[gilligan]

although they may seem conveinent they are pretty much unstable under heavy loads. if you are just using them for homeowner type stuff they should be fine but if you plan on using them for heavy loads on a regular basis i would not reccomend them. i got a roofer buddy who had them and in about a year they would shake front to back without any load. look in the parking lot of home depot or what not and find a truck w/ them on and give em a shake. i think that will answer your ?.
good luck.
p.s. i dont know where you are but Lynn Ladder in ma. had the best prices on all types of racks