Even in states that enforce an 80,000-lb. limit, you'll still encounter
trucks that will have a sixth axle as you describe. This sixth axle will
typically be found on trailers that haul products you can't move with a
forklift.
That is to say, standard van trailers hauling palletized loads will
almost never have extra axles, because the pallets can be moved within
the trailer (to a point) to better equalize axle load distribution.
A liquid load such as gasoline, using your example here, cannot be moved
within the trailer by forklift. Often times, one end of the trailer will
bear enough weight to require an additional axle at that end. It has to
do with permissible axle (or axle group) weights, not gross weight, in
an 80,000-lb. state like yours.
You also mentioned lift axles. On the one hand, more axles are required
for more weight; a good example is my native Michigan, where 9-axle fuel
trucks can weigh 130,000 lb.. On the other hand, when you have a
crapload of axles like that, the axles that are farthest away from any
steering pivot point (fifth wheel, or the mean center point of all
trailer axles) get subjected to horrendous side-to-side forces in turns.
At best, these side-to-side forces just eat tire tread for breakfast. At
worst, these forces have been known to snap axles in two. What all of
this means is that lift axles are lowered for greater weight capacity
when going straight or in gentle to moderate curves, but they must be
raised to make turns or to negotiate tight curves.
Lift axles don't require a ton of extra hardware; basically, all that
gets added is another airbag that acts on a pivot point to raise the
axle, a few more feet of air lines, and a switch on the dashboard to
control the axles.
--
Larry Harvilla
e-mail: larry AT phatpage DOT org
http://www.phatpage.org/
http://www.flickr.com/photos/larrysphatpage