I had a somewhat unnerving experience recently. I had bought a travel trailer (caravan for UK readers) and was towing it home when it started to sway from side to side. The oscillations built up to an extreme amplitude of 90 degrees, limited by the trailer hitting the side of the tow vehicle. I responded as I would to a rear-wheel skid in snow (steering in the direction the vehicle was traveling) and brought the vehicles to a stop. Now I want to understand why it happened, why it doesn't happen all the time, and how not to have it happen again. The tow vehicle was a mid-engine AWD minivan, which handles very well on its own. It weighs about 3800 lbs, has a 113" wheelbase and is rated to tow 3500 lbs (braked). The trailer weighs about 2800lbs and has electric brakes (drum brakes triggered by the brake lights of the tow vehicle). The pivot point is a 2" ball hitch about a foot behind the rear of the van, rigidly secured to the vehicle frame. Basically a normal caravan setup you can see anywhere in the UK (Canadians tend to use large pickups as tow vehicles, usually heavier trailers, and often a gooseneck hitch). The incident occurred when I was descending a slight incline on straight smooth dry tarmac at about 100kph. There was some wind, but not enough to notice had I been driving the van alone. There had been some swaying earlier - as I understand the motion, the trailer moving from side to side on the road, pivoting around a vertical axis through the hitch as seen from the tow vehicle - but these oscillations died down so I presume there was a damping mechanism in play. I don't have any witnesses, but I believe in the downhill incident the same motion became amplified through some positive feedback mechanism, though the hitch shows evidence of a twisting strain also. I have since fitted a friction damper (sway control bar) to the hitch, but I still want to understand the problem - this was a professionally built vehicle towing a professionally built trailer within the published specifications, at the speed limit on a good road in good conditions. I've seen countless vehicles with larger trailers traveling significantly faster in similar conditions, with no damper and apparently no problem.