Originally Posted by
mrleft2000
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).
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.