Hurricane Components Fork Up For 20mm Thru Axle $38.00
Mountain bikes have a problem with roof racks. The bikes, especially ones for gravity-assisted disciplines, don't really work well with them. Massive, oddly-shaped downtubes don't do well with downtube clamps, neither does carbon-fiber, nor ultra-thin wall aluminum. Thru axle forks are even harder to deal with. Jerry-rigging used to be a standard feature of getting a big hit bike on the roof. Thankfully, that is the case no longer.
The Hurricane Components Fork Up for 20mm Thru Axle works with all fork-mount racks, including Rocky Mounts, Thule, and Yakima. The tube fits a 20mm axle and is 110mm wide to work with just about all big hit, big meat forks. Just open up the rack's fork mount quick release, insert the fork up and tighten the rack's skewer. Safety tabs will help keep the mount in place when the QR is tight. The position of the Fork Up can be changed to better fit the bike in question onto the rack. And, when you don't need the Fork Up, just open up the rack's quick release to stow inside the car or with the bike.