Picture of Cheap Bicycle Panniers
I traveled to Hainan,China to go on a biking trip and needed some panniers. Didn't want to spend a lot of money so I went to a local market and bought two backpacks ($7 each - should be the kind that also have a strap that goes around the waist). I simply pulled the straps out of their buckles and put them in the respective buckles of the other backpack. Then I draped the two backpacks over my tent on my fender rack. Secured the bags to the side of the rack by using the waist-belts.
I could put the packs on and take them off the bike in about 30 seconds and I had no problems with them interfering with my biking. I secured everything to the bike with one long bungee cord.
First photo shows how I used them for 600 kilometer trip around the island of Hainan, China.
Second photo shows the two bags hooked together. 
Third photo shows the waist strap is wrapped around the bike rack spar and then cinched tight so that the bag will not move around or get caught in the spokes.
Video of my trip around Hainan:

Wow, I really like this idea. Does this work (with common school student type backpack) unloaded or with load in only one side? How does the bag stay centered? I saw on another Instructable, using vinyl plastic campaign signs to make boards to reinforce the insides of the bags. It used hooks to mount the panniers. Your idea looks much stronger though, since it uses the padded straps already sewn on the bag. Good job!
JimRD (author)  Yard Sale Dale3 years ago
In addition, I suppose you could use just one bag by attaching the straps to the rack somehow.
JimRD (author)  Yard Sale Dale3 years ago
Hi and thank you. You need two identical bags, one on each side, take the straps out of the strap buckles and put them in the other bags buckles. Then strap the two bags (that are now hooked together) over your rack (or over the tent on the rack). They are balanced by putting equal weight in both bags - if you dont do that you will have trouble. I use a bungee to hold the bag straps to the bike rack and this also keeps it centered. The bag waist straps are hooked to the bottom side of the bike rack to keep the bottom of the bags out of the wheel and away from your feet while you are peddling.I had no problems with this setup and in fact on Dec 28th, 2011, I am flying back to Hainan to do it again.
Simple + Cheap - Excessive Materials = Brilliant!
JimRD (author)  realanthillkit3 years ago
Wow, that word Brilliant gives me such a big head. Thank you.
zazenergy4 years ago
That's pretty cool, but I am not totally sure how you attached them at the bottom and without hitting the spokes. Sounds like a great hack -- if you have more pictures detailing the process that would be great to see!
JimRD (author)  zazenergy4 years ago
Thanks, I added a couple of pictures to clarify.