For 50 lbs mountain bike I think you might find a hitch rack more easy than a roof rack. For myself, I think it is highly likely that at some point I would forget the bikes are on the roof before I enter my garage.
I looked at a number of folding hitch racks that did not contact the frame and only the tires. I found that my mountain bike frame with the rear shock on the top tube would not fit on many of the other style hitch racks. I bought the 1-UP rack found in the link below. It is made in the USA by a small manufacture in Wisconsin and is only available through internet sales. Customer support is supposed to be good. It does not come from a big maker like Thule or Yakima. It is also not cheap but after you see the rack you see that you get what you pay for. The quality is much higher than Yakima or Thule. It has been rated the #1 hitch rack by Mountain Bike Forums for a number of years. I think the Kuat racks are also very good and would have been my second choice. Advantages of 1 Up that led me to go with it:
1) The base unit is for a single bike. Add on units can be ordered that can increase the capacity to 2 or 3 bikes for a 1.25" hitch. No other rack I found can be expanded to 3 bike capacity on a 1.25" hitch.
2) I often only need to carry one bike, not two. With the 1 bike base unit I can fold the rack up and still open the tail gate on my Forester. The 1 bike 1-UP unit does not stick out as far as the 2 bike Thule 916 ,917 racks or Yakima Hold UP.
3) The 1 UP unit is held into the hitch by a ball wedges itself into the hitch wall holding the rack in by friction and eliminating wobble of the rack at the hitch. This style of mounting may been seen as a weakness as there is not bolt that positively locks the rack into the hitch but there has been no reported instances on the internet of a 1-UP rack coming out of the hitch. I believe the Kuat also has a mechanism to wedge the rack into the hitch to prevent wobble and also as a bolt to secure the hitch to the rack.
4) The 1-UP base unit rack weighs about 25 lbs and can easily be folded up to be stored in the trunk of even a small car. That cannot be done with the Thule 916,917 or Yakima Hold Up hitch rack.
5) The 1-UP unit can be used on both 1.25" and 2" receivers, it comes with an adaptor.
1up USA Ultra Rack
Something to think about before buying a Thule 916 or 917.
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