We are hoping to take a self guided trip in Wisconsin. We would like to rent bikes there, but can't find a rental car with a bike rack available. Any ideas?
If you could rent a van or SUV you may be able to load the bikes inside the vehicle.
Most newer rentals have rear seat passthru. Take the front wheel off and you should still have one rear seat available for a passenger.
Sixt rental cars USA customer service says:
We do have bike racks available but it is based on the location as well as availability.
With any rental car agency, you should call and verify that they have the equipment you would like before you depart on your trip.
Some places rent folding bikes, which would remove the need for an external rack.
I would have thought with a few rachet packing straps you could place the bikes upside down on the roof, secure the handle bars to the frame (so they won't turn) then secure the bikes through the rear door or window (front window as a last resort).
As a disclaimer I have never seen this done but secured right it should work just like a roof rack and not damage the paint (but hey I reckon the rental company wouldn't check the roof). If the car has basic roof racks you could rig something pretty good up.
From personal experience with Enterprise, I brought a rack with me when I went to Colorado to ride. I just made sure to cover the contact points so it didn't leave and scratches or marks and put it on after getting to my hotel and before leaving. I think it's kind of a don't-ask-don't-tell policy.