by Collyn Rivers
Caravan Tow Ball Weight
For safe towing, a caravan must be tow ball heavy. Unless a caravan is tow ball heavy, it is potentially unstable. It is likely to sway. If swaying escalates, the caravan and tow vehicle may jack-knife. This article shows why having adequate caravan tow ball weight really does matter. It shows to assess the caravan tow ball weight your rig must-have. The required tow ball weight is related to the length and weight of a caravan. The longer and heavier it is, the higher the tow ball weight required.
The wheelbarrow effect
A caravan towing vehicle’s tow hitch is typically 1.25 metres or more behind its rear axle. The caravan‘s (essential) tow ball weight pushes down on that hitch. The effect like pushing down the handles of a wheelbarrow. It levers up its front. This reduces the weight on the tow vehicle’s front tyres. This, in turn, degrades the tow vehicle’s cornering ability. It also causes the caravan to sway in a ‘snaking-like’ manner.
Tow ball weight affects safe towing speed
The tendency for a caravan to sway is to the tow ball weight relative to the weight and length of the tow vehicle. The lower that relative weight, the lower the speed at which swaying is likely to occur.
A typical tow vehicle and caravan with 2% tow ball weight has almost no stability. It may sway at 25 km/h (15.5 mph). One with 4% tow ball weight may sway at about 50 km/h. The tow ball weight traditionally suggested (in Australia) is about 10%. In the USA it is 12-14% or more.
Use tow ball scales to check a range of loading configurations
While tow ball weight is essential, vehicle makers continue to decrease that allowed. In 2015, many reduced it from 350 kg (770 lb) to 250 kg (550 lb). The caravan industry ‘rethought its position’. It virtually overnight advised that lower tow ball weight, but few vendors indicated how to do this safely.
While better done at the design stage, you can reduce that weight by relocating and centralising heavy items (such as batteries) as close as possible over the axles. Doing so increases stability due to reducing yaw, or retains the existing stability at a lower tow ball weight. Recommended tow ball weights are only valid if the caravan is correctly loaded fore/aft of the axle/s. Never have anything heavy at the front, nor (especially) at the rear of the caravan.
Tow Ball Weight and Safe Speed
Tow ball weight is related to towing speed. The higher the speed, the greater the tow ball weight required. The lower that tow ball weight, the lower the safe speed. Be aware that yaw (snaking) forces increase with the square of the speed: i.e. they are four times higher at 100 km/h than at 50 km/h. Because of this, RV Books advises owners of typical Australian-made caravans never to exceed 100 km/h unless nose weight is at least 10%. Even then, it is advisable not to exceed 100 km/h. A disproportionate number of caravan roll-overs occur when overtaking a truck at speed.
Friction Sway Devices
No friction sway device is remotely effective when most needed (at high speeds). This is because frictional forces stay constant. The forces they need to control, however, increase with the square of the road speed. At 100 km/h the frictional effect of typical friction sway device is about 1%.
The Effect of Tow Ball Weight on the Tow Vehicle
Both vehicle manufacturer and tow hitch manufacturer set a maximum tow ball weight. The law requires you use whichever weight is the least.
As noted above, until 2015 most makers of vehicles used for towing had a legal maximum tow ball weight of 350 kg (770 lb). But in an attempt to reduce emissions (via reducing vehicle weight), almost every maker of such vehicles reduced chassis member thickness (from 3.5 mm to 3 mm). The vehicle makers reduced permitted tow ball weight accordingly but retained the previous typically 3500 kg (7715 lb) towing capacity.
Despite previously recommending 10% tow ball weight, many local caravan makers made no changes. They simply reduced that recommended tow ball weight almost overnight.
The term ‘towing capacity’ is rarely correctly understood by caravan buyers. It is defined, in SAE J2807, as a vehicle’s ability to safely tow by measuring braking distances, acceleration times, passing ability, grade-climbing ability, and physical load-carrying capability. Also specified is that the laden trailer does not change the cornering characteristics of the tow vehicle.
That often overlooked by caravan owners (and some vendors) is that only a very small proportion of vehicles used for towing pull caravans. By far the majority are used by military and tradespeople. They are typically about four metres and have a laden weight from 750 kg (1650 lb) to 2000 kg (4400 lb). They do not need high-tow ball weight.
Safe high-speed caravan towing stability, however, is substantially dependent on adequate tow ball weight. That long-accepted for typical Australian caravans of 10%, is still desirable, but decreasingly feasible as vehicle makers reduce overall weight.
Tow ball weight must be constant
In recent years, in order to achieve the desired tow- ball weight, a few caravan makers relied on front-located water tanks to be full while towing. Their change from full to empty could be as high as 200 kg (440 lb). This proved so potentially dangerous that most have been recalled for the tanks to be re-located.
Recently published photos of rolled-over caravans show that many had front-located water tanks. That incident can only cause wonderment of the caravan makers concerned seeming lack of even the most basic physics.
Weight Distributing Hitches (WDHs)
High tow ball weight reduces tow vehicle’s front axle loads. Using a Weight Distributing Hitch (WDH) to ‘remedy’ this can introduce undesirable tow vehicle stability effects. See the associated article weight-distributing-hitch limits cornering/
Tow Ball Weight – the right percentage
While not realised by all caravan owners, it cannot be over-emphasised that tow ball weight is directly related to safe towing speed. Further, despite towing speed limits (where applicable) maybe 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 caravans, the tow ball weight should ideally be about 10% of the trailer’s laden weight. In RV Books it is worrying that many local caravan makers are (with no apparent design changes) now advising tow ball weight as low as 4%.
UK and European built caravans are typically 35-40% lighter per metre than most Australian made products. Their tow ball weight can be 6% to 7% of the trailer’s laden weight.
The minimum tow ball weight of any correctly laden caravan is also related to its length. The shorter it is, the lower the required weight. For a typical camper trailer, tow ball weight can be as low as 6%.
It is legally necessary to recommend the tow ball weight advised by your trailer manufacturer. Make sure is within the maximum tow ball weight allowance of the tow ball/tow vehicle manufacturer.
How and When to Measure Tow Ball Weight
It is vital to know your tow ball weight for various caravan loading.
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 stable base. These scales typically measure tow ball weights up to 350 kg (770 lb).
Measure tow ball weight periodically to make sure no loads have been shifted or added and overlooked.
All you need to know about a tow vehicle and caravan stability is in our top-selling book Why Caravans Roll Over – and how to prevent it.