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Last changes 1/19/2007 |
We knew we wanted to outfit Red Rover with an "air-ride" hitch!
There is an interesting effect when using a Medium/Heavy Duty
Truck (MDT/HDT, Class 6-8), instead of an under-rated 1-ton dually
pickup truck, to pull a heavy trailer. With the pickup truck,
the trailer mass is greater than the truck, and road bump impact
is transferred to the truck. With the MDT/HDT, the truck mass
is about equal to the trailer, and the trailer gooseneck absorbs
the impact. Anecdotal evidence shows that the trailer can
sustain frame damage from this impact.
By using an “air ride” hitch, air bags are placed in between the fifth-wheel
hitch plate and the truck frame. This reduces the inertial impact
between the trailer and the truck.
Bill
Farlow was a leader in the discussion of the needs for “air ride”
hitches. You can scan the internet for articles written by Bill.
Another consideration for what fifth-wheel hitch dealt with the
frame height of Red Rover. The top of the frame rails are
about 40” high. A normal semi-trailer hitch plate is about
5” to 7” high. This yields a hitch plate height of 46”.
This is what we measured on several fifth-wheel trailers.
Finally, the traditional hitch used by semi-tractor is really
not suited for fifth-wheel towing. By nature, commercial trucks
are on flat pavement. While not rough terrain, campgrounds
are not always flat. The connection between the truck and
the fifth-wheel needs some lateral (side-to-side) motion.
Having some damping on this lateral action is also important for
towing stability. Unhampered, the lateral motion could allowed
the trailer to wallow behind the truck.
A current trend is to use a air cushioned trailer pin with a
traditional hitch. This does provide up-down cushioning but
does not handle the twisting action between the truck and trailer.
All the above issues brought us to a
Air Safe Hitch. |
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It is important to have the fifth-wheel plate of the truck and
the fifth-wheel pin of the trailer at a height where the trailer
rides as close to level as possible. If the front of the trailer
is held high, this puts more load on the rear axles of the trailer
and reduces the load on the front axles. If the imbalance
is enough, the rear axle could actually be over-loaded. This
is not good for axle wear and trailer stability. |
click on images to enlarge |
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A normal semi-trailer hitch only has to pivot fore and aft.
Therefore, the difference between the surface of the hitch plate
and the frame is minimal. With an "air ride" hitch, there
must be room for air bags between the hitch plate and the frame.
Most of the “air ride” hitches have minimum heights of 15”.
This is fine for pickup truck installations where the bed is much
lower than the frame of a MDT/HDT. There are a few "air ride"
hitches that have minimum heights of 10”. This is better,
but added to the 40” of the Volvo frame, it would be 50” high, far
too high for the trailer pin. |
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Raising the trailer could be an option. But most of the
trailers we were looking at were pretty tall already. So,
we decided to inset the hitch between the frame rails. We
have to give credit to Bob Speckman, who did this with his
Volvo. Pictures of his installation are at
Bob
Speckman's TrailerSaver Installation. The distance between
the Volvo rails was 26”. This limited the choices of “air
ride” hitches to two, the TrailerSaver and the Air Ride Hitch.
The decision to inset the hitch between the frame rails brought
another consideration into the process. The ideal location
for the hitch places the hitch pin over the center of the axle.
With a hitch mounted above the rails, this is very easy. Insetting
the hitch means we had to deal with items located between the rails.
The main consideration was a crossmember that is between the rear
hangers of the axle. In Bob Speckman's installation, he chose
to cut the crossmember across its vertical height. We wanted
to retain the crossmember intact. We therefore had to move
the hitch back to clear the crossmember.
Having the hitch pivot behind the center of the axle has two
effects. When backing the trailer, directional control of
the trailer will be more sensitive but it still will be far easier
than backing a travel trailer. A benefit of having the pivot
point farther back is that the trailer will track closer to the
track of the truck. |
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The other thing we had to be aware of was not to have the frame
extend too far behind the wheels. The issue here is if we
put boxes behind the wheels like we plan to do, if the length gets
too long, there is a chance of the boxes bottoming on a steep incline.
The decision was made to place the hitch as close to the crossmember
and behind it. We estimate that we can still clear a 11º ramp. |
click on images to enlarge |
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The TrailerSaver hitch uses a plate pivoted at the forward end
and the air bags at the back. This gives up-down cushioned
motion. This hitch had a very good track record. It
would have done the job. Get more information on the TrailerSaver
at TrailerSaver.com. |
click on images to enlarge |
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<update>
The original Advanced Air Hitch has been bought out by Air Hitch Technology and
it purchased through
Air Safe Hitch.
The Air Ride Hitch (ARH) has two models, the 25K and the 32K.
We choose the 32K to have extra margin in towing capacity since
we will probably be at the upper reaches of fifth-wheel weight. |
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The ARH 32KS has a plate with an air bag at two rear corners with a swivel
hinge point at the front. The swivel hinge is a large tie rod end like
ball joint. This allows vertical as well as right-left twisting.
This provides for cushioned and damped lateral movement as well as the up-down.
Get more information on the Air Ride Hitch at
Air Safe Hitch.com
Many hitches have a pivoting head but have no damping action
on the lateral movement. |
click on images to enlarge |
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We felt that since everything we own would be riding on this
hitch, the Air Ride Hitch would be the better choice. The
Air Ride Hitch costs a little more than the TrailerSaver, but as
a value proposition we felt the 4-way motion over 2-way was worth it. |
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