TOWBALL WEIGHT AND TRAILER STABILITY
Science outweighs the rule of thumb.
Why do the Aussies and Yanks insist on 10-15-percent towball weight, while the Europeans are content with around six percent? Towing speed is the main reason for the difference and that makes the Australian practice something of a misfit.
The purpose of this percentage is manifold: guaranteeing that the trailer won’t lift the rear wheels of the towing vehicle under acceleration or when climbing a steep grade; ensuring that the trailer tracks accurately behind the towing vehicle; and preventing trailer sway.
In North America virtually all trailer and caravan makers endorse the 10-percent rule and some go so far as to suggest the towball load should be as high as 15 percent. In the case of a 2000kg trailer that means a towball load of 200kg to 300kg.
In Europe it’s an entirely different story: the typical EEC car and 4WD towball load is between 50kg and 75kg, and even heavy trailers – above 3500kg – have towball loads around 100kg.
Why the difference, given that trailers and towing vehicles are pretty much global vehicles these days?
The North American scene
Published research on the ball weight topic is very hard to find, but we’ve managed to dig up some US calculations and real-world testing done in 2008 by Delphi and published by the SAE. These findings explain why the Yanks get lower trailer-towing ratings for globally available vehicles and why they insist on 10-15-percent ball weight.
If you buy a Toyota or Subaru, for example, in the USA you’ll find that the trailer rating is much lower than it is in Europe or in Australia – around half – and the reason is mostly to do with road speed.
Towing speed limits differ across the USA, but vehicle and trailer designers must assume the highest legal speed is their target and that is between 65mph and 80mph (105km/h and 130km/h). That's faster than trailer towing in any other country, which is why American-market towing vehicles have conservative towing ratings.
Also, as the Delphi study showed, heavier towball weights are necessary at 130km/h and above.
The Delphi study looked at variables, including the centre of mass, axle placement and vehicle speed, and calculated if the trailer was likely to sway. The predictions also noted and at what speed it was a decaying oscillation (taking care of itself) or an exponential oscillation (building to an accident).
Firstly, they assessed the stability of the tow vehicle, described by a dampening ratio, which when positive meant stability (decaying oscillations) and a negative damping ratio meant the opposite.
Up to 100mph (160km/h) and beyond, the tow vehicle had a damping ratio ranging from 1-0.5 – quite stable.
With a trailer in tow the tow vehicle’s damping ratio actually improved, but the trailer showed a damping ratio that ranged from 0.3 at 37mph (60km/h) and crossed into the negative at 71mph (112km/h).
This mathematical model was followed by a real word confirmation study with an adjustable trailer that tested three different configurations at different speeds.
In the first configuration the centre of mass was forward of the axles and the tongue weight was 10 percent. In the second configuration the centre of mass was moved rearwards, but still in front of the axles with the tongue weight at three percent. In the final configuration they put the centre of mass behind the axle line, giving a negative 10-percent ball weight.
In the first example, the truck and trailer remained stable up to and beyond 100mph (160km/h).
In the second example the truck and trailer crossed the stability threshold at 65mph (105km/h).
In the third example the truck and trailer crossed the stability threshold at 45mph (72km/h).
Note that the second example, with only three-percent towball weight was stable up to 105km/h.
Australia - USA towball weight at EEC speeds
Australia has largely adopted the North American model. Most trailer and caravan makers endorse the 10 percent rule, regardless of the trailer configuration and the number of axles.
Many car and 4WD makers hate the 10-percent rule, because it forces them to design heavier rear sections and stiffer rear suspensions than they need in most markets.
Some makers, like Subaru, limit the permissible towball load to a set figure (90kg for most models) and insist that if a Subaru is coupled to a trailer or caravan with a heavier towball load the owner must fit weight-distribution bars, to pull the towball weight back to 90kg.
We’ve towed a variety of caravans and camper trailers behind Subaru Outbacks, with towball weights less than 90kg and no weight-distribution bars and have had no sway issues at .
Why have a heavy towball weight in the first place, many people ask?
The marketplace is full of theories, but very light on actual test results. As the above US research shows, a heavy towball load reduces the chance of
trailer sway at very high towing speeds.
However, EEC trailer regulations mandate lighter towball loads, because European caravans mix it with high-speed traffic on multi-lane roads they’re restricted to 100km/h and 80km/h on secondary roads.
European and Australian light and heavy truck ‘pig’ trailers – drawbar trailers with centrally positioned tandem or tri-axle bogies – that are restricted
to 100km/h have very little or no towball weight and don’t have significant sway problems.
Ongoing vehicle manufacturers’ analysis and Society of Automotive Engineers’ testing has shown the factors that influence trailer sway include driver skill, speed, vehicle and trailer weight, vehicle and trailer load centres of gravity, number of trailer axles, aerodynamics, heavy-vehicle air turbulence, weather conditions, road surface undulations, wheel bearing condition, trailer brake adjustment, towball to coupling clearances and friction, tyre pressures and the suspension dynamics of towing vehicle and trailer.
