I was just wondering what the practical limits are for various rim widths and how the width of rim affects the grip/handling of a tyre.
I'm sure someone will come along and answer this more fully but if you run a fat tyre on a narrow rim, the tyre will be narrower so you have less grip.
If you look at the Mavic site, for every rim they give you a recommended tyre width, which is useful
Well mavic have guidelines but few stick to them.
I find wide rims and narrow tyres alter the shape of the tread that rides the ground-giving more drag and fatter tyre.
Wider tyres may not sit well on narrow rims.
For road it can be crucial and allow better ride.
Not sure for mud and xc. I guess having the sidewalls in their proper seating gives the ride the manufactere wanted?
Some tyres may improve while others become worse.
Hve a look at mavics site as a guide or even http://www.sheldonbrown.com/
Sheldone (RIP) has a great guide on this somewhere-I can't be bothered to find it!
Ok I did http://sheldonbrown.com/tire-sizing.html
I find that a wide tyre on a narrow rim ends up with a profile thatis too round. Plus, unless you run a higher pressure,it feels like the tyre is trying to roll of the rim in hard corners. That's the reason I like Flow rims. They're not silly heavy but they are really wide so I can run lower pressures for better traction but my tyre still feels stable in the corners.
Someone will be along shortly to tell you how they ride sillyfaston old MA40 rims with 3.5" tyres without issue so I'd best point out that what I've send above is just my opinion and I do not have any scientific test data to back up my hypothesis!