A tow vehicle that is short compared to the trailer is a wonderful thing when you are backing up. Just think of an 18-wheeler - they can back those things with pinpoint accuracy, and the short tractor (compared to the long trailer) is what makes it possible. Plus a lot of skill on the driver's part, of course.
On the other hand, think of yourself going down a long downgrade when you encounter a sharp curve. You turn the tow vehicle, and an angle develops between the trailer and the tow vehicle. The trailer tries to go straight ahead, and it pushes the back end of the tow vehicle in that direction. As it pushes the back end of the tow vehicle, the tow vehicle tends to swing around, making the curve tighter, and a jackknife can result. A longer tow vehicle resists that tendency better. There are some rules of thumb, as you discovered, for how long a tow vehicle should be compared to its trailer, and they are good rules, dervied from experience - but they are not magic. Of course, an 18-wheeler does not meet the trailer rules, relying instead on the skill of the driver and the massive dual rear wheels on the tractor, to stay out of jackknife situations. But when situations get out of control, a jackknife is a common result.
Hope this helps.
Bill
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