What can be creating resistance when I cycle? 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?
Make sure that:


The hubs aren't over-tightened
The tires aren't rubbing on the frame or brakes
The tires are at the highest allowed pressure
The freewheel isn't stuck


My guess would be that (at least when loaded), one of the tires rubs on the frame.