When folded, the bi-fold futon bed mattress has a wider seating area than does the tri-fold futon bed mattress. The bi-fold futon looks much more like a traditional sofa, whereas the tri-fold futon looks more like a two seat sofa. If space is very limited you may want to consider buying a tri-fold futon bed mattress, but if not, the bi-fold futon bed mattress usually looks far more pleasing to the eye.
Early designs involved a steel tooth design that would fit into pocket welds on the arms. These would snap off over use. The alternative design was to run a bolt through the arms and into the rails. While better it still did not address the issue of bending stretcher rails. Bent hinges also added another issue that would happen from use over time. While not as common as the other problems this issue still led to the failure of many black metal frames.
It was soon discovered though that these frames were not such a bargain that they appeared to be. The first problem with black metal frames is the round metal bars that make up the seat and back sections of these frames. Where as wood futon frames are using flat slats, black metal frames have hollow round bars. Thinner mattresses would slide through the gaps between the round bars. On wood futons this was never a problem.
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