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. In extreme circumstances that sway escalates and the caravan and tow vehicle jack-knifes. This article shows the caravan tow ball weight your rig needs. And why you need to have adequate caravan tow ball weight truly matters.
Tow ball weight affects the safe towing speed
There is a proven relationship between caravan tow ball weight and the speed at which such action occurs. The lower the tow ball weight, the lower the road speed at which a rig may attempt to change ends – i.e. to jack-knife. While extreme examples, tow vehicle and caravan with 2% tow ball weight has no inherent stability. One with 4% tow ball weight is likely to become unstable at about 50 km/h.
Use tow ball scales to check a range of loading configurations
How Much Tow Ball Weight Is Needed
Tow ball weight pushes down the tow vehicle’s tow hitch. That hitch is typically 1.25 metres or more behind its rear axle. The effect of tow ball weight is thus like pushing down on the handles of a wheelbarrow. It levers up its front.
Tow ball weight does the same to a tow vehicle. By pushing down the tow vehicle’s rear, that tow ball weight reduces the weight on its front wheels, and that degrades the vehicle’s cornering ability. The desirable tow ball weight is about 10% of a typical caravan’s fully laden weight.
The required tow ball weight is also related to the length of a caravan. The longer it is, the higher the tow ball weight required.
While tow ball weight is essential, it must be a compromise. You need the caravan’s axle/s to be further back than most, but that is only possible at the design stage. You can, however, reduce that weight by centralising the caravan’s weight over its axle/s. Move existing heavy items (such as batteries) as close as possible to (over the axles). Doing so increases stability due to reducing yaw, or retains the existing stability at a lower tow ball weight. Be aware that recommended tow ball weights are only valid if the caravan is correctly loaded. Never carry anything heavy at the front or rear of the caravan.
Tow Ball Weight Safe Speed
Tow ball weight is related to towing speed. 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 weight required. The lower that tow ball weight, the lower the safe speed.
Because of this, RV Books advises owners of typical Australian-made caravans never to exceed 100 km/h unless nose weight is 10%. Even then, it is advisable not to exceed 100 km/h.
Friction Sway Devices
Be aware that any friction sway device has next to no effect when most needed (at high speeds). The frictional forces stay constant – but the forces they need to control increase with the square of the speed. At 100 km/h the frictional effect is only 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.
Until 2015 most makers of vehicles used for towing had a legal maximum tow ball weight 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 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 towing capacity.
Despite previously recommending 10% tow ball weight, many caravan makers made no changes. They reduced that recommended tow ball weight almost overnight.
A relatively small proportion of vehicles, however, tow caravans. The military and tradespeople are the vast majority. The trailers they use are short. They do not need high tow ball weight.
That 3500 kg (or whatever) towing capacity claimed, is in effect, what the tow vehicle can pull at the end of a rope. There are also many requirements. These include the ability to stop and restart on a typically 14% gradient, and to be able to pull the fully laden trailer up a specified gradient at a specified speed.
Also specified is that the laden trailer does not change the cornering characteristics of the tow vehicle.
That not realised by many caravan owners is that towing capacity does not relate to supporting a laden caravan’s typical and desirable 8%-10% nose weight. The vehicle maker rarely reveals this issue.
Safe high-speed caravan towing stability is substantially dependent on adequate tow ball weight. The long-accepted (for typical Australian caravans) of 10% is still desirable, but decreasingly feasible as vehicle makers reduce overall weight.
A few caravan makers, recently relied on front-located water tanks to be full while towing to obtain that required weight! The change in tow weight can be over 100 kg. Such practice is so potentially dangerous that most have had re-located. Recently published photos of rolled-over caravans show that many have these front located water tanks.
Weight Distributing Hitches (WDHs)
High tow ball weight reduces the tow-vehicle’s front axle loads. Using a Weight Distributing Hitch (WDH) can introduce undesirable tow vehicle stability effects. See the associated article re this on this website.
Tow Ball Weight – the right percentage
While not realised by all caravan owners, it cannot be over-emphasised that the minimum tow ball weight is 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 be about 10% of the trailer’s laden weight.
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.
Measure tow ball weight periodically to make sure no loads have been shifted or added and overlooked.
A full explanation of the adverse effects of weight distributing hitches in our article Weight Distributing Hitches Affect Cornering.
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.