Thoughts and Concerns
About Towing Large Trailers
With a Bus
Updated 1/17/11 with updated links
Over the years, I have made many posts of various bus conversion bulletin
boards, expressing my concerns about towing large trailers with many types of
busses. I have pasted parts of three of my replies that will give you some
thoughts about my concerns:
There have been
many threads on towing trailers behind buses on this board. As most of you
know, I have committed to writing an article for George and Sue Myers on the
subject (even apologized to them at Bussin 2005 for being so tardy). (Editorial
Note: George and Sue no longer publish their bulletin - I still hope
to get this information into a format that can be published)
The article will address some of the issues involved and hopefully let the
reader make a better decision. I can’t begin to go into all of the details
here, but the following are some brief thoughts.
In general, buses were never designed to pull a trailer. Most do not have
strong frame rails like a truck. Instead, the trailer hitch must attach to
the engine cradle system. I won’t go into all of the problems that can occur
There are two major force components when we pull something: horizontal and
vertical. Pulling a toad is basically a horizontal load and an engine cradle
in sound condition with a well designed hitch can handle any reasonable
sized toad. It is when a vertical load is introduced that problems can
occur. Reinforced engine cradles combined with good hitch design and
equalizer hitches can handle reasonable sized trailers. Bear in mind that
equalizer hitches can introduce their own frame issues and care must be
exercised in how the hitch is attached to the engine cradle.
The big issue is
the structure of the bus. Any trailer hitch design will attach to the engine
cradle system on most busses. The cradle was not designed for some of the
large loads that trailers can impose on them. This can be made worse by
corrosion and fatigue which can reduce the ability to handle large loads.
There are two kinds of load: horizontal (straight pull as in a tow bar and
toad). The second is vertical load. In my opinion, most engine cradles that
have not been compromised by corrosion will handle reasonable horizontal
loads/pulls (i.e. most any size toad). It is the vertical loads that are a
concern. Any trailer should have at least 10% of its weight imposed on the
trailer hitch. Thus a 5,000 pound trailer will impose at least 500 pounds on
the hitch minimum. That is a lot of load on the engine cradle structure.
Some folks use equalizer hitches to take some of the load off the trailer
hitch. That works to some degree, but puts some different loads into the
system that can cause problems if the hitch is not properly designed.
Lots of folks tow large trailers and have had no problem. Having said that,
there are also folks who have reported frame problems. In my opinion, you
are really pushing the limit, if you have a trailer that weighs more than
Larger trailers can be used with auxiliary trailer tongue dollies or
specially designed trailers. As a part of gathering information for an
article I am writing, I have described some of those systems on my web site:
You mention GVWR. That is for the bus only and the trailer would not
normally count against that rating. The rating that includes trailers is
GVCWR where the “C” is combined which includes the bus and trailer. Since
buses were not designed to pull trailers, they do not have a GVCWR. The main
issue with the weight rating is to make sure that no axle is overloaded
(each axle will have a rating) and the trailer impact on the brakes. If no
auxiliary brake are used, you should not exceed the GVWR which is a real
challenge with some conversions.
There are some other issues that might come into the pictures. Overall
length can be a concern in some states on some highways. Having enough power
and gearing can also be a concern.
To build on Tom’s excellent post, you can also apply the
term “engine cradle” to many/most non-transit busses.
Most commercial motorhomes, as well as school bus and
truck conversions have a true frame system (two “C” channels running the
length of the vehicle). However, most over-the-road buses do not have that
type of frame. Some are monocoque (body stressed design such as GMC and
early MCI) and some are tube frame (Eagle, Prevost, and recent MCI). In
either type of construction, the engine is supported by a structure that can
be called a cradle. In most cases that is a tube structure tied into the
rest of the bus structure. It is designed to support the engine and make
installation/removal a simple process. In the “T” configuration, where the
engine runs for and aft, the cradle usually has two rails on which the
engine mounts rest. Once the bumper is removed, the engine/transmission can
easily be slid out the rear of the bus.
While it is not a part of the subject of this thread, you
will often see me expressing a concern, in various threads on the bulletin
boards, about pulling large trailers with busses which have engine cradles
(as opposed to “C” channel frames). Engine cradles were designed to support
the engine and mount the bumper, but they were not designed to support large
vertical loads on a trailer hitch. Lots of folks get by with no problems,
but it still is a concern to me.
As a part of researching the article, I have been looking for devices or
trailers that minimize the weight on the hitch (a significant concern for bus
application which were never designed to withstand large vertical tongue/hitch
loads). I am listing the results
of this research.
