Time needed = about 1 hour. Total Cost = Under $20.Materials Needed:30 feet 1" schedule 40 PVC pipe 18 connecting T’s 6 90-degree connectors (elbows) PVC pipe cementFor a 4-bike rack, see the parts list and cutting instructions at the end of this article!
Cut PVC pipe to the following lengths:6 – 18 ½” 6 – 16 ½” * 6 – 8 ½” 6 – 9 ½” 9 – 1 ¾”*the 16-1/2 lengths fit a Ford F150 truck bed. You may need to vary this length for your particular truck. You can assemble it without gluing and then make adjustments.
Assemble the frame as pictured at right. This is a view looking straight towards the wheel-slots. The horizontal pipe in the middle (seen behind the main assembly) receives the support struts. See the photos below.
Assemble the entire structure without gluing. This lets you adjust fit and set a correct angle for the elbows and Ts. Here we're assembling the main support frame. The two close pipes at the left of the photo form a slot, into which a bike wheel will slide. When complete, this part will be turned over so the Ts face down into the support struts.
Here the support struts are seen coming up into the support bar. Lying against the ground are the wheel-slots that will receive the bike tire. (In use, the structure will be turned over. The part that's sticking up here will be on the bottom of the truck bed.)
Glue joints one at a time, then let dry. In this photo, the support strut is ready to receive a T-connector.
Here the rack is complete, and has been turned over into its position of use.Drill a hole in the underside of the top connectors for bungee hooks (3 total). The bungee will hold the bike into the frame. Size the hole to fit hook style (plastic hooks work best.) An alternative is to wrap the bungee around the upper pipe and hook it back on itself.
Notes:
  • Glue connectors touching each other for standard mountain bike tires. Leave about ¼” space for 3” tires.
  • Use bungee cords to tie rack down to truck bed.
  • Hook bungee to seatpost or pedal to hold bike in place.
  • This design should clear an 8” rear brake rotor.
And here's another take on the rack:  Rick and Jeff Rodriguez sent these photos showing how they transport 5 bikes in their truck bed. They say the paint quickly gets scratched off. But consider paint if you plan to leave the rack in the sun -- PVC gets brittle and cracks after a year or two of sun exposure. 

Here's the basic rack, painted and

ready to receive the bikes.

On the ground, bike tires fall into the

slots for a nice stable (cheap) rack.

For greater lateral stability, secure the

brace on top of the bed liner.

Here's another look at the bike rack with the brace sitting on the bed liner. A couple of tiny holes drilled in the liner can serve as anchors for zip-ties that lock the rack in place. In this truck, the front tires of the outside bikes sit partially on the tire well, raising them slightly. Having the rack oriented upward as shown still provides a solid hold.

OK, so now we've got three bikes. Raising

the outer bikes eliminates handlebar clash.

Now, let's add two more bikes. They

sit upside down and backwards.

Toss foam pads between the center bike and

the upside-down bikes so they don't bash. 

Now toss a bungee or rope around the frames of your two upside-down bikes, so they won't tip outward against the outer forward-facing bikes. You're now ready to transport FIVE bikes to the trail in your pickup truck.