Water Tanks fill level: The Impact on your 4WD

By: AOT Staff Writer

Weight … What about the water?

If you’re in the 4WD and/or caravan scene you’d have to be blind Freddy if you haven’t seen, heard or read something on weights and loading issues of late. It’s a growing concern for a number of reasons including on road legalities, insurance, warranties, and also just general driving and handling issues.

Certainly the caravan scene has been subject to a lot more scrutiny lately including roadside measuring activities, to the benefit of all road users I might add. Many of us are now busily looking at ways of putting the van on a strict diet and removing the unnecessary and duplicate items we perhaps don’t need.

Something that can be often overlooked is the water content simply because it is a necessity of life and its importance usually outweighs its actual weight. But at a kilogram per litre it can soon add a significant amount to the laden weight of your van. But there is in fact a greater impact water is having on vans than just sheer weight. Its force.

Below is a diagram that will help in understanding the real impact of water on a towed van.

Water tank level

Most modern vans have two large water tanks on board, some bigger units even have up to four large water tanks, be it for fresh water, or grey and black water storage. But even a smaller single axle van will usually have anything from approx. 60L-90L or even a larger water tank slung underneath central to the longitudinal axis.
Manufacturers will generally put a tank over or just forward of the axle or axle group so any extra mass assists in maintaining a forward balance of the van with regards to ball weight.

However many of the water tank designs available for securing under a van are in my view poorly designed for the application.

I know that may seem like an outrageous statement to make given the number of similar tanks being fitted to vans every day of the week, but allow me to explain.

Water being fluid will easily move and ‘wash’ with the movement of the van as it is going along. The tanks employed in the van industry are usually blow-moulded or roto-moulded plastic, and can be over a meter long. They will (or should) generally have some baffle element to their design, but due to the method of manufacture this is often achieved by having the shape of the tank dip down in a couple of points across the width of the tank from the top. This works fine in a full tank.

Where the water in a tank is a real problem is in a partially filled tank.

Any solid vessel, completely full of water to the brim is less affected by the mass of the water and its fluidity simply because it has nowhere to go. The water will not compress, so the mass within the full tank is fairly static. An empty tank is of course no concern. But where a tank is say half full (or half empty depending on how you see the glass of life) then that mass of water will move according to the movements of the van due to inertia. When you then have that mass of water in a caravan tank where the minimal baffle effect protruding from the top can’t properly limit it in a sideways movement, or yawing, then that can become a real problem.  If you take a larger van with multiple tanks which are all maybe half full or more of water, which can be up around 200kg plus, and move it sharply to one side or the other, or worse still repeat the motion, the force of that fluid mass upon shifting across the tanks can be quite substantial. For the physics mathematicians, yes it can be deduced with a handful of formulae, but in this article I will refrain from putting the other 99% of readers to sleep. But it should be easily understood that a van under sway motion will have that motion exacerbated by the force of the water shifting across the axis of the van at the worst possible moment.

The van not only becomes immediately unbalanced to some degree, but just as the van is nearing its peak of yaw movement, another maybe hundred kilos shifts across and hits the furthest wall of the tank giving the van another little nudge for good measure. You don’t want that happening repetitively. Again this article is not going into the technicalities of yaw rates or yaw moment, it is simply important to acknowledge that a sideways movement of your van when towing can be worsened with a shifting mass.

So an easy remedy to this issue is to consider how much water is in your tank in relation to what you are doing or where you are going.

Because water is so essential to life and it is important to retain appropriate amounts of water if heading into remote areas, this is not to suggest depleting your minimum water supply. However if your trip plans include stops at well supplied points, then is it necessary to have the tanks full or partially full?

Certainly a full tank will add considerable weight and this will impact on ball weight, towing ease and fuel economy.

An empty tank may leave you in danger of serious water shortage if stranded in a more remote or even rural area on a quiet weekend. So deciding how much water you need and how much to have on board whilst travelling should deserve more than just a cursory consideration.

Perhaps look at your tank system plumbing and consider isolating otherwise combined tanks so that one is empty and the other still full, rather than two sitting at midpoint where this becomes an issue. Perhaps share your remaining water with others when finally heading home. There’s a number of options to consider.

There is a lot of concern around caravan swaying and much discussion has been created around loading, ball weights, and sway control devices. Modern vans are now being built with ESC or Electronic Stability Control in a similar vein to the vehicle towing them. That’s a great development for all road users. The point here is to properly consider how your water needs may influence those systems and your vehicle stability whilst towing.

So before just ‘topping up the tanks a bit’ think carefully about where you are going, what water is needed and how does it impact your van under adverse towing conditions. Make sure your water supply sustains life, not risk it.

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