Originally Posted by
Antifriction
I pointed out that failure happened where a piece designed to operate as a strut (axial load) was being loaded as a beam (from the side) - and had a big hole in it. What I pointed out was that turning the strut upside down - single hole at the top - would have eliminated the weak point - putting solid strut at the point of lateral load. So for one thing it would be good to advise people with the rack whose struts haven't failed yet to do that (and cut off the lower end beyond the hole they're actually using, if it interferes with anything).
For you: although the broken struts & the replacements you've found are both too short to use with your current panniers, if you bolt them together you can make a long-enough strut with no hole at the danger point - and avoid the need for new bags.
The problem with flipping it upside down is that The strut fits inside a C shape profile about 28.5 mm long, which counteracts the rotation of the strut at the bolt. The bottom hole that's meant for the eyelet doesn't have enough material at the end to reach the full height of the C shape profile, only about 5mm from the edge of the hole to the end of the strut. So if you did flip it upside down it will be very unstable rocking back and forth inside the C shape profile.
The C also 15.3 mm wide while the original strut is only 14.5 mm, so there's a lot of wiggle before tightening the bolts. The part I'm going to make will be 15.0 mm.
So I think it's not only the lateral force that caused the strut to break, but also the bending/rotational force applied by the C shape profile in the plane of travel.
The strut can't be doubled up because it sits on the inside of the C shape profile which would reduce the spacing in between the struts.