When we decided to start our online Brompton accessories store we didn't know it wich directions it would go. We started with the unique front rack frame that would allow me to add my backpack that has an " add-a-bag-sleeve". An "add-a-bag-sleeve" usually serves to slide over another suite case trolley.
It is "THE" solution for attaching a backpack, without a frame inside, to your Brompton. It is 20 cm wide (check your sleeve width) and 30 cm high. All the backpacks shown on our shop fit on this front carrier frame. The frame locks on the original Brompton front carrier block. (not included) but you can get it here from our store. This is a straightforward, hard-to-find solution for attaching a backpack to your Brompton.
Our largest backpack we found that fits to the front carrier frame has a capacity of 56 liter and expandable to75 liters.
You could call it huge. When it is full loaded my Brompton still behaved normally and I could take turns easily.