The prospect of negating all these factors by simply upping the towball weight is unlikely.
Interestingly, on the subject of driver skill, the EEC trailer towing regulations limit car-licence drivers to 750kg trailers, with or without brakes. To tow a trailer weighing 750kg up to 1750kg requires two days of driver training and an upgraded licence test.
Towing trailers that weigh more than 1750kg requires a truck licence. Can you imagine our weak-kneed pollies implementing such ‘repressive’ legislation?
European police regularly spot-check trailers for roadworthiness, gross mass violations and excess towball loading. All cars or 4WDs towing trailers above 80km/h, up to a maximum of 100km/h in Germany, must have a friction-type coupling to reduce the likelihood of trailer sway.
Additionally, North American and European vehicle makers are increasingly turning to electronic stability control (ESC) to counter trailer instability. Bosch, AL-KO and Dexter have introduced ESC systems Down Under.
As with solo-vehicle ESC the system applies selective wheel braking to towing vehicle and trailer, to prevent a ‘pendulum’ effect developing. Trailer ESC is activated when the trailer plug is connected to the towing vehicle.
We’ve driven a 42-tonne EEC-spec’ prime mover and trailer combination fitted with ESC on all axles and found the stability improvement almost unbelievable, but European truck makers stress the point that ESC won’t make up for a poorly loaded or badly driven combination.
European towball weight research
Until 2009 there was virtually nothing published in Europe on the topic of real-world, light vehicle and trailer towing stability, although it’s known that some vehicle makers have done considerable R&D in this area. In contrast, heavy truck and trailer R&D is well documented and all prime movers and semi-trailers sold in Europe can be ordered with stability control.
A paper entitled “An experimental investigation of car-trailer high-speed stability” was published in mid-2009 by the Department of Mechanical Engineering at Bath University in the UK.
The paper, by J Darling, D Tilley and B Gao, summarises the findings of tests carried out on a standard UK-built caravan and on an adjustable trailer, in which different dimensional and mass factors could be evaluated.
The tests began with matching the adjustable trailer so that it replicated the dynamic behaviour of the caravan, then altering one dimension change and one mass change at a time, to evaluate the results of the changes. More than 600 different trailer parameters were examined.
In summary, the engineers discovered that the three most significant parameters affecting trailer stability were trailer yaw inertia, nose mass and trailer axle position. Interestingly, the total weight of the trailer wasn’t a stability issue of itself, but weight distribution was critical.
Weight distribution in trailer design and loading
The researchers concluded that the best way to minimise trailer yaw inertia – the tendency for the trailer to sway laterally - was to position any trailer load at or near the centre of gravity. Loads fore and aft of that position increased the likelihood of towing instability.
The optimum nose mass (ball load) was found to be 6-8 percent of the trailer’s gross mass. This is quite different from the common ‘rule of thumb’ relied on in Australia and the USA, where ball loads of 10-15 percent are common.
Provided the measurement didn’t increase the ball weight beyond eight percent of trailer gross mass the greater the distance between the coupling and the axle, the more stable the trailer was in test manoeuvres.
The researchers evaluated car ESC, by performing stability manoeuvres with ESC alternately switched on and off. The trailers did not have TSC, yet even without this program, car-only ESC produced more stable behaviour than did the non-ESC tests.
Checking your trailer's ball weight
There are two professional ways to check trailer ball weight: split-weighing at a public weighbridge and using a ball-weight scale. Fiddling around with
a vehicle and trailer at a weighbridge takes time and split-weighing can be tricky at busy weighbridges, where truckies are anxious to get a printout
of their axle weights and get on the road.
Using a ball-weight scale means setting up the trailer in its loaded state, on a level surface and using the scale to measure the ball weight as load is wound off the jockey wheel. One such compact device - the Towsafe ball weight scale - is available from Repco and caravan accessory outlets for less than $90 - a low price for safety and peace of mind, in our view. An adaptor allows this scale to measure ball weights of trailers with any type of coupling.
The optimum device for measuring towball weight is the Reich Caravan Weight Control device. This European-designed and made scale measures axle weights as well, giving a total picture of tow vehicle and trailer.
The Reich CWC comes in two models, with wheel capacities of 1000kg (yellow finish) and 1500kg (orange finish), respectively. Being European, its ball weight calibration originally toppped out at the EEC maximum of 100kg, but the CWC has now been recalibrated for Australian conditions and can register ball weights up to 455kg.
The scale measures and records each wheel weight in succession as the trailer is driven onto the scale. Towball weight is checked by placing the scale
under the raised jockey wheel and then lowering the wheel onto it. When all axle weights and the jockey wheel weight are recorded the CWC displays
the individual weights and the total.
