DIY: Mass mat­ters

Un­der­stand­ing towball mass and its im­pli­ca­tions for tow­ing.

Motorhome & Caravan Trader - - Contents -

Un­der­stand­ing towball mass and its im­pli­ca­tions for tow­ing

Asafe towball down­load weight has al­ways been con­sid­ered to be in the range of 10-15 per cent. De­spite this ‘rule’ orig­i­nat­ing from a dif­fer­ent era of car­a­vans (the 1970s) the 10-15 per cent rule has per­sisted.

While this ‘rule’ is com­mon­place, it is not en­shrined in law. The only le­gal­i­ties re­lat­ing to towball down­load con­cerns the max­i­mum al­low­able towball mass (TBM) as stated by the ve­hi­cle man­u­fac­turer or tow­bar man­u­fac­turer, whichever is the lower TBM (it’s rare these days for the two to dif­fer, but it’s worth check­ing).

What has been in­ter­est­ing is the change by car­a­van man­u­fac­tur­ers to in­clude TBM at Tare on com­pli­ance plates. This has high­lighted the rel­a­tively low weight many car­a­vans im­pose on the tow ve­hi­cle – at Tare weight, at least. So, for ex­am­ple, a 2000kg Tare car­a­van might only im­pose 150kg on the ball. That’s about 7.5 per cent of Tare on the ball. And some vans have even less TBM than that – closer to 6 per cent.

Low TBM has long been a fea­ture of vans built in the UK or Europe. Their car­a­van markets are dic­tated by the need to have low TBM to suit the preva­lent lightweight tow ve­hi­cles, with many vans hav­ing a TBM of 4-5 per cent of Tare mass. These vans, with­out ex­cep­tion, use a fric­tion cou­pling to mit­i­gate sway, which is more likely with such a light per­cent­age of weight im­posed on the towball.

Tow­ing many Aussie vans with 6-8 per cent TBM for tow­ing tests or re­views, we’ve found that most tow per­fectly. The ve­hi­cle sits flat and there is no hint of trailer sway or ve­hi­cle yaw­ing, even af­ter long pe­ri­ods of tow­ing on a va­ri­ety of road sur­faces and con­di­tions.

What is a game-changer, of course, is what hap­pens when the car­a­van is loaded. Fill the front boot with gear, load up the water tanks (typ­i­cally mounted ahead of the axles) and you can have a much higher weight on the towball. In­stead of guess­ing what the TBM is with a pay­load, you should al­ways ver­ify it by weigh­ing with a TBM scale or at a pub­lic weigh­bridge.

With a much higher ball load than at Tare, you might find the dy­nam­ics of the rig have changed sig­nif­i­cantly. While proper pay­load bal­ance is vi­tal, this may still not avoid a high ball load sit­u­a­tion. A weight dis­tri­bu­tion hitch (WDH) may be in order.


A WDH is a great in­ven­tion to help dis­trib­ute weight in a ve­hi­cle and trailer setup. While they can be of great

ben­e­fit, it is worth in­ves­ti­gat­ing whether they are right for you and, if so, to un­der­stand the ba­sics of a proper setup.

WDHs are sim­ply two spring bars at­tached to each side of the trailer A-frame at one end and the tow hitch at the other. In the most pop­u­lar de­sign, there is a cou­ple of brack­ets to se­cure the bars to the A-frame, chain links at­tached to the bars to ad­just ten­sion and a spe­cial tow­bar hitch with holes to at­tach the end of the bars to the ve­hi­cle. So long as you have a tow­bar of the hitchre­ceiver de­sign that is strong enough to be used with a WDH (some are not), all you need is the WDH tow hitch and bar kit.

The WDH spring bars are set up so they bend un­der ten­sion, forc­ing the weight push­ing down on the rear of the ve­hi­cle to in­stead spread out to the front wheels of the ve­hi­cle and to the wheels of the car­a­van.

It fol­lows that the weight of the rig is spread more evenly over the wheel sets and will re­store steer­ing re­sponse, as­sist brak­ing con­trol, level-out ride height and gen­er­ally im­prove sta­bil­ity. As a bonus, it can also add lat­eral fric­tion to the cou­pling to re­duce sway, although WDHs are not in­tended for this func­tion, as such.

When driv­ing with a WDH, there is not much you need to do, pro­vided you ad­here to the man­u­fac­turer’s in­struc­tions. That in­cludes re­leas­ing ten­sion from the spring bars when en­ter­ing drive­ways or travers­ing spoon drains. Not many peo­ple do. Stop­ping in the kerb­side lane of a three-lane high­way, hop­ping out to re­lease the spring bars, then driv­ing over the drive­way into a petrol sta­tion, is eas­ier said than done.

