I have a gravel bike and I often ride 4hrs +. Sometimes quite remotely and getting extra drinks can be a challenge. I have a pair of bolts on the top tube and I wondered if anyone had any experiences of adding an extra bottle cage there.
If so do you have any advice on how to secure bottles given it would be horizontal?
This doesn’t answer your original question, but I was faced with a similar problem and found the best solution to be getting a lightweight hydration pack for these rides. It’s worth considering if you haven’t already.
Being able to carry 2 large bottles in my frame and ~2L of water of my back means I can ride for ~3-5h depending on weather before having to find somewhere to refill. I purchased a pack that is meant for racing so it really only serves to carry water, rather than having a bunch of storage for things like tools, snacks, etc.
I’d not thought of that. Thanks
+1 for the hydration pack.
What about TT-style bottle holders on the back / underside of your saddle?
I’ve tried putting a bottle on the top tube using strap on bottle holders. I placed it directly in front of seat tube pointing forward. I found it to be secure and easy to access, but it only increased my water capacity by one bottle so it wasn’t really a big game changer. I’ve also used a 2 bottle holder that mounts to the rear of the seat. It works well as long as the bottle cages used hold the bottle very tight. Otherwise, the bottles can bounce out when you hit a bump. Hydration packs work well, but what I have found I like better is a frame bag such as the Relevate Tangle that hangs underneath the top tube. It will easily hold a 2 liter reservoir and has a small opening at the front of the pack that is designed to run a hydration hose through. I use one of my wife’s hair ties to secure the hose to the bike’s stem. It keeps it out of the way but easily accessible when I want a drink. Cheers!
-1 for hydration pack. HATE THEM on my back. Back in the day I was doing long gravel rides I got a slim Revelate Designs toptube frame bag and put a bladder in it. Had a tube clip on my bars.
If there’s lakes or rivers along my route, I often bring a Sawyer mini filter. It’s small, compact and lightweight.
With a good tight cage it should stay put.
If you can orient it towards the front of your bike so that the bottle hits the stem on the way out, that can prevent accidental ejection. Makes it a little harder to get out…but it can act more as storage instead of constant use. So for instance use the one on the downtube 100% first, then swap with the top tube, so you only remove/replace it once.
Also, getting three 1 liter bottles instead of the 750ml adds some capacity.
A hip pack is another option instead of a back pack. I have this one. It’s pretty minimalist, but can carry 2 bottles. That, plus 2 bottles in cages is a lot of water.
I also have a low profile 1.5L pack, that’s better for MTB on technical terrain (can drink without taking hands off the bars), and also more aero (if needed) than the hip pack for long gravel rides. And I have a 3L pack for my longest rides in areas without option to resupply.
The Koala cage works great. I’ve used it on my MTB top tube and never lost a bottle.
I used a horizontal TT cage on a couple of long gravel rides. Mounted between TT bars (not on top tube unfortunately). I always had problems with water leaking thru the mouthpiece, that was pushed from the inside by the water bouncing back and forth.
I like however the accessibility of a bottle mounted so high, and the fact that it helps to keep the mounthpiece relatively clean (less grit, dust etc).
On my last rides (Inc Wisla1200) I mounted the cage vertically between TT bars (on Profile Design B-Tab) and it worked really good. I don’t mind loosing cargo space as I found all the front loaders decrease agility of the bike. I use 2 liters the top tube bag from Apidura instead.
Timon, What mount are you using?
This is what I use on my hardtail. Havent lost a bottle yet.
You could also consider the Wolf Tooth B-RAD System which includes a double bottle cage adapter. I haven’t used the adapter, but do use the system with the roll-top bag.
I love having the two bottles on my downtube thanks to the Wolf Tooth set up. I normally have a frame bag above them and the double bottle system means I can use normal cages rather than left/right entry ones too. No leg rubbing ever which is quite shocking when you first look at the setup.
I live in a humid climate and NEED three bottles on all my longer rides. And a hydration pack is like putting an oven on my back. 
Hi. Isn’t it too wide with 2 bottles mounted this way? Don’t you hit it with your legs?
It looks like you should hit your legs, but mine never do. Even when standing and swaying. A couple other riders in my club (Audax / Randonneuring) use the same set up to increase carrying capacity to great success. When I bikepack, I add a fourth bottle under the downtube but it gets crazty filthy.
Downside is a small aero penalty, but that’s less of an issue on gravel riding. And I need the extra water more than the aero gains.
Rather than having a hydration pack on your back, perhaps one in a frame bag would suffice? Or use soft flasks (placed in the bag and/or jersey pockets)
Re: hydration pack, I got one of those CamelBak pouches with a sleeve that had small clasps (English not my first language, can’t explain better than that). With some zip ties I was able to hang the hydration pack from the top tube , it did take a lot of the space inside the frame but I didn’t mind. I had no issues with this setup, I just stopped using it because my roadie friends didn’t like it 
