UKC

When frame space is taken up with bags, one option is to mount a bottle cage under the down tube near the bottom bracket. My bike even has the fixing points to do this. My question - how do you drink out of a bottle which has been covered in mud and worse crap thrown up off the road or trail?

You don’t. You use a bladder in your backpack.

I use the cage for a tool caddy which frees up space in the other bags for more water/food/stuff. Frame cages with 500ml bottles and 1 or 2 bar bottle holsters (650ml) gives me up to 2.4 litres and if I want to be a pack mule I can use a bladder (2lt) and pack or bum bag for another 652ml.

That's a lot of water when I normally can stop in a cafe for cake or a pub for chips.

You know those bottles with an additional cap over the drinking spout?

https://www.tredz.co.uk/.Elite-Fly-MTB-Water-Bottle_235283.htm?sku=779258&a...

I don't use frame bags for that very reason. Another option depending on frame size is a shallow frame bag that still leaves space for a bottle.

Ha! Never knew that existed. Might be a good alternative to risking a dose of the runs as I do now.

I use a a long slim 3l MSR bladder in a top-tube/frame bag with the drinking tube tied around the headset. While riding I generally only half fill but it's handy to have so much capacity for hot weather or evening meals. 

On my gravel bike I put the bottles on the fork, then can use my full triangle frame bag. Same issue - either go for bottles with flip over caps or give the bite cap a bit of a wipe first then hope for the best!

I use side loader bottle cages in the triangle if I have a frame bag.. Tool bottle goes on the underside of the downtube so there's little risk of road grime covering the drinks bottles. My bottles have caps on too as I'm prone to cold sores if I use bottles without.

Keeping as much weight as low as possible (in the triangle) helps stability. 

https://bikepacking.com/index/add-cage-mounts-bike/

You could mount a bottle on the forks? 

There are loads of cheapies around too, I just grabbed that link for an example. Glad it helped

Thanks for the suggestions everyone. I hadn't realised bottles with caps were a thing - I'll get one. I'd considered a fork mount - my bike has the fittings - but unless I carried two I was worried about balance (probably not a problem, I guess). With my half frame bag I can fit a normal bottle on the down tube inside the frame but it's a struggle to get it in and out when the frame bag's full. The other solution would be a side-loader (available) or a swivel mount. I haven't seen an example of one of those, so perhaps there's a gap in the market

I ordered a couple of these from eBay to add to existing bottles as I definitely get a bit if grit (and hopefully nothing else!) from muddy bottles. No idea if they fit existing bottles or actually work.

Title of eBay product is:

Bicycle Silicone Water Bottle Anti-Dust Cover MTB for Cycling Cycle