Many ve­hi­cle man­u­fac­tur­ers spec­ify that a WDH must be used when tow­ing, usu­ally when TBM ex­ceeds a cer­tain weight (although some, like Holden, tackle it from an­other per­spec­tive, say­ing that load lev­ellers must be used when the trailer weight is 750kg or more).

How­ever, in some cases, where such pre­scrip­tions don’t ap­ply, WDHs may not al­ways be nec­es­sary; if the TBM is not high and a man­u­fac­turer does not spec­ify their use, then per­haps you can do with­out. There is the ar­gu­ment that main­tain­ing steer­ing con­trol on the front

wheels and sta­bil­ity gen­er­ally doesn’t ac­tu­ally re­quire the ex­treme me­chan­i­cal loads ex­erted by WDHs un­til ball mass is very high (around 200kg).

The anti-WDH ar­gu­ment says that, pro­vided the ve­hi­cle ride height is not ex­ces­sively nose-up, that up to a point, it doesn’t mat­ter about the down­load mass ex­erted on the towball. Be­cause the TBM is act­ing like lever­age on the fur­ther­most point of the ve­hi­cle, the in­flu­ence on the front wheel set is min­i­mal up to a point. The ar­gu­ment is that you need a sig­nif­i­cant amount of down­load on the towball to cause enough lift to af­fect the steer­ing. If the ve­hi­cle’s body doesn’t droop more than 20mm at the rear or rise more than 20mm at the front, the sug­ges­tion is that a WDH is not nec­es­sary.

Ride height can be re­stored if re­quired with a self­lev­el­ling sus­pen­sion, such as stan­dard air springs or helper air springs, such as Polyairs.

There are some sit­u­a­tions where a WDH should not be used – if the ve­hi­cle man­u­fac­turer stip­u­lates that they are not used (or, like Land Rover, say they can­not rec­om­mend them) or if a light TBM van, such as a Euro­pean car­a­van, is be­ing towed.

In any case, be­fore buy­ing a new ve­hi­cle or van you should know what the man­u­fac­turer’s rec­om­men­da­tions are in re­spect of WDHs.


A WDH is no sub­sti­tute for good weight bal­ance ei­ther. If you are over­loaded or in­cor­rectly loaded, then a WDH is not go­ing to mag­i­cally cure the sit­u­a­tion. But if you are fac­ing a ve­hi­cle that points to the sky and have a van with a high ball load, then there is some re­lief at hand by way of a WDH.

There are a few dif­fer­ent va­ri­eties of WDHs, but the most com­mon is the an­gled-shank ball head and thick spring bars typ­i­fied with the Hay­man Reese units. For light-duty ap­pli­ca­tions (up to 110kg weight trans­fer) there is also the ‘shep­herd’s crook’ type.

Any WDH will need ad­e­quate A-frame length so the clamps can be fit­ted to the A-frame and the chains se­cured. Also, an in­board or cen­tre-mount jockey wheel makes fit­ting the spring bars much eas­ier. An out­board jockey wheel is not a deal breaker, but it can re­quire some deft moves to re­move or re­fit the jockey wheel to the clamp.

The Hay­man Reese type is avail­able in heavy-duty ver­sions of sin­gle and twin bars per side.

When you’ve fit­ted the height-ad­justable loadlev­eller tow hitch tongue and cou­pled up the car­a­van, you need to check the height ad­just­ment is cor­rect for the ve­hi­cle and van. De­cou­ple and ad­just as re­quired, then, be­fore you hitch-up the van again, mea­sure the dis­tance from the top of the wheel arch to the ground (as­sum­ing you have parked on a smooth, level sur­face).

Once hitched-up again, mea­sure how much body drop you have at the rear and how much the front has risen. Ideally, you need to ad­just the ten­sion of the lev­eller bars to re­store ride height to less than 20mm drop from orig­i­nal ride height.

Make sure you read all the in­struc­tions care­fully be­fore you fit the bars. Keep in mind that the bars store enor­mous ten­sion and if they re­lease too quickly they could do you some se­ri­ous dam­age. Do not leave your feet or any other part of your body un­der­neath the bar setup. Try not to re­lease or ten­sion the bars un­less the ve­hi­cle is on a level with the van and in a straight line, as the spring ten­sion may be much greater.

Clock­wise from top left:

Water tank lev­els and lo­ca­tion can af­fect ball weight; a cen­tremount jockey wheel and un­clut­tered A-frame beams make fit­ting a WDH eas­ier; know­ing your van’s towball mass is important so it’s best to ac­tu­ally mea­sure it; mea­sur­ing ve­hi­cle and van height be­fore set­ting up a WDH.

Above: Some ve­hi­cle mak­ers man­date the use of a WDH. Left: Man­u­fac­tur­ers stat­ing TBM on com­pli­ance plates clar­i­fies the ac­tual TBM at Tare.

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