>  The worst problem they had is the pintle hitch on the
> trailer, which is too loose, and the trailer tended to whip back and
> forth, limiting their speed to about 45 mph.
 
I don't know for sure what is happening in the above case, but in general
-- AFAIK --  the most common causes of sway are related to tongue weight
and/or tow vehicle unsuitability, although improper tire inflation
sometimes is a factor too. Pintle hitches with an inch or more of play are
often used around here on heavy units and never -- to my knowledge -- cause
sway.
 
(FWIW,  I have driven halfway to town with only the trailer *chains*
attached and not noticed much amiss (yup I have a red face here), but I
have often noticed sway problems when a load was incorrectly distributed on
a truck and trailer -- even though there was a properly connected ball
hitch with no slack).
 
Since trailers are often used in bee work -- for sideliners and larger
hobbyists as well as the pros -- I think the following discussion might be
of general interest, and could very possibly save someone's life.  I've
tried to get the important points in without getting too disjointed...
 
When more than a tiny bit of sway is noticed -- at any speed -- the only
thing to do is stop *immediately* and move the load around until the unit
runs true.  When the load is distributed correctly, any swaying that starts
due to sudden turns, etc. should diminish rapidly with smaller and smaller
wags and be entirely stopped before 3 or so swings occur.  Good mirrors are
helpful for observing this.  If higher speeds result in much more sway than
low speeds, the rig is *very* unstable and the load in
*serious* need of *immediate* adjustment.  Almost always, the trailer will
be found to be rear heavy -- a very dangerous condition.
 
With trailers, at least 10% -- and preferably 15-20% -- of the total
trailer weight *must* be on the hitch when stationary on level ground if
safe operation is to be expected.  That means for a 10,000 pound trailer
(axle rating), a minimum of 1,000 to 2,000 pounds should be on the hitch.
Interestingly enough, this can result in an increase to 11,000 or 12,000
pounds in trailer payload, but means a corresponding decrease in the
truck's maximum cargo must be made.  This also means the truck has to be
able to support 1,000 or 2,000 additional pounds at the back bumper -- or
wherever the hitch is -- safely.
 
It also means the hitch has to be up to the job.  Most have a rating
stamped on them and the rating means exactly what it says.  Typical
pickup bumper hitches are good for 500 pounds hitch weight and that is all.
They *will* hold more, but once the unit begins to move, dynamic forces
occur which result in far more load than is obvious at the curb.
 
It is critical to also know that although hitch weight is important,  with
a small, light duty tow vehicle *too much* weight on the front of a trailer
can be dangerous, since it will overload the tow vehicle, bend the hitch,
or even lift the tow vehicle front wheels during sharp braking or on a
steep downhill resulting in loss of control.
 
In contrast, when using a heavy rated tow vehicle more load on the front of
the trailer is better.   Even loads well over the 10% - 20% *minimum*
recommendation and up to the safe rating of the tractor unit are desirable,
provided the hitch is properly designed and located.
 
Since a hitch is normally a measurable distance behind the rear wheels of
the tow unit and thus has considerable leverage over the truck, cargo on
the truck itself must be reduced at the *back* of the truck deck
accordingly.  I am assuming here that the truck is capable of comfortably
handling the necessary trailer tongue weight on the back end.  If it isn't,
a load leveller hitch, a gooseneck trailer or a different towing unit is
absolutely necessary.  Goosenecks move the hitch further forward.  Load
levellers have a similar effect, but do not work well in rugged off-road
conditions.
 
When sitting ready for driving on good roads, the truck itself should
appear level, not aimed at the stars.  If the trailer has more than one
axle it *must* be level too for proper distribution of the load to both the
truck and trailer axles.  And I guess this is obvious, but worth
mentioning: trucks that are overloaded often have the headlights pointed at
the sky. This results in poor night vision and blinding of other drivers.
 
Although the tow vehicle *can* safely weigh less than the trailer when
properly configured, if the trailer is much heavier than the truck a hitch
expert should be consulted in all cases and special care must be taken when
driving such a combination.  Good (independantly operable) brakes are
essential to long term survival of the driver and other users of the road
(and ditches).   Such minimally rated configurations *can* function safely
on level roads if set up properly, but may fail off-road.
 
