The smaller rims have two basic designs, one a little safer that the other. The best design has a removable ring on the outside of the rim, with a narrow split in it. The other, somewhat less safe type has a full-circle removable ring, with a couple of small relieved areas to allow it to be pried off the main wheel assembly. This type has a pretty narrow locking area, and it's pretty susceptible to rust and damage from improper tire-mounting procedures. My 1959 3800 had that type of rim, in the 18" size. A couple of them were VERY difficult to align properly while inflating the tires. The split lock ring rim is pretty idiot-proof if you use just a little common sense when mounting the tires, and the more air pressure is applied, the tighter the ring locks into place.
If a sectional-rim tire is more than just a few pounds low on pressure, don't attempt to air it back up without breaking things down and finding out why the pressure is low. NEVER attempt to re-inflate a flat on a sectional rim!
Jerry
