I recently bought a foldable bicycle.
Everything seems fine and smooth, but when I cycle I can feel quite a lot of resistance - not air resistance, but resistance on my tires - to the point than when going downhill the bike brakes (itself) and stops.
My first suspicion was that the brakes might be broken, I thought maybe they were too tight. However that's not the case since I tuned them, and at one point I even released them completely to be sure it wasn't the brakes.
When I spin the wheel it seems to run free. It does not spin as much as I would expect but nothing really serious seems to be slowing it.
What could possibly be happening? Maybe when I'm cycling my extra weight affects the wheels somehow; is there some other tuning or test that I can do to find out and fix this problem?