Here are some pictures of the piggy-back frame I made to allow me to carry Ortlieb panniers on my Brompton. This was a proof-of-concept frame I knocked up in a couple of hours with no access to tube bending tools. I never got round to tidying up the welds properly nor have I painted the frame. So be warned, it is not a pretty sight. However, it has functioned perfectly whilst fully laden all summer (1100km including the Rallarvegen). The total load on the luggage system was 16-18kg depending on how full my water bag was.
1) The view from the rear of the Brompton touring bag. The frame is attached at the top with a pair of jubilee clips. Most of the weight is transferred directly to the mount at the centre of the Brompton bag frame. This mount, in turn, sits directly on the Brompton luggage block. This seemed to be the most sensible way to do it at the time. I am not sure I would do it differently now, as it still seems to make sense.
2) Close up of the lower mounting point. There is an L-section piece of metal welded onto the middle of the bar. The bottom of the L protrudes forwards and sits in a little trough at the top of the plastic mount. I don’t know if newer Brompton bag frames have the same feature.
3) View of the top of the frame and how it is fastened to the Brompton frame. The jubilee clips work a lot better than the fastenings I made myself. The clips pull the frame down so that the L-shape tongue is clamped to the plastic Brompton bag mount. One of the problems of this design is that later Brompton touring bags cover the Brompton bag frame almost completely. You’d have to unpick part of the stitching at the top of the bag to be able to get a jubilee clip or other fastener round the top bar of the Brompton bag frame.
This is one of the reasons why I plan to do away with the Brompton bag completely in the Mk 2 version. The other reasons is to be able to use Ortlieb front panniers on both sides of the frame and save the weight of the Brompton bag frame.