After a somewhat traumatic event, I spent some time looking online to see how motorcyclist's secure their bikes on hitch mounts. They share a nervous creativity that I found in trying to secure my bike to my honda accord.
Using the Thule Pro XR as a base, I used the front retracting wheel holder on the bike frame, mostly to secure the bike while I used ratchet straps to secure the handlebars down to the base of the rack. The idea for ratchet straps had come from Chris Nolte at Propel. The result was a very secure set up. The Thule Arm does something but very little. The rear wheel base secures the rear wheel at a single point, while the ratchet straps pull back and down on either side of the handlebars to the steel frame of the Thule rack. The straps at the handlebars prevent the bike from falling to either side and prevent the front wheel from turning to either side or moving forward.
Another idea to add more strength to the set up is to ratchet down the rear of the bike by running a strap to the void at the battery attachment. When transporting this bike, I always remove the battery. This would add back up in case the rear wheel hold failed.
I previously had owned the Thule Pro XT to transport a specialized Vado. The Vado had rims as well and I always wondered how the Thule arm was to work with the retracting arm. With vado, I place the retracting arm in front of the rim. It worked, but I never really felt that confident about it. With the purchase of the Boost, I had determined the wheelbase of the boost would work with the Pro XT, but had not taken in consideration how the front wheel support would work with a larger rim. Recently I found in the Thule XTR Pro (same model with a wheel for ease of transporting) literature, a diagram showing the front arm is to be within 1" of where the front wheel meets the frame. That would have been a nice diagram to have for the Pro XT.
This bike was a pandemic purchase. We have put 1500 miles on it and love it, I will write more at a later time.
Any questions or concerns about this setup would be appreciated.