I used to ride motocross bikes up a 10" wide 8-foot ramp into an old F-150. Then one time I went to do that and the bottom of the ramp shifted over so that the front wheel made it up, but the rear was still on the ground. I got off of the bike without dropping it, rolled the front wheel back off of the tailgate onto the ground, and pushed bikes in from then on.
I rode a Ducati Monster M900 up that ramp a few times too. After once feeling the urge to put a foot down when half-way up I quit doing that too. I just powered into the truck bed rather than dropping the bike, but it was clear that I was taking a chance doing that.
A few months ago I had a flat rear tire on my big KTM and needed to get it up a ramp and into the back of the truck belonging to the local shop to go get it fixed. It's heavier and more awkward than any other bike I've owned, so I was having a tricky time powering it up the ramp while standing to the side of it. After the ramp shot out the back once, we tied the end of the ramp to the bumper and I let the guy from the shop get it in there. That was another case of the front wheel ending up on the tailgate.
It seems to be a lot easier to get a 250 pound MX bike up a ramp than a 500 pound adventure bike.
__Scott R. Nelson, 2008 KTM 990 Adventure, 2001 Honda XR650L, Folsom, CA