Trek (and Gary Fisher) do something I don't really like. They offer a disc specific model, but the Non-Disc model does NOT have any disc tabs welded to the frame. The two holes at the drop-out are for a pannier rack. At least the 2002 models are like this. A co-worker bought a 2002 GF Tassajara and wants to upgrade to discs and cannot.
I cannot tell from Trek's website, but this appears to be the same for Treks.
So, if you want to upgrade to discs, you need a new frame. NICE!
What is confusing, is that the Trek 8000 has "disc ready" wheels.
Attached is the best picture I could find of a 2003 Trek 8000 and could NOT see any tabs welded to the frame.
Also, if you have a Rock Shock Fork, then you have an ISO (International Standard Organization) type disc tab on the fork. The Avid's use the post mount standard (like those on a Manitou fork) and will need an adapter to bolt to, regardless of rotor size. The rotor size WILL however, determine what type of adapter you need.
Attached is a picture of the Trek's seat stay. If you can find the disc tabs, point them out to me!
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"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming, "WOW, What a Ride!" - unknown
"Your Bike Sucks" - Sky Yaeger