikes where a) the cable routes on top of the bottom bracket and chain stay instead of under (Personally I don't know about the 620; you will know that of course), and b) if the larger ring is anything less than approx. 50t, which yours isn't. My Miyata has this problem (the derailleur is sitting a few mm higher than optimum, but luckily it still shifts quite well).
2) The outer cage may rub the inside of the crankarm when shifting to/in the largest ring. It depends on the crank and the derailleur combination. You will just have to try it to find out.
-Leigh