However, some people won't invest in a ball-weight scale, so a set of bathroom scales can suffice, in conjunction with a sturdy piece of wood or steel tube between the scale and the ball receiver. It's necessary to use a load-spreading pad of wood on top of the scale, so you don't damage its upper surface.
Where ball weight is likely to exceed the capacity of the bathroom scale it's possible to use the 'ratio method' to measure ball weight. In this procedure a strong piece of wood, around 100mmx50mm thick, or square steel tube is used between the scale and a fulcrum that's the same height as the scale's upper surface. If the load-supporting piece of wood or steel tube is positioned exactly midway along this piece of wood or steel the reading on the scale will be half the actual ball weight. A piece of horizontally-laid dowel, thin square tube or pipe on top of the fulcrum and the scale allows accurate distance measuring.
If the half-ratio measurement would exceed the scale's capacity you can vary the position of the fulcrum and scale, so that the ratio alters from 2:1 to 3:1 or even 4:1. In the photo and diagram the ratio is 3:1, because the distance from the fulcrum to the ball is one third of the distance from the fulcrum to the scale.
Water tanks and ball weight
Australia's widely respected Caravan Council conducted a survey of its members and found that many respondents asked: "Just how much do full or empty
water tanks change ball loading?"
Many caravans and camper trailers have two water tanks. Ideally these are located close to the axle(s), with one in front and one behind, but some have both ahead of, or both behind, the axle(s).
Some have tanks positioned a long way ahead of, or behind, the axle(s) and this layout causes ball weight to vary appreciably, depending on whether each tank is empty or full. These changes to ball weight often result in handling and stability problems, and the weight can exceeding the ball-weight-rating of the towing vehicle or its towbar.
The Council has provided this shocking example of a badly designed van that has a massive unladen ball weight of 360kg. That's already dangerously and
unnecessarily high, but is made worse by the fact that both its water tanks are located ahead of the axles. When filled, they'll add significantly
to the ball weight, making this combination overloaded, illegal and lethal.
Below are the Caravan Council's simple drawings and formulae for calculating the increase or decrease in ball weight, depending on the contents of each water tank and the LPG cylinders, as well as a formula for calculating the effects of moving spare wheels.
Knowing the magnitude of this weight disribution effect enables caravan and camper trailer owners to load their trailers correctly, to achieve optimum
Formula for determining how much the ball weight varies, depending on whether the Gas and Water tanks are empty, or completely full. Note: dimensions from
the centre of the axle-group to the left are positive… dimensions to the right are negative.
When filled LPG and water tanks ahead of the axle(s) increase ball weight; tanks behind the axle(s) decrease ball weight.
The Actual Mass of the ‘van will increase by: G + W1 + W2 when the empty tanks are filled. One litre of water weighs one kilogram.
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 = 18 kg; W1 = 45 kg; W2 = 90 kg; LG = 3.0 m; LW1 = 1.5 m; LW2 = 1.5 m; LC = 3.5 m
Change in ball weight = ( (18 x 3) + (90 x 1.5) - (45 x 1.5) ) / 3.5
Change in ball weight = ( (54) + (135) - (67) ) / 3.5
Change in ball weight = ( 122 ) / 3.5
Change in ball weight = + 35 kg
Worst Case 1: W1 empty; G & W2 full
Change in ball weight = + 54 kg
Worst Case 2: W1 full; G & W2 empty
Change in ball weight = - 19 kg
Caravans and camper trailers should be designed so that there is the least possible change to ball weight when tanks are full or empty. Multiple water tanks should be positioned as close as possible to, the axle(s), whether in front or to the rear.
The formula can also be used to calculate the change in ball weight when an appliance, or heavy article, is installed.
Spare wheels and ball weight
Formula for determining how much the ball-weight changes, when the spare wheel(s) is moved from the A-frame to the rear of the ‘van. The mass of the ‘van will not change… but the ball weight certainly will.
Moving the spare wheel(s) from the A-frame to the rear of the ‘van will significantly reduce the ball weight.
More importantly, it will increase the stresses, deflections and vibrations of the rear portion of the chassis… possibly by a serious amount.
The spare wheel(s) will now be on the end of a cantilever… rather than between the end supports of a beam.
Calculate the “Moments” – Mass X Distance – around the centre of the axle (or midway between tandem axles):
Note: Dimensions from the centre of the axle-group to the left are positive… dimensions to the right are negative.
Example: T = 30 kg; LC = 3.5 m; LF = 3.0 m; LR = 2.0 m
Ball-weight currently contributed by the spare wheel(s) being on the A-frame:
Ball-weight = (T x LF) / LC
= (30 X 3.0) / 3.5 = + 25.7 kg
Ball-weight decrease by the spare wheel(s) being on the rear of the ‘van:
Ball-weight = (T x LR) / LC
= (30 X – 2.0) / 3.5 = – 17.1 kg
The overall effect on the ball-weight is a reduction of 48.2 kg
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