Many hitch-mounted bike racks require a flat top tube. This is a top tube adapter to fit a mountain bike to a hitch-mounted bicycle rack. The cheapest one I've found is $20. I thought it would be simple to make with household junk and $7 worth of pins.
This is my first Instructable, feel free to comment.
Step 1: Parts and Tools
Most of this was found around the house...
Segment of chain-link fence tubing (PVC pipe would've been much easier if available)
2 Chainlink fence tensioning brackets
2 Locking wire pins (about $3.50 each at Lowe's, in the same cabinet as Clevis pins)
Dog (gotta have a good distraction)