ACHIEVING ON-ROAD STABILITY THE EASY WAY
Tom Olthoff looks at on-road stability in caravans
Words and photographs by Tom Olthoff
Getting a rig to tow in a safe manner is a bit like baking a cake. If the right ingredients aren’t used in the first place, it’s no good adding them later. It is the same with a caravan or any trailer. If the axle position hasn’t taken in to account the layout, location of various items of equipment or the loading, very little can be done afterwards.
Contrary to caravanning mythology, getting a caravan or camper trailer to behave itself is not difficult. Yet everyday we hear stories of caravans that become unstable, drivers who are frightened of what might happen and accidents that could have been avoided if a little care had been taken from the outset.
Over the years it has been established that in order for a trailer to travel along in a stable manner at normal open road speeds, about ten percent of the total trailer weight needs to be supported by the tow ball. This is generally referred to as ball load, coupling load or vertical towball load and relates to an empty as well as a loaded trailer.
When it comes to trailers with high bodies like caravans, there is also the matter of the amount of body area behind the axle compared to that in front. The area behind can behave like a big sail that is only supported at one end while that section of body in front of the axle is supported by the tow vehicle as well as the trailer wheels.
It may be possible to load a trailer that has its wheels located in the centre of the load carrying area so that the 10 percent ball load is achieved. However, this doesn’t necessary guarantee stability as there would still be a considerable length of body behind the wheels that can be affected by wind pressure.
In addition to the 10 percent ball load that places the Centre of Mass in a desirable position in front of the axle, there should also be a considerably larger area of body in front of the axle centre. In the case of a tandem it is the mid point between the two axles.
Another myth is that these ‘rules’ only apply to a single axle van and not a tandem. Nothing could be further from the truth. As a tandem caravan is generally longer than a single axle one, there is usually a much greater area of body to catch the wind as well as the possibility of undesirable weight a long way behind the wheels.
WHY 10 PERCENT?
Most of what we do in Australia stems from North America where towing safety has been a major concern ever since they started building fast freeways and caravanning became popular in the 50s. And as we know, Americans like things big – cars as well as caravans. The importance of stable trailers soon became evident.
Those with a bit of a technical bend and those who paid attention in school will have heard of the Centre of Gravity or the Centre of Mass (C of M) in regard to objects. Take for example a motor car. If the engine is at the front and drives the front wheels, most of the car’s weight will be in front of centre. We can then say that the C of M is in front of centre. When the engine is mounted at the rear, the C of M is behind centre. As such the two cars would handle entirely different. Fortunately, though, there is usually a wheel at each corner.
A trailer with two or four wheels near the centre of the load carrying area is a different story. To ensure stability, the C of M has to be in front of the axle or axles. But where exactly? Let’s look at a box trailer that has four sides that are all the same height. If we filled the trailer with sand up to the top of the four sides, we could say that the load was distributed evenly from side to side and front to rear and the C of M would be in the centre (ignoring the drawbar). If we now place and axle under the centre of the trailer the C of M would be over the axle.
While a centrally located C of M is appealing as it makes it easy to raise the front and hitch the trailer to the tow car, the down side is that the trailer can more easily yaw, often referred to as snaking, tail wag or sway.
One way to reduce the chance of yawing is to move the C of M forward. But how far is enough? Sometimes advice given refers to 60 percent of the total trailer weight being in front of the axle and 40 percent behind. If we had a box trailer filled with sand, the axle position to achieve this would be quite easy to calculate. But when the load distribution is uneven like in a caravan it becomes more difficult.
However, it works out that if a trailer is loaded in the recommended manner, about 10 percent of the total load will be at the coupling. This is normally called the ball load. Yet another myth is that the caravan has to be loaded before it becomes stable. Any trailer, if designed correctly, should be stable when towed in an empty state.
If a caravan has about 10 percent ball load to start with, it is much easier to maintain this balance when the van is being loaded. In most cases the cause of insufficient ball load when a trailer is empty can be found by looking at the axle location.
More or Less?
Although we tend to talk about the 10 percent factor, what are the implications if we go away from this? While it is not suggested that eight or nine percent is bad, it is worth remembering that as the percentage of ball load reduces the likelihood of instability occurring increases as the C of M moves further rearwards. As to what the bottom limit may be depends to some extent on the layout and the length of the drawbar. (refer to the section on Layout below)
Ball load higher than 10 percent are quite acceptable and can be beneficial to stability. The maximum amount of ball load that can be used depends on the specifications of the tow vehicle and the capacity of the towbar fitted. Take a Holden Commodore for example. If it was fitted with a 1600kg towbar and we towed a 1500kg caravan it would be fine to have 150kg on the ball. When the same car has a 2100kg towbar, the 1500kg van could have 200kg of ball load without causing any problems.
