Originally Posted by
Lanovran
Unlike the 3500 Disc, the standard Trek 3500 does not have the mounting compatibility on the frame nor the stock fork for disc brakes, so it would be costly to change the front, and impossible to change the rear. If you want to upgrade the front brake to disc, then you would have to get not only the brake system (calipers, rotor, new/longer cable & housing), but also a disc-compatible front wheel and a disc-compatible fork. Add on labor costs for a mechanic to get it properly installed, and it could be pretty expensive.
Bit of wrong info there.
The Trek 3500 Disc frame has a bolt on IS disc brake mount, this bolt on mount will fit on the 3500 frame too.
Bolt on Post Mount:
https://www.sellutionmtb.com/Product...ctCode=BA-GFTP
Bolt on IS Mount:
https://www.sellutionmtb.com/Product...ctCode=BA-GFTU
Most disc brake calipers are now days are post mount, Trek uses the bolt on IS mount on it's bikes, with the bolt on IS mount an additional PM to IS adapter is needed to fit a post mount brake caliper, with the bolt on Post Mount you can directly attach a post mount brake caliper onto it.
Bolt on Post Mount with post mount brake caliper directly attached.
Bolt on IS mount with a PM to IS adapter to attach the post mount brake caliper.
* Upgrading this bike to front and rear disc brakes will most likely cost you more money then what the bike is worth.
2013 Trek 3500 - New and Used Bike Value