571 mm aka 650c width?
 

What is the most narrow tire I can use on a 571 mm / 650c rim? Is it 25 mm (1" as in 26x1) or is there rims that can take 20 mm tires? And are those tires commonly available or...? I am going to do some touring so I dont want to end up with no tires in the middle of nowhere.

You don't want 20s for touring(loaded).

Yeah but I want to have the option when I am not touring on it :) Did check out continental and they have some nice Gators that is 23 mm. Guess that is what I want. And I will only do some touring on this one, not loaded touring, that I use the MTB for :)

are you sure this is a bike you want to use for touring? i haven't followed the whole story surrounding the frame, but most bikes designed for 650c wheels are built for light weight and performance, not comfort and loaded riding, in mind. plus it wouldn't surprise me that you would have some trouble finding tires, at least in small-town LBSs.

650c is a racing rim and tyres are available from 13 to 25mm You will not find anything wider. You can use 650cx25 for fast, light touring if this is what your bike takes. This size is not recomended for carrying loads and the availablity is restricted to race-oriented quality bike shops.
If you are chosing a wheel size for a touring bike then MTB 26" makes much more sense. The narrowest size is 1" which is good for fast rides. Medium touring tyres 1.25-1.75 are widely available and you can buy an MTB tyres at any bike shop.
Road-oreinted bikes with MTB wheels are not common but they are made. Check out Thorn bicycles at sjscycles.com.

Loaded in this case is more like a saddle bag containing two or three pair of socks, two or three pairs of underwear, a extra t-shirt, a small towel, razor and after shave :)

But thanks alot for the replies on tire selection, that it can be scarce. Guess I have to pack atleast one extra tire then. (Do they make those tires foldable?)

Anyway, is there a risk that they abandon the 650c size any soon or will it be possible for many years ahead to find tires and tubes (in well equipped racing stores)?

Actually this is not going to be my primary touring bike, I got a wonderful MTB for that purpose that got racks, fenders and so on. But, I was planning on going over the Alps from Munich to Lake Como on this roadie, mostly to have so little weight as possible on the way up and to be able to enjoy the fast ride down.

And on that trip I will be in fairly civilised places so I hope to be able to wash clothes and use hostel facilities as much as possible and also eating out.

i sort of see where you're coming from. just to let you know, not that it necesarrily applies to your bike at all, but on my 650c bike, it takes virtually no effort to lock up the rear wheel; the angles are so steep there's very little weight on the rear wheel. so you might have to be more mindful on the descents if yours is designed similarly.

If you're riding on 650 x 20 tires with a touring load on the bike, take at least two spare tires. Small cross-section tires don't support the dead weight of luggage very well. FYI, there is a Panaracer-manufactured 650 x 28 tire that is sold by Terry.

Thanks for the advice on the braking, I will most certainly explore that before I flung myself down the alps :)