I seldom use the rack to carry stuff. I have not yet had the rack stays buckle. I primarily carry loads on the front block. I have carried up to 30 pounds, not recommending it. Steering gets dicey over 25 pounds.

Yet I too worry about over stressing the rack, primarily when using the bike in shopping cart mode. In order to facilitate shopping I added an outrigger wheel such that the center of mass stays inside the wheels when in shopping cart mode. It makes for a big improvement. Most of the time in shopping mode the rack carries half the load of the bike and the half is on the rollers not attached to the rack. However to steer the "cart" I have to lean it back so that the front rollers are in the air and the back rollers then take the load of the bike and the contents in shopping back which is way over the rack allowable of 10kg. I can feel feel the flex. So far my set up works but I would like to give it extra support at the cantilever roller. A strut would be ideal. I bought an 8mm AL strut but I would need to to do a very tricky bend to keep it from hitting the front wheel in the folded position if it were to go from the roller down to where the struts attach to the frame. It would also compromise the capacity of the strut as bent columns are not as strong. Here are some pics.



I plan now to provide support by adding a cantilevered beam, a 1/4" x 1/2" beam cantilevering off the rack at 45 degrees and attaching at the cantilevered wheel. I will also run a tube between the rear wheels to tie it together. Note the cantilevered wheel does not extend out beyond the distance of the fixed pedal