I started this thread because I was puzzled by the fact that my folder has dramatically less trail than full sized bikes as well as hearing that other folders also seem to have much lower trail figures than FSB's and I wondered why that was. I tried the question in the Framebuilder's Forum and did finally get an answer to my question. If any of you have wondered the same thing then there's a new-to-me geometry quality called Castor Angle (which, in spite of it's name, is NOT the same as trail) that creates a "Stability Index" figure from the ratio of wheel radius to trail. Therefore, assuming all other factors are equal (which they never are), the "correct" amount of trail for "neutral handling" on a given bike is directly proportional to it's wheel radius. So, yes, bikes with 20" wheels need to have proportionately less trail.

AND if you understand this concept you can compare the trail of your bike with what it ideally should have for "neutral handling". For example my BF Pocket Rocket Pro appears to have ~ 35mm of trail but for the tire size i'm using it should have been 43mm for "neutral handling" and closer to 48 - 50 for my particular limitations. So it's not surprising that the bike doesn't work for me.

Much more into at the Framebuilder's Forum > "Geometry gurus ?" Thread. (Sorry, i'm not allowed to post links yet)