For Bike Friday on their touring bikes, when a regular rack is used, which you must, due to the rear panniers are just that tall,

There is plenty of space over the wheel for a storage bag under your rear rack , then...



Make, not shop.


Your OP is a beam rack , how much space under it depends on how high you mount that rack above the rear wheel on the seat post..
then again a long seat post would help.. ..



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