I have a 2010 Diamondback Insight 3. I'm trying to mount some fenders and I noticed the horizontal tube connecting the two seat stays (where you would mount a side pull brake caliper) has been welded between the two seat stays with the threaded hole through it vertical- basically the hole is 90 degrees off from where I'd expect it to be. In order to mount my fender, I will have to bend the bracket 90 degrees.

Is this a manufacturing error or is it normal?

Why would you want a hold drilled vertically through this part of the frame anyway? What purpose would it serve?

It's welded correctly... or at least correctly for that company and that particular model of bike. I've seen a few MTB frames with the hole sitting vertical like that. I suspect that the designers or maybe the fabricating company wasn't thinking of us poor slobs that like to run with fenders.

Any idea what they were thinking of? What would you mount on this hole?

you could just drill a hole into the fender and thread a bolt with washer that fits into the slot.
that's how some older bikes had their fenders mounted.

Good idea, thanks.

My carbon fiber cyclocross frame has a vertical hole in the brake bridge section of the seat stays too (though there is a braze-on fitting instead of just a hole there). Use a screw through the fender as has been suggested. If the resulting position of the fender is too high off the tire then use spacer above the fender to move it down.

Ironically, a lot of custom bikes built for fenders all have these holes on both the chainstay and seatstay bridges radially aligned to the wheel. The idea is that you can eliminate the L-bracket and make the installation "clean". The bridges are also positioned to have an identical and appropriate bridge-to-axle distance so a good fender line is assured.