Although reasonably useable on improved roads and highways, when operated
off-road on undulating terrain, light duty combination units are subject to
complex dynamic and static weight shifting between axles and over-rated
units are much preferred to allow for high momentary overloads that occur.
Off-road speeds must also be kept very low to reduce this dynamic factor.
 
In the USA, vehicles are rated by the manufacturer for towing limits and
penalties are in place in most states for exceeding them on public roads.
Weigh scales are provided along the roads to weigh truck axles to ensure
safe loading.  Usually, however the authorities who operate them usually
concentrate on the large commercial tractor-trailer units and let the
little guys go by -- often badly loaded.
 
Most laymen have no instinct for correct loading since proper loading
requires fairly complex calculations or use of a scale and it is amazing
what can be seen going down the road.  A trip over a scale -- one axle at a
time -- can teach a lot and I always pull over at the self-weigh ones here
in Alberta to see how good my guess was in loading.  My life, and those of
my helpers, depends on getting it right every time.
 
When loading and unloading trucks and trailers that are near the limit of
their weight capacity, consideration must be given to the balance problem
and what happens to the tow vehicle if the front of the trailer is loaded
without placing appropriate counterweight on the back.  Excessive weight
may be placed on the hitch and the rear of the tow vehicle,  In extreme
cases, breakage or bending may ocur. If the unit is driven between yards
while being progressively loaded up from front to back, damage, severe
instability or even a wreck may occur.
 
Something else to consider: if a trailer is loaded incorrectly, chances are
that a few tires are overloaded too.  Each tire has a maximum load marked
clearly on the sidewall, with a recommended pressure to be used at that
load.  For short trips, even gross tire overloading usually does not result
in immediate failure, but on a highway trip, heat builds and failure
typically ocurs after about 50 miles.  For an unstable truck/trailer
combination, tire failure can be the final straw that results in a wreck.
Frankly,  I don't hesitate much to overload a spring a bit, but never
intentionally overload a tire.
 
I realise that all this sounds a bit alarmist and perhaps overly technical
to some, but FWIW I've been there more than once and I didn't learn all
this the easy way.  Here is some more food for thought:
 
1.) I just spent the last 3-1/2 working days changing one of our
trailers -- welding and re-designing -- so that we can load and unload our
forklifts in less than a minute, including dropping the ramps, unchaining
and driving off or on.  One of the tests after construction was driving the
combination at speed on loose gravel roads. The combination in question is
a 2,000 pound two axle (12,000 pound rating) trailer behind a 'one ton'
dual wheel truck with 16 foot deck.  (The truck is stretched to 10' cab to
axle or I would never consider towing with such a long deck).
 
As it happened, we were not happy with a very minor stability experienced
with the original position of the Swinger on the on the 16 foot, two axle
trailer (20' overall).  We expect 100% stability but noticed a little self
limiting sway at 50 MPH on loose gravel.  I suppose it would be acceptable,
but we found that moving the centre of gravity of the Swinger (3,500
pounds) only one foot forward of the initial test position by turning it
around end for end resulted in *much* more stable performance.  Additional
weight at the front of the trailer (1,500 pounds) resulting from loading
during the day did not result in much change in handling. That's the way we
like it.  I hate white knuckle driving.
 
2.) Even with all our experience using trailers, constant vigilence and
attention to detail is essential *every* time a load is changed even a
little.  Last year, one of our people managed to flip a loaded 3/4 ton
truck and trailer combination right in the middle of a gravel road.  The
cause there was too much weight on the hitch for a small truck.  They had
unloaded the back of the trailer and not the front.  The resulting tongue
weight pushed the back of the truck down so much that the steering (front)
wheels on the truck were not properly loaded on a steep downhill and an
uncontrollable oscillation started up.
 
I've written before on moving partial tanks of liquids on small trucks or
trailers and the weight shifts that can ocur, so I won't go into it here
again.  Probably a search for 'trailer' would find it fast for anyone
interested.
 
There is a lot to the matter of using trucks and trailers. Although the
public takes trucks and trucking for granted, it is actually an area that
requires considerable training and study to do properly and safely.
 
I hope that careful thought, analysis and consulting experts where
appropriate will prevent potential tragedy and make driving much more
pleasant for all -- including people who live near a road and wouldn't like
a truck and trailer full of bees in the living room :).
 
Allen
 
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