[A/S] FW: Weight equalizing hitch bars - and wheel balancing

["Oliver Filippi" <ofilippi@xxxxxxxxxxxx>]

It seems to me that the recent post relating the capacity of the spring bars
to the load capacity of the tow vehicle is completely off base, although
there are some elements of truths interspersed in the discussion.  Hence, I
think a clearer understanding of the mechanics of a modern weight equalizing
hitch is in order.

First lets look at the function of the WEIGHT EQUALIZING SPRING BARS.  As
their name implies, the purpose of the spring bars is to transfer SOME OF
the hitch load of the trailer TO THE FRONT WHEELS OF THE TOW VEHICLE. Under
optimum conditions, with the spring bars properly adjusted, one would have
the chassis of the tow vehicle essentially in the same plane (approximately
parallel) to the pavement, as the vehicle would be if there were no trailer
hitched to it. Obviously, with the added hitch weight, the tow vehicle will
now sit slightly lower on its suspension (both front and rear) than it would
without the trailer.  This is the equivalent of putting the load on the tow
vehicle between the wheels rather than on the rear bumper. Thus we don't
create a "Wheelie" attitude, but, instead, maintain an "even keel" and the
designed suspension geometry built into the tow vehicle by its manufacturer.

Note that thus far, we have said nothing about the load capacity of the
vehicle (or in the case of a truck, perhaps its cargo weight capacity).  It
should be obvious that the tow vehicle must have sufficient load capacity to
handle the hitch weight of the trailer.  (in addition to having the
capability of pulling the total weight of the trailer and the cargo and the
passengers carried in the  tow vehicle).  Note too, that the hitch spring
bars DO ABSOLUTELY NOTHING to increase the load capacity of the tow vehicle.
(If you wanted to split hairs, you could say that the spring bars have
actually reduced the load capacity of the tow vehicle - by the amount of the
weight of the spring bars).

If the springs on the tow vehicle are too soft, of deflect too much under
load (assuming they are otherwise still adequate to the task) then the fix
is to install stronger (stiffer springs) or air lifts.  The hitch springs
have nothing to do with this.  If your tow vehicle has inadequate springs,
your best bet would be to get a different tow vehicle.  Installing, for
example, air shocks would be a real mistake, as shock absorbers are designed
to dampen oscillations, not support a load. Likewise, so are their mounting
brackets on the vehicle.

Now lets look at the CAPACITY of the hitch spring bars.

When you buy a weight equalizing hitch, you will find that they are
available with different capacity spring bars which the manufacturer will
tell you are to be selected TO MATCH THE HITCH WEIGHT OF TRAILER. Remember,
in use, the hitch spring bars will be sufficiently tensioned to transfer
some of the hitch weight from the hitch in the rear to the front wheels of
the tow vehicle.  Clearly, the spring bars must be strong enough (have
adequate capacity) to take the load of the hitch weight and transfer it
forward. Further, hitch manufacturers will recommend the lightest spring
bars that will do the job.  We will look at this issue momentarily.

So, what about getting the maximum capacity spring bars and just tension
them less?  Not a good idea!  It reminds me of the saying of an elderly
neighbor when I was a small boy, "If a little is good, more should be
better - as the soup came out too salty."  Shakespeare put it more
eloquently in describing honey.

The problem with overkill spring bars is that they are stronger and stiffer
than the preferred size ones (in mechanical engineering terms the heavier
springs have a larger "k", which is the amount of force it will take to bend
them a given distance) which can cause some unintended consequences.  First,
let me say that an excess capacity spring bar tensioned properly for the
hitch weight will work just fine - as long as you are traveling over a
smooth road with no dips or rises.  However, as soon as the terrain changes,
note what will happen:

  If you go over a rise the tow vehicle ball (and spring) mount will tilt
down relative to the trailer (and the spring tensioning mechanism) and
REDUCE THE TENSIONING on the spring bars which in turn will increase the
load on the rear of the tow vehicle and decrease the weight on the front
wheels.  Conversely, initiating a climb (or coming out of a dip) will
increase the downward force on the front wheels and decrease it on the rear
wheels. With the proper capacity spring bars, this fore and aft shift in
loading of the tow vehicle will be within an acceptable range.  On the other
hand, excessively stiff spring bars will cause more pronounced shifting of
the weight on the tow vehicle, and could become dangerous under some
conditions, especially slippery conditions or ones with compromised traction
(creating problems controlling the tow vehicle). This see sawing of the
weight fore and aft could also cause a less comfortable "choppy" ride.  I
doubt that this would be sufficient to pop rivets on your trailer, but it
could be a minor factor. (the reason we got into this discussion in the
first place.)

Now lets look at the impact the characterizes of a tow vehicle suspension
could have on the trailer.

Here, one must recognize that the VAST MAJORITY OF THE WEIGHT OF THE TRAILER
IS CARRIED BY THE TRAILER'S OWN SUSPENSION (AXLES, SPRINGS, SHOCK ABSORBERS,
AND WHEELS AND TIRES), which, in turn, are primarily responsible for the
ride (and vibrations) the trailer experiences.  Thus, the place to reduce
vibration in the trailer is to fine tune the trailer's running gear as much
as practical. [Mostly balance the tires and wheels, properly inflate the
tires, and have good shock absorbers.]

Sure, a rough riding tow vehicle can contribute some to the trailer
vibrations, but is not the major factor.  We have all heard the saying,
"This vehicle rides like a truck!" meaning that the ride is hard or stiff
and bouncy.  Recognize that this does not have to be the case.  Vehicle
suspensions have come a long way in recent years, and there are ways to have
a comfortable ride and still have substantial load capacity.  Without
getting into a debate or partisanship over makes and models, it is probably
safe to say that half ton pickups USED to ride a lot better than 3/4 ton
models. Since both vehicles had to operate both fully loaded and empty, the
added spring capacity of the 3/4 ton often gave it a harsher ride when
empty.  But modern variable rate (our old "k") springs have changed all
this.  If you have test driven American pickups these days, I think you will
find that Dodges ride the harshest followed by Ford and then Chevy/GMC.  The
best riding pickup of all is the Ford with the OPTIONAL INDEPENDENT REAR
suspension borrowed from the Expedition. [Ford hired a suspension engineer
from Porsche to fix the unstable Explorer - for which he designed a four
wheel independent suspension. Then they put him to work on the Explorer.
The main motivation was to get a lower interior floor (which can be done by
not needing the clearance space for a solid axle to go up and down). In the
process (and in my opinion, for the wrong reason) Ford got a far superior,
modern suspension. Now, I am sure, all of the automakers will likely
follow.]

One final note on wheel balancing:

The same referenced post also extolled the accuracy of the on vehicle tire
(and hub) balance. Note that this referenced balancing method ONLY provides
a STATIC balance.  It does not (and cannot) address the DYNAMIC balance I
went into in such detail on a prior post (namely the side to side wobble
caused by an imbalance between the inner and outer rim and sidewall.

And a final note on brake and hub balancing:

So far as balancing hubs and brake drums (or rotors), I think the long term
solution would be to pressure Airstream to offer balanced hubs in the first
place (and perhaps offer a factory balancing service.  You could even block
up your trailer and ship the hubs to Jackson Center for balancing, assuming
they could provide acceptable turn around.

Smooth trailering,

Oliver Filippi