The large ring at the top (#3 in the diagram you posted) is what holds the stay in; tightening the screw pulls the stay up into that ring so it can’t move. Your rack is missing those pieces, so there’s basically no clamping on the stay, allowing it to move around.
You should also have at least 10mm of stay sticking past the bracket, so after you install those parts you’ll want to move the rack forward.
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https://preview.redd.it/ew873n7szlmf1.jpeg?width=3024&format=pjpg&auto=webp&s=34c650c093fdc55d6bd633eecdd9732c188804d1
it came with rubber caps for the roundstay ends, but they looked cosmetic, was i wrong??
I just pulled a Tubus rack off an old bike this afternoon. It should look like this:
[https://imgur.com/a/LfAUi8U](https://imgur.com/a/LfAUi8U)
The large ring at the top (#3 in the diagram you posted) is what holds the stay in; tightening the screw pulls the stay up into that ring so it can’t move. Your rack is missing those pieces, so there’s basically no clamping on the stay, allowing it to move around.
You should also have at least 10mm of stay sticking past the bracket, so after you install those parts you’ll want to move the rack forward.