Update 1/17/11: The description
below was written several years ago. A review of the updated website
suggest that they have a 3500 HD unit for $3195. It appears that the
smaller units are no longer available.
This is a recently marketed product that has been under development for quite
a while. It is covered by US Patent 6,820,887. There are two
versions. The first has 8 inch tires and is rated for 2,000 pounds tongue
weight (priced at $2695) and the second has 10 inch tires and is rated at 3,000
pounds tongue weight (priced at $2895).
The unit was developed by Bob Riggle (owner of the patent) who is a drag
racing driver/owner famous for his "Hemi Under Glass" exhibition car.
The axle is pivoted (small amount of rotation) and the unit is mounted to the
bus/motorhome via a direct connection (no pivot). The unit comes with the
equalizer bars and an extra tire with cover.
The marketing folks say that it backs up well for short distances without any
change to the unit (wheels are not castered), while longer backing can be easily
accomplished by locking the back-up arm.
I talked to Rick at Penn Glass Fenders (1-814-344-8110 ext.4) and he said it
has been tested behind motorhomes with large overhangs and the unique solid
mount system with the pivot axle tracks very well.
I have briefly reviewed the patent and I am impressed with the design of this
Tuff Tow Trailer Dolly
This device has been used by the racing industry for years.
It has been installed on OEM trailers. There has been concern expressed
over the tire size and the ability to handle large vertical movements that occur
with buses and motorhomes with large rear overhangs. The manufacturer has
been asked to respond to those concerns. I will post the response when I
Note, this link (http://www.dallasmotorsports.com/)
is still active, but it appears that they no
longer carry this item. A search of the internet would suggest that it is
no longer available. I will continue to list it, as do-it-yourself folks
might want to consider the concept.
The Easy Tow is a complete system with articulated drawbar
(with air bags) as well as an air bag suspended set of castored wheels. It is
essentially a custom built unit. It is designed for a tongue weight of up to
3,500 pounds. It uses two Goodyear 13 inch trailer tires.
They have been installed on some custom built Champion
stacker trailers - mostly for racing teams. I did not ask about price, but the
person I talked to said “it ain’t cheap”.
They went with the articulated draw bar design to handle
large movement of the rear of a motorhome when going through dips – they felt
that the suspension of the dolly wheels could not handle the full travel
required for these applications.
One interesting design feature is that they dump the air
bag for the wheels and add air to the draw bar so that dolly wheels are raised
off the ground for backing up. This puts the full load of the trailer tongue on
the bus for a short period of time.
This dolly would probably not be a practical retrofit, but would be
a great solution for a person building their own trailer or ordering a large
As was the case with Easy
Tow, the manufacturer of this product seems to be out of business. A
search of the internet would suggest that it is no longer available. I
will continue to list it, as do-it-yourself folks might want to consider the
The Hitch Buddy unit is primarily aimed at the goose neck/fifth wheel market.
They do show a tag hitched to it. The likelihood of a bus or motorhome towing a goose neck
trailer is not high. If it were used for a tag trailer, it would be an extra
piece of equipment to find a place to store in a campground. Also, it adds to
the overall length in a tag application and that could be a real issue in some
I downloaded the video and was surprised to see that the unit would allow
backing up. I am not sure how they do that, but it apparently is possible. Both
the Tuff Tow and Dallas Motorsports unit allow backing up.
Special Designed Trailers
Competitive Trailers (no web site
Note: as of 1/17/11, this phone
number seems to be current.
This company makes various size stacker trailers under the name
"Bus Companion". It has wide spread axles (much like the old hay wagons),
with the front axle being self-steering. The front axle has built-in
castor which is hydraulically reversed when the tow vehicle is placed in
reverse. A good friend has one and is very impressed with how it handles
on the road and backing up. The price is not for the faint of heart (over
$50K), but it is the perfect answer for someone who wants to take all of his
toys with him.
German Semi Trailer with pivoting front axle
Front view of front axle suspension
Rear view of front axle suspension
We were in Germany in March of 2005 and saw many semi size
trucks pulling trailers like the one shown above (photos are thumbnail and can
be enlarged by clicking on the photo). Tag type trailers used in the US
have a separate fifth wheel type dolly, while these trailers have the pivoting
front axle mounted to the trailer. The axle itself is suspended by a
trailing arm and air bag suspension. This type of trailer design would not
put any vertical load on the hitch and would be very easy to hook up. I
would suspect that it would not be as easy to back up as a normal tag trailer.
It would be relatively easy to design and fabricate a scaled down version of
this trailer for use behind a bus.