2_i 

Components: Osprey Stratos 34 (loaded to full capacity), Trigo S-Bag Frame (Ali), Scooter Carrying Handle (Ali)



Components


Frames connected with handle's straps


Front


Rear

Jipe

Did you ride with this setup with a loaded bag?

My experience is that the bag fastening on the bag frame must be totally rigid without any play otherwise when the bag is heavy, it generates shimmy in the bike steering.

Too wide heavy front bag (45-50cm or more) also create shimmy in the steering (I think its the reason why the biggest Brompton bags are 40-42cm wide).

Test rides with an empty or lightweight bag doesn't show the problem.

2_i 

Yes, I rode quite a distance at night with the pack loaded as normally for work - around 13kg. My interest was in any sagging of the pack over the lamp, but it could have worked better than with a loaded T-bag. This and other Ospreys have a bow shape, so the center of gravity enters pushed in more than in other arrangements. At first I attached the pack at four points to the S-bag frame, but then I realized that just two points at the top are enough. If any problems develop over time, I can go back to the four attachment points. The beauty of the bowed Osprey frame is that the S-bag frame can stay tucked in, while carrying the backpack on one's back, without affecting in any practical way the comfort in carrying the pack.

For the moment, this works far batter than trying to turn the T-bag into a backpack. The Stratos has a zipper on the side that you orient up in the transverse orientation, so you can take stuff out and put it in, without the need to take the pack off the bike. Of course, I will be monitoring for any problems that may develop. One worry is how the Osprey's frame will hold over time in such operation.

Winfried

Nice trick.

I would add a large/a couple of thin straps to keep the bag tight against the frame instead of relying on its "sleeve" on the back.

With such a large load in the front, it'd be interesting to see if the bike rides different on long rides than having the bag sitting on the rear rack à la PathLessPedaled.

2_i 

I rode again extensively trying to look for the shimmy that Jipe mentioned, but there was none. Maybe the pack has at this moment too much load for that. The width is ~63cm. I'll be riding to work, of all places, tomorrow, presumably with a bit less load, so will be trying again to see whether I can run into any instability.

The beauty of the current mounting is that attaching the pack to the bike is as easy of not easier than a T-bag. There is less interference with the handlebars and gear there than for a loaded T-bag. The issue of those extra thin straps, suggested by Winfried, is the same as of those two extra attachment points I was mentioning. Maybe, again, the problem surfaces for a less loaded pack. For the moment, I will just practice riding with the pack and see whether any issues come up. Somewhere farther down the road I am think about adding extra curvature to the S-bag frame.

2_i 

Update: I rode by now with the pack lightly filled, heavily filled and medium. I rode uphill and downhill, sidewalks, street, gravel, etc. There was neither shimmy, nor other problems. Two suspension points appear sufficient. I was expecting problems riding side-by-side with others and passing pedestrians. No problems whatsoever.

The pack is more practical than the T-bag or Borough. It leaves more open space by the handlebars and beats the Brompton-branded bags off the bike by miles.

Maybe, I should add a potential correction. The vendors use the term S-bag frame for all the frames they sell, but I doubt they can all relate to the particular bag. The designation they use to distinguish the particular frame from other is RX30x20, which stands for 30cm height, 20cm width and no handle.

At this point I turn to potential refinements. I look at some quicker attaching of the frame to the pack and making that frame slightly curved to make it tuck in better within the backpack.

Falconista

Looks like a good solution. Would be interesting to hear if it makes any difference to the steering if the bag is vertical rather than horizontal. It would reduce any turning moment on the handle bars, especially if the bag wasnt loaded equally top to bottom.

Careful of those straps in the front wheel...

Geepig

And careful of those straps in case they catch something you ride past...

2_i 

On Brompton the front luggage attaches to the front of the frame, which decouples the steering from the load. Many owners, and manufacturers too, adapt Brompton system to their folders because of this convenience.

As to the vertical orientation, the Brompton T-bag for that front mount extends vertically about as far as one can go without impacting the turning of the handlebars. In fact, the T-bag does not even work with all of the handlebars that are possible on Brompton, because of the interference with the steering. I started out, in trying to find a luggage solution for commuting, by converting the T-bag into a backpack. As a backpack, its handling is far easier off the bike than originally, but it is is still a very poor backpack. It is too short as a backpack, poorly profiled and its center of gravity is too far away from the body. For small loads it does not matter, but for larger loads with longer carrying times, it becomes a torture, hence the continued quest for a solution. In my opinion, the turning of the orientation for a bag that works equally well on and off the bike, at least on Brompton, is a must. There are commercial solutions that take this route, such as Brompton Freitag, but I have not seen yet any where the backpack functionality looked good.

P.S. Here is a commercial pack for Brompton, from Bromptonic, that mounts upright. Interference with the handlebars and raised center of mass are the negatives. Division into compartments and style are negatives or positives, depending on the rider, but generally lock the user into choices unrelated to the bike. Lumabag Urban Traveler and Bromfort basically work like an overloaded T-bag.