Is there any "rule of thumb" for when to replace/repack wheel bearings on a utility trailer? I normally do my boat trailers every year, as they are repeatedly dunked in salt water. The trailer I use for bikes has a good number of miles (and tiny little 8" wheels that spin at high rpms), but the wheel hubs still stay cool on the highway, and there is no excess free play or any other signs of trouble. Should I just leave things alone, or is there a timeline for replacing these things? Thanks.