Is it wrong to install a bathroom fan in a wall even thought the instructions only explain installation between trusses, in a ceiling/floor?
I can't run the vent duct horizontal for the stated minimum length of 2-3 feet, but if I mounted the fan in a wall I could run the duct straight up without any bends.
Anybody got any ideas?

Most bath exhaust fans can be mounted vertically in a wall, in fact some installation manuals will mention that wall mounting is an option. I often mount fans in interior, never exterior, wall stud spaces because it avoids an additional large penetration through the "vapor barrier" in high-moisture area of the house and it doesn't reduce depth of attic insulation at that point (I'm in a cold climate in Wisconsin). The fan can be ducted to vent up or down, depending upon shortest route and most practical location for a wall cap. I avoid vents through the roof. The fan won't vent with a foot of snow over the roof cap.

Great.
Thanks

If the instructions don't mention wall install, then I would not do it, as the bearings will burn out quicker

That's what I was fearing, I imagine these fans were designed and balanced specifically for horizontal placement unless otherwise stated.
I thought about calling their support line but I imagine the person on the other end would know even less about the technical aspects than most people on this forum.

I'd contact the fan manufacturer for their opinion. If they say it is OK, get it in writing.