I can relate; usually using a 33L Dakine rucksack for most things (including work).
However, this doesn't play well with the 8 miles on the Brompton that now forms part of my commute as it offers no breathability for the back and is really too large / deep / heavy to be comfortable over distance; causing my neck to ache.
So, I too am looking for a Brompton-friendly alternative of similar capacity. Options appear to be:
None-Brompton-compatable rucksack
Same problems with carrying all load on back (aching / discomfort). Ventilated options are about (I have a small Osprey which is great), however this limits capacity and few if any of their cycling bags go to 30L. Not a very viable option IMO.
Single, Brompton-compatable rucksack
On paper a good approach - easy to carry and unobtrusive when off the bike, can be hung off the front to relieve load on the upper body when on the bike. The problems are capacity and availability. To fit the back rucksacks are portrait-oriented, while this is not ideal on the Brompton as tall bags foul the bars when steering.. so this typically limits capacity. Brompton get around this with their Borough waterproof backpack by mounting it sideways on the bike; however capacity is only 17L, it's expensive (c. £180) and appears to have relatively limited provision of packing options (I too love to compartmentalise everything). This approach would be my favoured route if I could find an appropriate bag.
Single, Brompton-compatable shoulder bag
Lots more choice here as the shape of the bag for either application is very similar. Typically bags for the front of the Brompton top out at about 20-25L, so perhaps a bit smaller than ideal. There's a reasonable amount available but again none really ticks all the boxes for me. A single shoulder bag should be manageable when off the bike, however probably not as much as a rucksack; especially if it's a pretty big one. Big bags seem to look a bit excessive hung off the front of hte little bike. This is the route I expect I'll probably take, but I have my reservations.
Small rucksack, Brompton-compatable shoulder bag
What I've been running for the past two days:
- 12L Osprey Syncro for bike-specific bits (puncture repair, tools, lights, ride-specific clothing - which all live in there anyway) and personal stuff in a separate drawstring bag inside the Osprey (phone, wallet, keys etc).
- The front bag / basket (Brompton Borough) takes a bagged change of clothes (which I haven't currently used, but will if I begin to stink too much / need to become more presentable / happen to unexpectedly sh*t myself), lunch, water bottle and deodorant.
On the bike / at work this works nicely - splitting the load into manageable chunks both physically and mentally; allowing better compartmentalisation and separation. The down-side is when off the bike; having to carry / drag it as well as lugging two bags. The trolley approach with the bag still attached might be worth investigating. Also, I'm a bit less keen on the rucksack as it (and all others) tends to abrade my shirts somewhat. That aside it's a really nice little package for carrying / organising the necessaries for a typical ride.
The Borough bag / basket came with the bike and is more suitable for casual urban stuff than a longer commute (open-topped which has already seen an empty eggbox jettisoned over a speed bump and a lack of waterproofing), however it's offered value in allowing me to try a few things and get a feel for what I can pack into a given capacity.
I appreciate that a lot of the above's not directly answering your question, however what you're proposing is an approach I'd have considered before I became educated in the benefits of on-bike luggage (which you apparently already are).. the above thought process being where I am currently when faced with a similar situation to yourself.
At the moment I'll continue to run two bags while I try to get my head around the problem. It would be great if a suitable on-bike rucksack appeared, but this is probably unlikely in an appropriate timeframe. Likewise a shoulder bag that ticks all the boxes.
FWIW I've also been looking for a replacement for my Dakine for ages now (capacity and feature set are good, but it has a few shortcomings, is well worn (holed beneath with slightly dodgy zips) and tbh I really don't like the aesthetic much. Really stuggling to find anything apporpriate to replace it with, however
Good luck - I think you'll need it
