How to Load a Caravan Safely - updated June 2019
The above shows the effective mass of locating a 20 kg item at various distances from the axle/s of a caravan - it is not linear.
Knowing how to locate mass correctly along the length of a caravan is vital. That applies both to its design and the owner's loading.
As with a see-saw, two children of the same mass sitting at equal distances from the central pivot balance each other. Likewise an adult sitting near the middle on one side can balance the far less mass of a child at the far end of the other side.
A centre-axled caravan behaves like this too. Where you locate stuff along its length has exactly the same effect. Its distance from the axle/s increases its 'effective weight'.
But whilst a caravan behaves like a see-saw in the vertical plane (i.e. pitch up and down), it can also yaw (i.e. sway) sideways under tow.
Locating a (say) 20 kg tool box on top of a see-saw's pivot will not cause the see-saw to move. Likewise placing it directly over a caravan/s axle will not affect it if that caravan were to pitch or sway. But if that tool box is half-way toward the back of the caravan (and that caravan then pitches or sways) that 20 kg tool box has an effective mass of 40 kg - because it is further away from the axle/s.
If that tool box is on a 10 kg external rack at the rear of the caravan, it acts as if weighed around 80 kg and the rack an effective 40 kg. Each spare wheel carried on the rear of a caravan has the same effect. A tool box and rack thus acts as if were four times heavier at the back of a 7 metre caravan than in the middle. It not only exerts additional force when the trailer pitches and yaws (sways): it induces that pitching and swaying.
The effect is similar at the front. The down and side forces on the tow vehicle causes it to pitch - and yaw sideways. That side force literally steers the tow vehicle by distorting its rear tyres.
Never have heavy items, particularly tool boxes, spare wheels and full jerry-cans, at the very front or rear of a caravan.
Always load the tow vehicle to its maximum legally permitted weight. Keep the caravan's loading as light as possible - and as close to the caravan's axle/s as possible.
Further - there is a common and very dangerous belief held by many caravaneers that it is fine to have a heavy load on the rear as long as it is balanced by a similar load at the front. That is virtually a recipe for jack-knifing. The rig will seem ultra-stable much of the time, but if sway sets in at speed it may be impossible to stop it. The sway is likely to escalate and the rig jack-knife.
The caravan needs to be about 10% nose heavy, but do that by locating heavy stuff just ahead of the axles - not like a weight lifter's barbell (that has all virtually all of the weight at the ends). This is why RV Book's articles and publications so strongly condemn locating heavy spare wheels etc at the extreme rear of long caravans.
Be aware that a few long and heavy caravans are being made with all water tanks ahead of the axle/s. With these the critical tow ball mass will vary (and potentially dangerous if water is drawn). They appear to rely on the tanks being full whilst towing. Some have since been recalled and modified.
For a truly thorough plain English explanation of all this (and how to assess the stability of your own caravan) do see our book 'Why Caravans Roll Over - and how to prevent it'.
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