That clearance thing makes no sense. Road bikes have razor thin clearance on the chainstays in relation to sidewall. The 6mm clearance is in relation to yolk where chainstay and seat tube meet and also along the entire seat tube. I am pretty sure the issue is with the isospeed moving the seat tube too much which could cause it to rub on a bigger tire.
I wouldn't even call the checkpoint a gravel bike anymore with a 35mm tire clearance. It's just another CX bike at this point.
This is what's listed on the 2020 so basically they making you use a specific tire so they can get by saying it can take 40c. That way if you have an issue and you not running that tire at 40c you out of luck.
Max tire size40c Bontrager tires (with at least 4mm of clearance to frame)
Last edited by gus6464; 02-18-19 at 01:48 PM.