How much is enough when it comes to tire clearance in the frame? I'm specifically interested in how that may change when dealing in bikes aimed at gravel. Not all manufactures include a recommended max size, and when they do, they generally veer on the safe side. Does it matter if the tire is a slick or knobby(more prone to pick up a rock)? I'm also interested in how your local gravel conditions influence what you would consider safe.
I know this post is a few months old, but I'm definitely interested, too. I'm not far from needing to replace the tires on my bike (All City Macho Man Disc), and I'd like to go with something wider than the 35's that it came with. AC says 38s will fit "with clearance for mud". The mud around here is like peanut butter and sticks to tires so bad, I just stay off the dirt when it's that bad anyway.
Based on measurements I've taken, I could get a 43 in my AC MMD, and it would roll... but wheel deflection would definitely be an issue.
I've found that rim size can also swing tire size a few points. For example, I run 40mm Clement MSO's. Most people have found that they mount fairly true to size, but on the narrower Alex rims that I have on my prospect, they measure at 38mm at the widest point. Not sure what the Macho Disc comes with, but if the rim is narrow you might be able to sneak something in. For what it's worth, my Civia Prospect had a recommended max of 35mm.
With Clement 40's on my SuperX, there is only 1-2mm clearance, I don't like to ride them if it's muddy as there is so little space, I think 37-38 is good this gives 5mm.
I have 40mm Kinards on my gravel bike and would actually like to go wider. The gravel around here has a tendency to get really soft when it is wet, and the extra width is helpful for tracking straight and climbing strong in those conditions. I don't plan on winning any races, so I'll trade hardpack speed for stability+comfort on the squishies.
