Aluminum racks are the lightest but they also won't carry as much load as a cromoly rack which weighs more like the Tubus that I have. The Cromoly ones are rated at 88 pounds and weigh about a pound and a quarter. The other advantage to heavier and stronger racks is that they do stiffen up the rear of the bike so if you ever had sway on a loaded touring bike with aluminum racks switching over to Cromoly will probably eliminate it and at the very least reduce it by 75%, so a good rack will improve a loaded bike's handling. There is also stainless steel that weighs a bit more than cromoly which is pointless since it has the same 88 pound load carrying capability unless your concerned about rust, but it may be stiffer than cromoly. Aluminum racks are known to crack at the welds while steel will last a lifetime, but aluminum will be fine if you travel light and well below it's supposedly rated maximum load carrying capability. It's one thing for a rack to carry weight without jostling around but once you start moving the load will bounce up and down and on the downward movement whatever load weight you have is suddenly greater, so you could be traveling with 50 pounds but the downward force could exceed 60 pounds which the rack is not design to take for a prolong period of time.