I think if you’re hung up on the aero watts lost to a top tube bag you’re picking the wrong battles in bikepacking… this coming from an aero-weenie. Ironically, the taller it is, likely the “faster” it is in terms of aero drag. You only need to look at the wacky bottle placement that has become popular in the Tri world. Filling that void between your chest/arms and the cockpit of a bike is ideal.
So, to your question, I’ve never really liked the rear “jerry can” type bags, as they’re just a bit small to fit anything other than a small “spares kit”. They also cut into standover which isn’t a huge deal, but they do make dismounting and remounting a bit more work on tired legs three days in, especially for mountain biking, where you’re over your bike more than on a gravel or road setup. But with that said, I did make a full length top-tube bag that I really like for certain terrain where I know I’m not on-and-off as much. It’s two compartments, with the front half being zippered, and then the rear half being a simple split opening with stretchy fabric. The rear is sized to hold a 24oz water bottle, or it’ll fit 8 SIS gels…
Overbiked and slow vs underbiked and fast?
(Dithering over whether I should dial in my rigid MTB for touring or try to use my carbon roadie for a ~6wk trip this winter. I don’t expect a real answer, just something else to talk about ITT.).
What’s the terrain? Also, what’s the largest width tires your road bike can clear? Does the road bike have enough gears / ratio?
Tool Kegs
Anyone using tool kegs or something like the Wolf Tooth TekLite Roll-Top? If so, which ones? What do you store in it?
More generally, what do you store on the bottom side of the down tube near the bottom bracket?
What’s the terrain? Also, what’s the largest width tires your road bike can clear? Does the road bike have enough gears / ratio?
Mostly paved (and most of the pavement is…mostly there. If I bring the rigid MTB I’ll dabble in dirt roads, if not, probably not, or much less.). I believe manufacturer spec on the road bike is 32 but it’ll probably take 35s. Cheap alloy wheels with quite a few spokes. I think I’ll be okay with the gearing on most pavement, yeah.
I’ve also thought about trying to rig a Bob trailer to the road bike. Sounds crazy but I’ve heard of it being done and working well, and it might take some weight off the frame.
It depends, for the last trip I used a Zefal tool bottle that cost only a few £ and think it contained:
- 2 x tubulito MTB inner tubes and repair kit
- multi-tool
- Pedros tyre levers
- Wolf tooth WT-1 chain lube (0.5 oz)
- curved needle and thread
- tenacious tape
- quick link
- Nitrile gloves
- batteries (for powermeter)
- zip ties
- kitchen towel (doubles as padding)
This was stored on the downtube but for trips where water is more sparse, have a 64oz klean kanteen metal bottle.
This past weekend, I did a 3 day bikepacking trip in the Black Hills area of South Dakota. I hadn’t spent much time in this part of the country before, and I was surprised at how beautiful it was. I’ll definitely be back in the future.
130 miles and 13k feet of climbing. A mix of weather - from below freezing and snow to 70F and Sun. A mix of trails - from graded rail trails, to chunky 4x4, to sweet single track.
There were enough sources of water along the trail where I had to carry a max of 4L on the bike at any given point. Helped lighten the load vs desert riding, but my bike still probably weighted 60+ lbs.
Everything went well - my bike, equipment, fitness all worked out. A great three days on the bike 
Chiming in to agree with @spinout, panniers are the way to go imo. While I have limited experience with bike packing I did not enjoy the “sway” a rear pack created despite doing everything I could to mitigate the problem.
I used an alloy tailfin rack with panniers for my 2nd trip which was much longer and with more rowdy terrain. The bike felt stable the entire time.
