Scooper

Thanks, RK. I love 'em all.

I taped pieces of paper to the chain stays at the point where the tire casing is the widest, and taped a millimeter scale across the chain stays at that point, marking as closely as I could, the inside surfaces of the chain stay tubes on both side. There is a little parallax from the camera, but those marks are within a fraction of a millimeter.

The 25 mm tire casing is actually 25.7 mm wide inflated to 115 psi. There is about 6.5 mm clearance (6 mm on one side and 7 mm on the other - the wheel must be a little out of true). It's not 5 mm as indicated above when I attempted to measure it without removing the rear wheel. The distance between the inside surfaces of the two chain stays at the point where the tire casing is widest is approximately 39 mm.

So, 39 mm - 25.7 mm = 13.3 mm, or 6.65 mm per side.

A 35 mm tire might fit if the casing of the inflated tire isn't much more than 35 mm, but it would be extremely tight. 39 mm between the stays - 35 mm casing leaves only 2 mm of clearance on each side. 32 mm wouldn't present any problem.

One other factor to keep in mind is that a 35 mm tire casing will be at its widest about 5 mm closer to the BB shell than the widest part of a 25 mm tire casing; the chain stays will be even closer together providing even less clearance.