When mounting to factory installed crossbars, it is imperative that the crossbars themselves are strong and intact.  Most factory crossbars are not adequate to carry bikes because of the amount of torque put on the crossbars with a tall, heavy load is mouned to the roof.  If you are unsure, please call the shop for advice.

The Ride-On Adapter works well for many applications, but does have some drawbacks, as it makes the bike racks sit about 3" higher, and about 6" towards the middle of the car.  Keep this in mind when mounting on tall SUV's.  This may be a good reason to invest in a multi-sport rack system.