20" rims are not 20" in diameter
A 20" rim is around 406mm = approx 16" in diameter
A 20" diameter = 506mm which is actually around the size of an actual 24" cruiser rim diameter (507mm)
So if you had a 20" x 1 1/8 rim it will be thin and 16" diameter (406mm) and the tire would be thin too so maybe it would fit into an 18" fork (16" for the rim and upto 2 inch for the tire)
Thinner the rim width then smaller circumference fo the tyre. Plus you have to add the tread depth on top so maybe 18" for forks is right!
Plus because the rims are narrow.............I dont think many people would run super chunky tires with huge tread depths on them as they'd probably pop off the rims are make it wishy washy in the handling.
18" is about right.................but I'm no expert..........so dont hold me to it!!
