A quick word on why trucks jackknife.

[jmbrodsky@domain.elided]

     Yeah, I know I probably didn't spell jack-knife correctly for one 
     word, anyway.
     
        A sliding wheel has less drag than a braking wheel.  A rolling 
     wheel has even less drag than a sliding wheel.  As the rear 
     wheels on a semi lock up, they begin to want to travel faster 
     than the rest of the truck, due to decreased friction.  When this 
     happens, AND the truck is not ideally and perfectly straight, the 
     rear end will travel out and around the braking wheels.  Most 
     times this happens, the truck is attempting to stop for some 
     obstruction.  If the Truck could let off the brakes (like the 
     obstruction would somehow get out of the way for 25' of tractor 
     and 53' of trailer) the rear wheels would then be slding and the 
     front end would be rolling, thus the front would travel faster 
     than the slding rears, and the trailer would correct to center, 
     and everything is fine.  Bad news is that:
     1) the obstruction for 70'+ of truck never get out of the way.
     2) the trailer after a little bit of deviation from center will 
     try to tip over and pull the tractor over too.
     
        Trucks have great responsibility and only the rookies 
     tailgate.  Either rookies, or Darwin's examples.
     
     -Joel
     
     PS how come that header drap is attached to my mails/posts, only 
     sometimes?