Not sure if this is B42 specific, but I can't see a way to craft RunFlat Small Tires. I can craft the large version, and I can disassemble the small ones (into a small rim + small rubber), but I can't assemble them again into a RunFlat Small Tire. And these trailers won't even accept "universal" tires. So if one of the original tires is missing or disassembled, the trailer becomes unusable.