After buying the minvan to haul the dogs, we bought the PetStep folding ramp
http://www.petsmart.com/product/ind...ferralID=3ac15bb2-410f-11e2-b779-001b2166c62d . This ramp has a cross-ribbed walking surface with a rubbery polymer that makes for excellent grip. We live in Oregon so it rains a LOT, and this ramp surface worked well even when wet. "Moose" was a big boy -- 245# and the first ramp broke after about 3 years. (it actually broke while Moose was walking up the ramp -- fortunately he was not injured) I spoke with the manufacturer and they made arrangements to pick up the broken ramp. They came back and said that the plastic must have become weakened over time and exposure to UV (we kept the ramp in the minivan all the time). We bought a second PetStep and after about a 1.5 years, it seemed like it was weakening, so we bought the TopPaw ramp mentioned above. The construction seems to be more robust, but the walking surface is not as good as the PetStep. Moose was not as sure-footed on the TopPaw.
I would say that if Bob doesn't tip the scales at more than about 200# then the PetStep would be the way to go.
The PetStep comes with "suggestions" on how to train your dog to accept the ramp -- they say to start with the ramp flat on the ground and have your dog walk on the ramp (either on leash with firm control, or with treats). Once the dog is comfortable walking "between the lines" of the ramp when it is horizontal, you can then move to the inclined position. Moose was good starting with the inclined position.