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Last Changed 1/19/2007
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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. |
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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. |
click on images to enlarge |
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 ET Hitch came onto the market since we bought our hitch. This is a
very robust hitch designed for HDT trucks, mounting directly
to the frame rails. The major
difference from the TrailerSaver is the use of parallel link arms to
provide a pure vertical movement of the hitch head on the air bags.
The design also prevents over-run if the trailer should ever be
pushed into the truck. Get more information on the ET Hitch at
ETHitch.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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