Originally Posted by
alan s
My understanding is that wider tires on narrower rims can lead to the rim splitting/failure due to excessive outward force of the larger tire, which can be limited by running at lower pressure. For example, I have succesfully run 60mm tires on 19 mm rims at 2/3 max pressure (40 psi on a tire that can be inflated to 60 psi). Too low pressure can lead to poor handling, so you need to find the proper balance.....
The outward stress the tire exerts on the rim is called "hoop stress". The formula is very simple, pressure
X width, so a 2" tire at 50psi, creates the same stress as a 1" tire at 100psi. Since tires are usually ridden a pressures inversely proportional to their width anyway, it becomes a wash.
That said, I'd be a bit leery of pushing the limit on narrow sport/race rims where the makers were watching weight, and expecting that only narrow tires would be used, especially if said rims were nearing the end of their brake track wear life.
My commuter uses 2" tires on 21mm rims at about 60psi. The brake tracks are well worn at 25k miles, but so far so good. I will say that they can get squirrely if I have a slow leak and pressure drops significantly, but I've learned (the hard way) to watch for that. If the front wheel doesn't have the right "bounce" on road bumps, I check.