Bicycle Mechanics - New fork help?

I looking for some information on replacing the fork on a 2002 Trek 6700 mtb. The bike came with a Rock Shox Duke which has turned out to be a huge POS.....looking to replace it with a Marzocchi Marathon S, with 105 mm of travel. I believe the max travel on the 2002 Duke is 100 mm. The guy at the bikeshop today #1 didn't really know that much about forks but kinda had to help us because there wasn't anybody else who could; and #2 totally freaked us out by saying we had to get something no bigger than 80 mm because it wouldn't work on the bike and would void a warrenty, blah blah blah. I guess my question is how much max travel is really going to affect the geometry of the bike? Will 5 make a huge difference or could you go higher? Also, if anyone could recommend a possibly different/better fork than the Marzocchi, I am open to suggestions. The only stipulation is we are trying to make the purchase before the end-of-the-year sales are over! This is strictly for a hobbiest, there will be no racing or dropping off cliffs done on this fork! Any help is appreciated!!

Cheers! :beer:

20mm can be a jump. That said, it may affect the handling favorably. I added suspension to my hardnosed Sedona and gained a handling sweety, I upgraded the 80mm to 100mm on my Xtc and tamed it out. Try it and see.

the genreal rule seems to be do not increase the travel on your fork by more than 15-20% of the original travel. its really up to you - an increase in travel will be most noticable on climbs. if you dont ride steep technical climbs then you can probably get away with it. ride them with as much sag as possible and perhaps slow the rebound a little.

Conversely, on technical descents, the longer travel has a favorable slackening effect. A lot less endos

The shop could very well be right about voiding the warranty on your Trek though, a longer fork increases leverage on the headtube.

The shop could very well be right about voiding the warranty on your Trek though, a longer fork increases leverage on the headtube.

True, but we don't think there is any warrenty left on this one, so thats not a main concern. We are more interested in knowing if spending $300-400 is going to be worth it. Its not like you can hop on a bike at the shop in interchange all the forks you want to try out before purchase, so we must rely on reviews and information from various sources!