By
Mark Penlerick
Engineering Team Leader
Blue Ox Towing Products
In
the
Spring 2003 issue of RV technician I touched on the subject of height
differences between motorhome receiver hitches and baseplates on
towed vehicles in an article entitled "Tow Bars wear out?"
Due to the importance of this issue, I feel this subject deserves
a little more face time than it received in that article.
Seeing
the hundreds of towing set-ups that cruise in to the Blue Ox factory
for tours and visits to the Blue Ox RV park, coupled with having
been to several rallies over the years, I've seen some strange hook-ups,
even some dangerous ones that I've spoken to the owners about correcting.
As a dealer, you should take it upon yourself to correct unsafe
situations when Rver's stop by your dealership.
The
angle of the tow bar is probably the easiest thing to spot, and
one of the most dangerous if left unresolved. A few years ago, it
was easy to convince an Rver that their ball coupler needed to be
parallel with the ground to keep the coupler from prying itself
off of the ball. Presently, in the days of motorhome mounted and
stored tow bars
some Rver's do not feel the angle of the tow
bar is as important as it used to be. When Blue Ox first brought
motorhome mounted tow bars on the RV scene they were new to the
industry, dealers, Rver's, and even the manufacturer. Back then
the sky was the limit so to speak and severe tow bar angles were
somewhat common. Now, you say; why does this matter? Well, if an
Rver has a 10-inch height difference and never has to make a panic
stop or slow very rapidly, they will likely not have a problem,
but if they must brake hard, it could spell T-R-O-U-B-L-E.
Blue
Ox's recommendation is that the receiver hitch of the motorhome
should never be more than 4 inches higher than the baseplate attachment
points. Four inches or less keeps the tow bar level with the ground
or slightly angled up towards the coach from the car. The tow bar
should never be angled "up" towards the car from the coach.
If an extension is added for say a bike rack it is even more imperative
the tow bar not be at a severe angle. Not only does it increase
the leverage, but each connection adds more slack in the whole setup.
It's all geometry and physics. Please, no groans, I know you enjoyed
those classes! The farther back you move the pivot point of the
tow bar from the center of gravity of the coach, the more vertical
movement you get and the more leverage is applied to the receiver
hitch, tow bar and the baseplate. See, that wasn't so bad!
Lets
get a better visual of a potential problem. First I need to be perfectly
clear that each coach and towed vehicle set-up on the road today
is different. They have different suspensions, weights, centers
of gravity, and brakes. They are loaded differently; some have front
engines, some rear. Long overhangs, short overhangs. Different wheelbases,
you name it, they are all different. Even two coaches of the same
make, model and year could react differently from each other. Towed
vehicle suspension as well as weight and the presence or lack of
a towed vehicle braking system, also play a big part in this formula.
Now, having said all that, what we are about to discuss is a model,
and only a model to show you the potential for problems. For the
record, the coach in the model is 36 feet in length and has a 12-foot
overhang past the rear axle. The towed vehicle is a small car that
would weigh approximately 3,000 Lbs.
Light
Braking
In
a setup where, on level ground, the receiver of the coach is six
inches higher than the baseplate attachment points. Seems pretty
harmless, doesn't it? You've probably seen worse things right in
your own parking lot! Then light braking causes the height difference
to change from 6 inches to ten inches. Believe it or not, the drawing
only reflects a one-degree change in the angle of the coach and
the car. Think about that for a minute! Not only does the suspension
of the coach give upward, allowing the front end of the coach to
dip, but the suspension of the towed vehicle also gives downward
and allows its front end to dip as well. This is now starting to
take its toll on the suspension components, alignment and tires
of the towed vehicle.
As
the situation progresses to moderate braking, you'll notice that
since the towed vehicle is gaining leverage on the coach by pushing
up more and more, the height difference increases about 6 inches,
where it only rose 4 inches during light braking. Now, we're seeing
a much more pronounced impact on suspension components of the towed
vehicle, but that's not all. The angle of the tow bar and the weight
of the motorhome pushing back down on the towed vehicle may start
flexing the baseplate installation. Depending on the distance from
the baseplate mounting points to the attachment points of the baseplate,
you are starting to get a large spike in leverage, force and stresses
on the baseplate, its hardware and the frame or unibody of the vehicle
to which it is mounted. Depending on how the baseplate attaches,
you may actually see the frame of the vehicle flex then return to
either its original position or at least close to its original position.
Repeated stops in this fashion may eventually lead to fatigue in
the vehicles frame, loosening of the bolts, as well as fatigue in
the baseplate itself, depending on its design characteristics. OK,
I've got to get a plug in here. Over the last few years here at
Blue Ox we have changed our philosophy on baseplate design to include
letting the baseplate itself torsion, much like the receiver hitch
on the back of the coach does, to absorb a lot of these types of
stresses. However, when forced into these positions repeatedly,
no baseplate or bracket design will hold up forever. Just remember,
that coach can weigh up in the 20,000 to 35,000 Lbs range and gravity
is looking to bring it back to earth. Gravity does not care that
the coach is pushing down on the baseplate of the towed vehicle.
Just
as we saw the jump from four to six inches between light to moderate
braking, now with heavy braking, we see the drama unfolding. That
six-inch difference sitting out in the parking lot has just turned
into a very scary situation as our customer just stood on the brakes
to avoid Bambi running across the road. Although it takes a pretty
hard stop from a six-inch difference to make the towed vehicle end
up under the rear of the coach, I have seen it happen before. Granted,
that set up was closer to 10 to 12 inches off to start, but have
you seen one in your lot that far off
probably have, and so
have I.
Correcting
the problem
This
is the easy part. Blue Ox manufactures a complete line of drop receivers.
They range in drop from 2 to 10 inches in increments of 2 inches.
These drops can also be inverted in the event that the baseplate
is higher than the receiver hitch on the coach. Along with the drop
receivers Blue Ox also manufactures what we call a hitch immobilizer
which can be bolted to the drop receiver and the receiver hitch
to keep the slack from allowing the drop to move around too much.
I would encourage all dealerships to have several of these drops
and immobilizers on hand at all times. They are a quick sale, and
an even easier install. It sure beats dealing with issues down the
road that were caused by too much height difference.
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