I was motoring along southbound on a four-lane Oregon road, barely overtaking another motorcyclist who I'd first seen over a quarter-mile ahead of me. By now the distance had shrunk by about two-thirds and, even over the wind noise I could tell from the considerable rumble that he was riding a big twin. I was still a few hundred feet behind when he gradually overtook a van towing a big house trailer. This rider had been favoring the right side of the lane, and when he got right up behind the trailer, he made a quick flick into the left lane, again on the right side, close to the van-trailer. It was going almost as fast as we were, and I could hear that he didn't accelerate when he drew alongside of it. He had probably been in passing position for almost a minute when the van-trailer signaled, then began to move over (a deer carcass lay in the road ahead). The rider apparently didn't see the signal and didn't react until the vehicle began crowding him. Then he pulled ahead next to the driver and gestured angrily. I saw the brake lights on the trailer, then the bike, illuminate, as the rider apparently tried to stay next to the vehicle. But the driver still had the obstacle to avoid and continued to move over while trying to brake behind the bike. The rider wanted to express his anger though and didn't get out of the way until he was finally on the shoulder, and both he and the other vehicle had slowed to less than 30 mph, and I'd had to brake to stay clear.