I seem to remember that the standard System luggage was only rated to about 8Kgs each side.
You could probably lay a Lomo 20 ltr dry bag on top of each pannier extending alongside and strapped to the rear carrier and the pannier mounting rail. That would keep the weight lower and further forward in much the same way as the people with alloy luggage strap stuff on top of them.
Otherwise take less clothing and use laundry facilities on your travels.
Thanks for the idea, i like this, and it's cheap with no modification, and when empty the bags and straps take up minimal space.
I have some 20 litre dry bags on order (for inside the panniers) so I will give this a "dry" run when they arrive.
Cheers



