Why are pups so wiggly?

Technically 3, tractor fifth wheel, lead pup pintle hook, dolly fifth wheel.

 

Ruts, high wind and improperly loaded trailers are the three main reasons I find them to wiggle. The loads we get are sealed loads and all we can verify is weight of trailer on manifest. More often than not when I get to destination and pop open door the freight is nose loaded high and tight in front half or a heavy shipment of product on rear of trailer.

 

The driver can't prevent wiggle only increase it.

 

Yes they can get off phone or move to another lane.

 

They bounce nice when empty

 

cause they are watching trailers in the mirror to much

 

The path of the last axle of last trailer will tend to have a greater displacement than the more forward axles. This is called "rearward amplification".



Rearward amplification is also more extreme with shorter trailers which causes pups to have far more wiggle than longer doubles as can be seen here.

Note how Turn Pike Doubles (TPD) are pretty close to a single 48' trailer.


There have been some attempts to reduce the yaw in Canada IIRC like dollies that have dual pintle hook connections but I think they add significant amount of weight because the trailer needs to be significantly beefed up.

Several companies have argued to move the size of Pups to 33' to increase productivity and safety but getting over the ISTEA freeze politics is difficult. ISTEA(Intermodal Surface Transportation Efficiency Act of 1991) froze the maximum length to the maximum lengths that were in effect in a State on June 1, 1991.