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...

PARTS:
Segment of chain-link fence tubing (PVC pipe would've been much easier if available)
2 Chainlink fence tensioning brackets
Electrical Tape
2 Locking wire pins (about $3.50 each at Lowe's, in the same cabinet as Clevis pins)

TOOLS:
Hammer
Punch
Hacksaw
Drill Press
Knife
Marker
Vice
Dog (gotta have a good distraction)

Step 2: Measure Twice, Cut Once

First, measure the tube length you'll need between the headset and seatpost, leave a little room for play. Mark the pipe with the needed length and stick it in your vice. Cut the section of fence tube to length. File sharp edges and wrap ends in a couple layers of electrical tape to protect paint and carbon fiber parts.

Step 3: Hammer It, Bend It, Make Holes in It

Take the fence tensioners and flatten them both out. Make sure to be careful, hold them tight and wear ear protection if using a metal surface to flatten them.

Next, measure the diameter of your headset and seatpost. Mine were about 1-1/2". Leave a little extra room and bend the flattened tensioners into a nice U-shape. I used a vice and shovel handle to keep the bend from getting a point. Wrap them in electrical tape if you want to protect your expensive bike parts.

Don't forget to cut the tape out of the pre-drilled holes in the tensioners. Your pins will go through these

Step 4: Puncturing Your Pipe

Set the pipe up on your top tube, and using rubber bands, place your U-shaped brackets where you want them. Mark through the holes in the U-brackets to the pipe for where you need to drill holes. Find a drill bit slightly larger than your pins, use your punch to make dimples at the starting points and go to your neighbor with the drill press. Drill holes through both sides of the pipe to keep them even. Make sure you securely fasten the pipe before you start drilling. After the first set of holes is done, use a LONG rubber band to go around the pipe and mark the second set of holes; this will insure all the pins wind up on the same plane.

Step 5: Voila!

Assemble the adapter, attach it to your bike. Now stick your bike on your rack and go find a new trail!

Notes:

If you have thick PVC, use it. It is easier to drill through than the metal. Just make sure it doesn't flex too much with the weight of your bike.

Make sure your locking pins secure very well, I think i should've gotten longer ones to insure they don't vibrate open.