Caravan Tow Ball Weight - how much (updated 9 December 2019)
Tow ball weight affects safe towing speed. Here's why and how
Tow Ball Weight Definitions
Tow ball weight is the weight placed on the tow hitch of a tow vehicle. This weight is technically known as tow ball mass - but for the purpose of this article, mass and weight may be seen as identical.
Why the Need for Tow Ball Weight
That a caravan be nose heavy is vital for safe towing. As with an arrow or a billiard cue thrown light-end first, they rapidly change ends. Unless caravans are nose heavy they’ll attempt to do the same.
There is a proven relationship between that front end (tow ball) weight and the speed at such action may occur. The lower the tow ball weight the lower the speed at which the rig may attempt to change ends - i.e. to jack-knife. Whilst extreme examples, a rig with a tow ball mass of 2% has virtually no inherent stability; one with 4% is likely to become unstable at under 50 km/h.
How Much Tow Ball Mass Is Needed
Tow ball mass is imposed on the tow vehicle's tow hitch - that is 1.25 metres or more behind its rear axle. The effect of tow ball mass is thus like pushing down on the handles of a wheelbarrow - that weight levers up its front. Tow ball mass does the same to a tow vehicle. It removes weight from its front wheels and that degrades the vehicle's cornering and braking ability.
The desirable tow ball mass must thus be a compromise but can be reduced having the caravan's weight centralised over the axle/s - and as far back as possible. It thus makes every sense to move any existing heavy items (such as batteries) as close as possible to (over the axles). This results in either increased stability due to reducing yaw, or retaining the existing stability at a now lower tow ball mass.
The required tow ball mass is related to towing speed. As yaw (snaking) forces increase with the square of the speed (they are four times higher at 100 km/h than 50 km/h) the higher the speed the greater the tow ball mass required. And, or course, the lower that tow ball mass, the lower the safe speed.
Because of this inherent effect, RV Books advises owners of typical Australian-made caravans never to exceed 100 km/h unless nose weight is at least 8% and ideally 10%. Safer, however, is never to exceed 100 km/h anyway.
Do be aware that any friction sway has next to no effect when needed most (at high speeds). This is because the frictional forces stay constant - but the forces they need to control increase with the square of the speed. At 100 km/h that frictional effect is only about 1%.
The Effect of Tow Ball Mass on the Tow Vehicle
Both vehicle manufacturer and tow hitch manufacturer set a maximum tow ball weight. The law requires you use whichever is the least.
Until 2015 most makers of vehicles used for towing permitted had a legal maximum tow ball mass of 350 kg. But in that year, in an attempt to reduce emissions (via reducing vehicle weight), almost every maker of such vehicles reduced the main chassis member thickness (from 3.5 mm to 3 mm).
The makers reduced permitted tow ball mass accordingly, but retained the previous (typically 3500 kg towing capacity). Their reasoning is that only a tiny proportion of such vehicles tow caravans. The vast majority are used by the military and tradespeople. Both may use trailers - but those are short, compact and thus have no need for high tow ball mass.
That 3500 kg (or whatever) towing capacity claimed, is in effect, what it can pull at the end of a rope - and its ability to stop and restart on a typically 14% gradient. It does in any way relate to carrying some part of that weight via the towing vehicle.
That 'towing capacity' does not in any way relate to supporting (or reacting yaw forces) of a laden caravan's typical and desirable 8%-10% nose weight. This is rarely revealed by the vehicle maker, and hardly-ever realised by caravan owners.
Regardless of previously recommending a realistic and desirable 10% tow ball mass, and making zero engineering changes, when vehicle makers reduced that permitted tow ball mass, most Australian caravan makers reduced their 'recommended tow ball mass' virtually overnight to suit.
Safe high speed caravan towing stability is substantially dependent on adequate tow ball mass. The long accepted (for typical Australian caravans) of 10% is still desirable, but decreasingly feasible as vehicle makers reduce overall weight.