Another example is a fifth-wheeler. Some of these may have 25 percent of the total load at the front without causing any concerns. The reason is that the load is taken directly on the tray or chassis of the tow vehicle. This allows the axles of a fifth-wheeler to be positioned well towards the rear to enhance stability.
With the Centre of Mass above the axle there can be a tendency for instability as speed increases |
When the Centre of Mass is in front of the axle the trailer is very stable |
When the Centre of Mass is behind the axle the trailer can become unstable at relatively low speeds |
A 60:40 load distribution results in a ball load of about 10 percent |
INTERNAL LAYOUT
While caravans can come with many different internal layouts, they basically fall in three groups – front, centre and rear kitchen. On the basis that the heavy items should be carried near the centre and over the wheels, the centre kitchen is preferred if maximum stability is called for. With front and rear kitchen models, the position of the wheels becomes much more critical.
When the kitchen is at the rear of the van, the wheels need to be a considerable distance back to not only place the C of M in front of the axle but also prevent the additional weight at the rear causing the van to wag its tail. As long as the tow vehicle can cope with the ball load, a rear kitchen model can be quite stable on the road.
On front kitchen designs, the wheels are often closer to the front to support the additional weight and keep the ball load within acceptable limits. While that may suit cars with low ball load limits, there is the danger of having too much body behind the axle which in turn can be affected by wind pressure and cause the van to be less stable.
SOME SUGGESTIONS
Before being too concerned about a particular van, it is necessary to have some facts. A visit to a weighbridge is a must. By firstly placing the caravan wheels on the weighing platform and then the entire caravan, the ball load can be determined by subtracting the first from the second reading. For example, if the load carried by the wheels is 1800kg and the total weight is 2000kg, this means the ball load is 200kg. Dividing the 2000kg by 200kg gives 10 percent.
When shopping for a caravan always check out the wheel position by looking at the side of the van from some distance away. If the wheels are positioned back from centre that is a definite plus. On the other hand if you feel you need a tape measure to determine the wheel position relative to the length, there may be a potential problem.
If the manufacturer provides the ball load with the other data on the trailer plate, remember that this refers to the empty ball load. Compare it to the tare weight stated and see if it is about 10 percent. If it comes out a little on the low side, consider the wheel position as well as where most of the content will be stored. Obviously items placed in a front boot are going to add to the ball load but as some manufacturers limit the amount that can be placed in the boot this can be tricky of the ball load is low to start with.
Summary
We started by saying that getting a trailer to tow in stable manner is not difficult. But the ingredients have to be right. Although we don’t want to dismiss any particular design or layout, everything considered a centre kitchen model has a lot going for it. The bedroom can then be at the front or rear. For a full-height van a front bedroom is an advantage as it allows a more aerodynamic shape at the front.
Having a smaller area of caravan body behind the wheels than in front is most desirable. This means that buffeting side winds or air pressure from passing trucks will have less on an influence on stability.
Loading the van so that the desired balance is maintained is also important. The only way that you will find out is you have achieved this is a trip to the weighbridge when everything is onboard.
Don’t add heavy items to the rear, particularly if the wheels are close to the centre. Nor should heavy items be added to the drawbar as this could overload the towing vehicle and place additional stresses on the drawbar.
Reputable and professional dealers will make sure that you rig is set up safely. Nevertheless the saying “buyer beware” should not be dismissed. Just like you wouldn’t use plain flour for a cake when self-raising is called for, you wouldn’t start with a van that has insufficient ball load hoping to fix it later.
If you want more information on any of the points raised in this article, ask the question in Tom’s Tech on the C&M forum http://www.candm.com.au/forum/
With a well designed caravan behind, towing is a breeze |
Having the largest area of the caravan body in front of the wheels helps stability |
Adding weight to the rear can cause a van to become unstable if the ball load is already on the low side |
With an ATM of nearly 2100kg and a tow ball load of 127kg a lot of weight would need to be added to the front to reach the 10 percent mark or about 200kg |
Extra items added to the drawbar can make the ball load too high |
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