I modified the suspension on our small fifth wheel just so I could run 16" LT tires on it. You want to stick to ST or LT tires, and LTs only come in 16" and bigger. For 14" and 15" another option is to switch to better brands of ST tires--Maxxis is popular and that is what I use for 14" and 15" trailer needs. If possible it would be a good idea to increase the rating of the ST tire that you're using by going to a higher ply rating (such as switch from C to D) or a wider tire (215 to 225). Search for threads on tires--probably one of the top 3 topics. In fact this thread will probably explode just after everyone wakes up.