They mount between the hub and wheel. I installed my own (so it's easy). The hardest work was installing them on the dully wheels because I had never taken one off before, there is a slight trick to hold the inside dully wheel so it is not creating pressure on the outside one preventing it from coming off. I also had to cut the access hole on the rear truck wheel centramatics, for the air stem, slightly larger so I could get my hand in there for the air chuck--I just used a tin snip for that job and a metal file to get rid of any rough edges. I have steel rims with simulators so it's easier to get my hands in the access area. The aluminum rims have smaller holes. If I had aluminum rims I would install longer air stems.