Originally Posted by Lava

I posted this in the Topstone thread, but I'll add my experience here too in case people come across it searching.

A Cannondale rep confirmed the clearance issue in a thread, here. Fender stays seem fine, but big/protruding rack tubes seem to get in the way of the skewer. It may depend on the rack. I spoke with another owner who installed this Bontrager rack, and because the attachment points are flat, the skewer seemed to clear the rack mount points. My Tubus Logo Evo got in the way, however.

I went through this with my rack-equipped Topstone 105 recently. The axle levers are adjustable, so theoretically you can loosen it little by little, adjusting the lever every time, but that's a time consuming process.

I bought a Robert Axle rear thru axle using their axle finder guide. Ended up with a Lightning Bolt-on Axle - Rear 12 mm x 162-167 mm x 1.0 Thread. It's longer than the stock axle, but uses a spacer to get the correct length. It's allen-key removable, so interference with the rack and fender mounts isn't an issue.

I struggled to get it threaded when I first swapped things over, but I didn't have much thru axle experience and didn't know the derailleur hanger had to be well lined up before the threads would catch. Once I figured that part out it was pretty smooth to install, and seems to be going strong!