This is my backup chair, a Quickie GPV. I replaced the existing 37-540 tires with Kenda 2″ knobbies (50-540). I figured out that it would be ok to mount them on the existing wheels by reading Sheldon Brown’s article on tire sizing. Thanks to this, I didn’t need to buy new wheels (expensive!) or worry about the axles.
Then I replaced the 5″ hard roll casters with 6 x 1.25 soft roll casters. This raised the front end of the seat up about half an inch, which is fine, because the chair doesn’t have enough dump as it is. It also meant that the caster barrel housings had to be re-aligned to be vertical – my beloved did that, because his tools are cooler and he’s better at precision than I am.
I found that the knobby tires were intruding into, shall we say, my personal space a little bit, so I increased the axle spacing. That made the chair 27.5 inches wide from handrim to handrim, sadly. I’m going to get rid of the camber (currently 3 degrees), which should allow me to bring that back in about 1/2 an inch on each side.
I took it out to the park last week, and definitely noticed a difference over bumps and grass – I didn’t have to wheelie up once going across the grass. Nice.