The importance of good load balance in caravan engineering

In part two of How well engineered is your caravan? we look at the importance of good load balance in caravan design and engineering. If you missed part one, it’s here.

It is not only the ability to not fall apart around your ears that marks a well designed and engineered van. It should be a given that the caravan does not sway and should ride well, regardless of whether it is empty, partly loaded or fully loaded up to its ATM rating.

This is done during development by ensuring that mass distribution for both empty and loaded conditions stays within the acceptable range, to ensure that the extremely critical towball loading stays safely within the required narrow range. While CAD/CAE input helps, ultimately this can only be determined by conducting real-world road testing.

Caravan engineering

Fit-outs can be complex, but if all is drawn up in CAD first you can avoid dimension or component position mistakes

An important design requirement is that all heavy items need to be located as close as possible to the axle(s), and not at the ends of the van, especially not at the rear end. A caravan with a weighty rear end might tow okay, but as soon as evasive action is required, basic engineering principles will tell you that it’ll want to sway more quickly than a better-balanced van.

Caravan engineering - Bushtracker
CAD drawings allow for plumbing and electricals to be accurately placed, as in this Bushtracker build

How to calculate theoretical towball download changes

It’s an important part of good chassis balance to know what effects water tank and gas bottle volume changes have on towball download. Those changes should result in minimal change to towball download. You’ll need comprehensive measurements from the caravan manufacturer, or take your tape measure and get the measurements yourself. An excellent formula for calculating towball download is shown below (with thanks to Colin Young, of the Caravan Council of Australia).


The formula for determining how much the ball-loading varies, depending on whether the gas and water tanks are empty or full.

Note: Dimensions from the centre of the axle group to the left are positive… dimensions to the right are negative.

When containing water/LPG, tanks ahead of the axle(s) increase the ball loading; tanks behind the axle(s) decrease the ball loading.

Effect Of Contents Of Tanks On Ball Loading Diagram Reference

The Actual Mass of the van will increase by: G + W1 + W2 when the empty tanks are filled.  One (1) litre of water weighs one (1) kg.

If a third tank is fitted, add it to the drawing and measure the (+ or -) distance from the centre of the tank to the centre of the axle(s).

Calculate the “Moments” – Mass x Distance – around the centre of the axle (or midway between tandem axles):

Change in ball loading  =  ( (G x LG)  +  (W2 x LW2)  –  (W1 x LW1) )  /  LC  

Example: G = 18kg; W1 = 45kg; W2 = 90kg; LG = 3.0m; LW1 = 1.5m; LW2 = 1.5m; LC = 3.5m
Change in ball loading = ( (18 x 3) + (90 x 1.5) – (45 x 1.5) ) / 3.5
Change in ball loading = ( (54) + (135) – (67) ) / 3.5
Change in ball loading = ( 122 ) / 3.5
Change in ball loading  =  + 35kg  

Worst Case 1: W1 empty; G & W2 full
Change in ball loading  =  + 54kg  

Worst Case 2: W1 full; G & W2 empty
Change in ball loading  =  – 19kg  

Caravans should be designed so that there is the least possible change in the ball loading when each tank is full or empty. Multiple water tanks should be positioned as close as possible to, and each side (front/rear) of the axle(s). The formula can also be used to calculate the change in ball loading when an appliance, or heavy article, is installed.

In part three, we talk about chassis and suspension design and ratings.




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