The motivation behind this page is the misinformation I have been repeatedly given by people from companies and
organisation who should know better and an apparent lack of consensus among these professionals.
We will start with some basic definitions pertaining to towing a trailer or caravan.
Tare
This is the unladen
mass of the trailer/caravan as would be measured by placing it
wholly on a weighbridge.
GTM
(Gross Trailer Mass)
This is the laden
mass of the trailer/caravan supported by the wheels.
It does not
include the load supported by the coupling of the towing vehicle.
ATM
(Aggregate Trailer Mass)
This is the total laden
mass of the trailer/caravan as would be measured by placing
wholly it on a weighbridge.
Ball Weight
This is the load supported by the coupling of the towing vehicle.
The maximum load you can legally tow is usually determined by your vehicle manufacturer.
In the absence of any manufacturers requirements the maximum of 150% of the
towing vehicle applies. This is now uniform
throughout Australia.
However, don't rely solely on what you read here or anywhere else, check with your vehicle manufacturer or their
authorised Australian agent and
your State vehicle registering authority.
Notifying your
insurance company would also be a wise move.
The vehicle manufacturer usually will supply two figures.
* Maximum towing capacity
* Maximum tow ball load
I will use my Holden Jackaroo as an example with figures from the Owner's Handbook.
* The maximum towing capacity is 2250 Kg braked
(750 Kg unbraked).
* The maximum tow ball load is 120 Kg.
It is well known (or should be) that the tow ball load should be at least 10% of the loaded caravan/trailer mass for safe stable towing.
The higher the ratio of caravan/trailer mass to tow vehicle mass, the more
critical this becomes.
With a 1:1 ratio I prefer closer to 15%.
This gives me a safe stable outfit that doesn't wander around in cross
winds or with the passing of semi trailers.
The problem here is 10% of 2250 Kg is 225 Kg and the maximum allowable is 120 Kg. To pack the caravan so there is only 120 Kg on the tow ball, only just over half the accepted load, would seriously compromise stability.
This is where the misinformation and confusion arises and despite what you may have read in magazines or have been told by industry "professionals", the answer
can be simple and unambiguous.
Despite information to the contrary given to me by a leading towing
equipment manufacturer, according to General Motors/Isuzu, providing the
attitude of the vehicle, that is the height of the body from the ground at
the front and rear remains the same in relation to each other, then they do
not specify a maximum ball weight providing the vehicle is not lower than
the rear would have been with the 120 Kg and that the maximum ball weight
rating of the tow bar is not exceeded. In my case this is 250 Kg.
This is achieved with a weight distribution hitch as show on the right.
This is basically a pair of torsion bars attached to the towbar such that
they can move in the horizontal plane only and attached by 100-150mm of
chain to the caravan/trailer drawbar.
The load carried by the torsion bars is adjusted by selecting the most appropriate
chain links so as to raise the rear of the vehicle until the attitude of
the tow vehicle is the same as before hitching up the caravan.
However, I believe the present design of these leaves a lot to be desired.
For example, in my experience . . .
* They are very noisy particularly whilst turning - despite liberal use of
dry lube.
* They have a high wear rate - also despite liberal use of dry lube.
* They must be released over rugged terrain.
(Large "spoon" drains impose enormous
additional loads.)
* They can be a pain to fit (literally) in the 250 and 350 Kg versions.
* I have been advised
they can not be used with 360 deg. off road
couplings.
Despite the shortcomings of these agricultural devices, in the absence of
something better, they do achieve the aim.
If you would like to
see the the type of wear I have experienced, click
here.
Update Dec 07
Despite advice
promising accelerated wear due to the retention of dirt and debris, for
the last 2 years I have been using wheel bearing grease on all
contact points including the chain hooks and whilst this is very messy to
handle, it has reduced wear (and noise) substantially.
Although this leveling could be achieved with adjustable (air) shock
absorbers, the weight distribution hitch is the only practical solution
since it does not just raise the vehicles rear but in doing so transfers a
large percentage of that load to the vehicles front axle, the remainder is
transferred to the caravan/trailer wheels.
(The actual percentage transferred to each is proportional to the ratio of
the distances between the axles.)
This results in a better balanced vehicle and better steering and braking.
(Also, there are a number of devices on the market to counteract
trailer/caravan swaying if you have a badly designed or balanced
outfit. You may need one of these anti-sway devices in this instance
or if you like to load your caravans' rear bumper with spare tyres, jerry cans
etc. However, for stable towing, any significant mass should be
concentrated around the caravan axle/s or preferably forward of the
axle/s.)
When you are towing something that can
be 150% of the tow vehicles mass, you need everything going for you
especially when road conditions and/or weather is less than ideal.
I would strongly advise you to do as I did and contact your vehicle
manufacturer and ascertain whether or not they are happy with this arrangement
and if so, ask for them to give you a written approval exonerating you from
their usually pitifully inadequate maximum tow ball load when using a
correctly sized and fitted load distribution hitch.
It is important not only from a safety point of view but in the event of an accident or mishap, if you have not complied with manufacturers load ratings and specifications, you are technically un-roadworthy and in all likelihood, you will not be covered by your insurance company and will leave yourself open to a civil claim for damages as well as possible criminal
proceedings.
A link worth looking at for tips is from the South
Aust Caravan Ass. |
