Brompton (Radical) C-bag: As bulky and aerodynamic as a large house, but if you care about aerodynamics why are you riding a Brompton?
Enough volume for standard commuter stuff, with room left over to grab some shopping on the way home. Or for use as an overnight bag. Or for a laptop, geek tools and a big tangle of wires. Or a quick trip to the local shops. Or a full set of winter cycling kit, including bike shoes (for throwing in a motor vehicle alongside the Brommie). That sort of thing.
It's a really nice bag. It's got the same two bidon-sized rear pockets that hang either side of the stem as the T-bag, which neatly solves the water-on-the-bike problem for multi-modal journeys. There's also a side pocket that'll hold pens and stuff (but is usefully exactly the right size for a spare tube and pack-and-forget toolkit), a large compartment in the flap, and a small mesh pocket for quickly stuffing things in on the other side. Internally it's one big compartment with an internal pocket about the size of an early-2000s laptop (the rigid frame handily means that items in this position aren't going to be under any bending stress), and a zipped compartment for loose bits and pieces. The inside is all bright yellow, which is surprisingly useful when you're groping around for things in poor light.
There's a messenger-style strap, which is very comfortable in use (though over-shoulder comfort of the bag is largely dictated by what you've got in the rear pockets). Sadly this doesn't detach, so has to be stuffed into the main compartment when the bag's on the bike.
Sadly this bag isn't waterproof, though a little drizzle on a 2-mile ride to the station isn't going to be a problem. It comes with a fluorescent yellow 'shower cap' which seems to do the job as well as any shower cap does: That is to say that it's awkward to fit when you're wearing gloves, hampers access to the luggage at the roadside, inevitably doesn't get fitted until too late, and is a real pain when it's windy.
The bag frame is the larger size which isn't designed to work with the S-type Brompton. In practice you can get away with it if you amputate the plastic carry handle (perhaps fitting a flexible one in its place) - unlike the T-bag you can't overfill it to the point it will foul the brake levers. I've extended the bars on my S-type so this isn't an issue.
If you're going to only have one piece of do-it-all Brompton luggage, chances are it'll either be this or the T-bag. On the other hand, the S-bag *is* a better size for normal commuter use.
Kinetics Pannier Rack:Basically, it's a S-bag frame with the sides lopped off and a cunningly bent stainless steel tube fitted in their place to form a rack which will hold two regular panniers. That's all there is to it, which makes it both elegant and underwhelming in equal measures.
What it means is that you can put a couple of Ortlieb Back-rollers (photoed) or equivalent on the front of your Brompton, ideally without taking the piss with the 10kg weight rating of the front luggage block in the process - realistically, this is more about carrying your sleeping bag than your weekly shopping. As the weight is on the bike frame it doesn't affect the steering, though things do look rather odd with large panniers on the front of the bike and nothing on the back.
Lifting the rack off with panniers attached works, but depending on weight distribution may be a little awkward. Much less awkward than removing a beam rack, though, which is the nearest functional equivalent. The bike will fold with the rack attached, but obviously it sticks out at the top.
There's room inside the frame that could be usefully used to bungee a waterproof jacket or dry-bag. Maybe even a water bottle, if you don't already have provision for that elsewhere.
If you have an S-type Brompton, you may need to consider how much the panniers stick up above the level of the rack, with regard to fouling of brake levers. Panniers that hang down below the line of their hooks are likely to be more successful. I haven't actually tried my enormous Altura Orkneys on it, but that would probably only be sensible if you were delivering bubblewrap or something.
Also, full-size panniers are going to reduce the side visibility of the front light. The actual beam is still fine for seeing the road, but you'll want a blinky on the handlebars/stem/front of the luggage rack to be seen at junctions. (TBH, I reckon this is a good idea with any of the larger Brompton front bags.)
Is it worth it? If you already own panniers, and want to be able to carry voluminous stuff on the Brompton from time to time it compares well with most of the popular luggage bodges. If you're touring this could be usefully combined with a rear rack bag and/or saddlebag to give a respectable volume of stuff. As daily luggage - eg. with a pair of 'front' panniers - it lacks the elegance of the C or T bag (which would give about the same volume), so would only really make sense if you had some niche bike-swapping routine or really liked the panniers. If you have a non-Brompton bike with a Brompton block fitted (I'm thinking Circe Helios) it might be a positively sensible way to expand the carrying capacity. But if you're looking to buy some luggage for a Brompton, and don't already own panniers, then it's a very expensive and not especially neat solution.
Carradice Carradura rack-top bagI'll just mention this for completeness - it's in no way Brompton-specific.
Fits the Brompton luggage rack just fine, though I think it works better facing 'backwards', as this keeps the back pocket from obscuring the rear light, and having a wedge shaped object on the back of a bike with the narrow end at the front (without good reason, such as fitting under a saddle) offends my sense of aerodynamics.
Anyway: Nylon, 4 velcro straps, decent zips, expanding compartment, side pockets, crap zip-pullers, not waterproof but supplied with yellow shower-cap. Does what it says on the tin. Not as nice as the Prima that came before it.
Brompton do a Radical rack-top bag with a bit more capacity, but the fundamental problem of putting stuff on the rear rack of a Brompton is that you lose the ability to flip the wheel under and use it as a stand. As such, my preference is that this is the last place that you plan to attach luggage to, and if you're doing lightly loaded bike rides something like a mini-O bag would be a better choice.