Too much nose weight, however, reduces the tow vehicle's front axle loads (and using a Weight Distributing Hitch (WDH) can introduce undesirable tow vehicle stability effects. See the associated articles re this on this website.
A caravan's weight, including payload, should be as central as possible. The heavier loads should close to or over the axle(s) and low down. Ideally, the A-frame should carry no load. Most importantly, nothing heavy should be at a caravan's rear - yet a few makers locate spare wheel/s here. See here as to why and here for further information on correct trailer loading.
Never add extra weight at the rear of a caravan to reduce tow ball mass. Instead, redistribute weight inside the trailer as centrally as possible until the correct tow ball mass is achieved. If feasible, locate the spare wheel beneath the caravan (or even in/on the tow vehicle).
A few caravan makers have (recently) relied on front-located water tanks to be full whilst towing to obtain that required mass! This is potentially so dangerous that most have been recalled and the tanks re-located. (Recently published photos of rolled-over caravans show that many have front located water tanks).
Studies have shown that, for stable towing, a caravan's correct i.e (centralised) weight distribution is more important than their laden weight..
Recommended tow ball weights are only valid if the caravan is correctly loaded.
Tow Ball Weight - the right percentage
Whilst not realised by all caravan owners, it cannot be over-emphasised that the minimum tow ball mass is related to safe towing speed. Further, that despite towing speed limits (where applicable) may be 110 km/h, it is far safer to limit it to 100 km/h.
For a typical, medium-sized ( 6 to 7 metres long) Australian built caravan, tow ball weight should be about 10% of the trailer's laden weight if it is to travel at 100 km/h.
For a generally lighter UK or European built caravan, tow ball weight should be around 6% to 7% of the trailer's laden weight - not least as the towing speed limit is mostly 80 km/h. .
For short camper trailers, tow ball weight can be as low as 5%.
It is only legally possible to recommend following the tow ball weight advised by your trailer manufacturer, but first make sure that is within the maximum tow ball weight allowance of the tow ball/tow vehicle manufacturer.
How to measure tow ball weight
It is vital to know your tow ball weight for various caravan loading. This is particularly necessary for caravans that have water tanks in front of their axles.
Tow ball scales are far from expensive (around A$75) and are safer to use due to their moulded tow ball shape at the top and solid base that ensures the 'van will not roll off. They typically measure tow ball weights up to 350 kg.
If the required tow ball weight is under 185 kg it is usually possible to measure it by having a length of timber under the trailer hitch (such that the caravan stays level) on top of a set of bathroom scales. Raise the jockey wheel slowly as weight on the timber and bathroom scales increases, but keep the jockey wheel just clear of the ground in case the timber collapses. (Do not measure weight via the jockey wheel as this will give a false reading).
But as having the right tow ball weight it vital why not do the job properly and buy those scales. Surely your safety is worth spending the price of two bottles of half-decent wine!
For a full explanation of why having the right tow ball weight matters so much buy my book 'What Caravans Rollover - and how to prevent it' via the Bookshop on this website.
Tow Ball Weight - when to measure it
When you first buy your caravan or trailer, measure its tow ball weight for a range of situations. This might be a light configuration (for weekends away), a medium configuration (for weeks away) and a heavy configuration (for months away). To see the effect on tow ball weight by moving items within the trailer Click here.
Also check tow ball weight with water tanks empty and full. See above also re caravans with front-located water tanks. That (in RV Books' opinion) dangerously limits the required tow ball weight to being only when the water tanks are full.
Once you become familiar with the correct loading patterns for your caravan, continue to measure tow ball weight periodically to make sure no loads have been shifted and overlooked. Keep a set of tow ball scales handy and use them often, especially when you take on new or unusual loads such as bicycles or other new accessories.
If/when discussing the above with anyone who knows physics, you will score Brownie Points by referring to tow ball weight as tow ball mass.
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