Hot answers tagged luggage
I would suggest splitting the weight in half and putting it low on your bike for stability. Here is my daily commuting configuration (2 matched 6 gallon plastic waste baskets mounted by hose clamps to a standard bike rack.) (Your emptied backpack could be rolled up and placed on top if you need it for off-bike use.)
There is more than just air friction to consider. If you extend the rack back then you would have too much weight aft and it would make the font wheel lite (it may even bring it off the ground). Vertical you don't have base size to secure and now you have weight higher. The bike would be wobbly. You are not going to have a lot of speed. I would go ...
This is more of an extended comment. If you're mounting it on one side, that's likely to be quite a lot of weight off-centre, which will also affect the handling. I'd aim for vertical, but you'll need something solid underneath to support the weight - like 2/3 of another rack inverted and bolted/clamped to your rack. Alternatively you might get this to ...
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