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I can't seem to stop my Alpkit toptube bag flopping about. The only way I can stop it is to have a velcro strap over the whole thing which seems to negate the whole point of quick access. I have tried one of the Dr Jon steerer widgets but the back half of the bag still flops about. It is an oldish design, so would a newer bag behave itself better?
Gas tank bag? Good ones have reinforced stiff sides for the whole of their length.
I've got a couple - Restrap and Apidura, and they are solid as a rock. I suspect your issue is poor design and construction.
Is it the strapping mechanism or the rigidity of the bag itself? If it's the former, you may be able to sort it with a bit of inner tube around the top tube, if it's the latter, you could shape a couple of bits of thin plastic and stick them to the sides of the bag on the inside.
I think it's sometimes just down to how much you are carrying in it. My old Revelate bag has always worked well, with only the two velcro straps round the toptube, plus one around the headtube. I bought a Cannondale bag for my Topstone which is bolted on and that works really well too, but obviously not an option if you don't have the mounts. Some sort of stiffener is always going to help.
Alpkit ones are really lightweight, so forego the extra reinforcement of others. Just got to think about how much you want to try and put in it, then how to best pack it.
A 76 Projects clip helps a bit for attaching it to (near) the top of the steerer tube
The clip in question: https://76projects.com/products/ass-anti-strap-system
Pack more into it...that'll give the sides a bit more support to stay upright.
I have added a layer of 4mm fairly stiff foam into the sides of mine - I glued to the bag and this has massively helped the structure and avoiding it flopping around. Paired with a decent strap to the steerer this makes it better than any other solution I've used for added snacks etc when trying to ride packless
I also suffer with an old floppy Alpkit top tube bag. I have a cut up plastic milk bottle inside as a stiffener, basically cut the top off it and copied the shape so it has a bottom, two sides and the bottle base is against the steerer. Works well, didn't cost anything
I had a Restrap size large that sagged in the middle due them skimping on a centre strap. My old Revelate extra large has no such sag as it's got plenty of straps.
Get one with plenty of straps!
My tailfin top tube bag stays put with just the two straps around the top tube (or bolt on) they must make the rubber base out of limpets or something. It's also the only top tube bag I don't hit with my knees
I have the Topeak Tri Drybag and a rubber Specialized Remora aero one for long distance TTs. Both have been fine. As mentioned above, try some internal plastic first.
We started with alpkit, but found them floppy too. The Restrap ones are better. The Brooks is far more again
I have literally just designed and 3D printed a platform to support my alpkit bag. I have not ridden with it yet to see it solves the problem. It uses the head tube to stop it rotating and has a slot for the strap to fit through
Tailfin is the expensive but brilliant solution
My Alpkit one is particularly bad at this, I don't remember having issues with previous Blackburn ones - I've since bought a Tailfin one, which seems like a much neater solution, but I've not tested it yet.
I finally decided to go against my cheapskate nature and buy the Tailfin, after all it is going to be on the bike for every ride. So far I am very impressed. The middle sized one takes an inner tube, multi tool, tyre levers and small snacks and doesn't flop at all. I haven't done anything too demanding yet but it's all looking good.
Little mini Voile straps instead of the velcro ones can help as long as its not too top heavy
+3 recommend for Tailfin. Rock solid with the rubber feet, rubber top tube straps and internal reinforcement.