roccobike

There's a dude on this forum asking the same questions about upgrading a Trek 820. The 3500 is pretty close to the 820 and has similar limitations. I'd ride one on a medium level single track, technical trail, but stay away from big jumps. You need to know your bikes limitations. As for upgrading the fork, you've got another problem to consider. Changing that fork will raise the front end and could impact the handling of the bike. Forks are expensive, even for a good used fork. If you want to upgrade don't go any higher than 100mm. If you look at the 2014 specs for your bike, TREK upped the size of the fork on your model to 75mm. It looks like the frames were not changed much if at all. That means your bike could handle a 80mm aftermarket fork no problem. Maybe a 100, but I most certainly would not go larger than 100.