Collector of Useless Info
Join Date: Aug 2009
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99% of perceived roughness on a bike is due to the tires. Even on a supposedly "cushy" steel-framed bike, the flex is mostly in the fork (which is not made of aluminum in 99.9% of bikes), and that's only a mm or two. The best way to tell if a bike will be too harsh is to ride it and see.
20" tires, for the same tire inflation pressure and size, will feel rougher than 700c or 26" tires. That's because the tire has a slightly harder time going over a bump than a larger tire- and it transfers the bump into more forward-back force. But 20" wheels come with larger tires with lower inflation pressure, so it ends up being more about exactly the bike rather than generally about wheel size on pavement.
So, if folding and comfort are the most important things, then FietBob's suggestion is a good one- get a birdy with front and ear suspension. If you want to try it and see, then a reputable bike shop (maybe City Bikes in Sac, although they might be more specialized in road bikes) should have a loaner or two. If folding is less important, then maybe a comfort-hybrid might be more appropriate.