I hear people toss around numbers like 40-50 lbs weight limits for their rear racks and sure enough, that's what most manufacturers spec their racks to be. However, I've sat on my rack, straddling it with pretty much all of my weight (150 lbs) and did not notice any flexing. Is this not the right way to determine the maximum load of a rack? Are the forces put on it significantly different than when the bike is moving and runs over a pothole? What exactly are the signs of rack failure if I am near the weight limit? What are realistic weight limits of most aluminum racks?

For the record I have some Toppeak rack that I bought 11 years ago. I don't know the model but it is made of anodized aluminum and is a little heavier, (over 1.5 pounds) while I notice newer racks come in a little lighter (under 1.5 pounds).

There's a big difference between just putting weight on the rack while standing still, and subjecting it to the vibration, potholes, and other dynamic forces it will experience out on the road with that same load on it. If you actually ride the bike with anything like 150lb on the rack that's rated for only 40 or 50lb don't expect it to last very long at all. The signs of rack failure will be cracking at stress points if you're lucky, if grossly overloaded it's more likely to collapse with little warning as soon as it breaks at one point and the additional load on the remaining points overwhelm them.

The previous poster covered the difference between a static and dynamic load, and why your test is invalid.

As for "What are realistic weight limits of most aluminum racks?" - it's whatever the manufacturer says it is. The material isn't of much concern. An aluminum rack can be far stronger than a steel rack, depending on construction. Or vice-versa. A rack with high weight limits will generally be built with more careful attention to reduction in the number of stress risers, better bracing of load-bearing components, etc. You can't generalize anything to all racks of a given material.

Look at how a Tubus Cargo, for example, compares to a typical low-end rack. It uses steel rods instead of thin stock to mount to the seatstays, etc.

While I very much concur with the fact that your static test tells you nothing about the realistic weight limits of the rack while moving, I can't totally agree with the idea that the manufacturer's stated load limit necessarily represents the actual (design) load limit of the rack -- the marked limit almost certainly has a safety margin built in and the design limit could differ by as much as 100%, given the litigious nature of modern society, at least here in the US. That said, the practical limits could be higher or lower than the stated limits, depending on a number of factors, such as where and how you ride, how a load is attached to the rack, the age of the rack, etc...

Another factor is your risk tolerance - how willing and prepared are you to deal with a sudden catastrophic failure?

Regularly exceeding the load limits of the rack will certainly put extra strain on the welds and other stress points. So will regularly loading it to its nominal limit, but riding extremely aggressively (think about it -- stand on a board, it flexes. Jump up and down on that same board, and it bends much further, and is likely to break...) So if your commute involves a lot of curb hopping, off road adventuring, or other excessive stress, you might be wise to substantially decrease your expectations of its load capacity. Likewise, if you only ride silky smooth tarmac, with slow, steady acceleration and deceleration, only, you might be able to get away with loading the rack beyond the stated limit for an extended period of time.