i have built a 35l ish tank from 2mm ally that is level with top of the diff frame and sits on a grp frame by ditching d13 i could make it simply longer
i think the grp tube work is strong enough without d13 im 80kg and i can stand on the tip of it and jump up and down with no bending or movement
the hole idea was to keep a full size stowage area in the back and get a bigger tank so touring would be possible
if you have a metal back end then you would need to redesign the support as the orignal only has two thin strips of metal and d13 that support the backend
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