AIRBAG SUS­PEN­SION An al­ti­tude ad­just­ment for your 4wD

If camp­Ing lux­u­rIes are weIgh­Ing heav­Ily on your 4wD, per­haps rear aIr sprIngs are the so­lu­tIon.


There was a time when a car­a­van was a sim­ple ply­wood struc­ture with min­i­mal fea­tures, and a camper trailer was just a box trailer with a tent and bed on top. Both weighed very lit­tle and could eas­ily be towed around by even the mod­est ve­hi­cles of the day. Then we, the buy­ing pub­lic, be­gan to ask for en­suites and big fridges and hot wa­ter ser­vices and air­con­di­tion­ers and tele­vi­sions and all the other im­ped­i­menta of life which fills our homes to bursting. Those homes on wheels grew ac­cord­ingly.

Th­ese days, our car­a­vans and campers com­monly start at around a tonne-and-ahalf in weight – and fre­quently more – even when empty, and our tow ve­hi­cles have to be large 4WDs or sim­i­lar. Aside from the fuel con­sump­tion and aero­dy­namic loads they im­pose on our ve­hi­cles, it’s the sheer weight, even when they are stand­ing still, that makes them a ma­jor bur­den on those ve­hi­cles.

One of the more ob­vi­ous loads im­posed be­comes ap­par­ent as we lower the hitch on to the tow­bar. The cou­pling makes con­tact as we lower the draw­bar with the jockey wheel, caus­ing the back of the ve­hi­cle to sag a lit­tle. We keep wind­ing the jockey wheel up, but it stays firmly on the ground, caus­ing the back of the car to lower in­stead.

If this is the case for you, maybe you should read on, be­cause you could have some prob­lems to con­sider. All that weight

on the back of the tow ve­hi­cle has over­bur­dened the rear sus­pen­sion and caused it to sag. Push­ing down on the rear end not only low­ers that part of the ve­hi­cle, it causes the nose to rise, light­en­ing the weight on the front end. As this is where the steer­ing takes place and most of the ve­hi­cle’s brak­ing is done, this is not a de­sir­able out­come.

Re­duced front wheel con­tact will am­plify any sway in­duced by the towed mass, in­creas­ing the chance of a jack-knife sit­u­a­tion on the high­way. At night, your head­lights will be il­lu­mi­nat­ing tree tops, and you’ll be the most hated driver by ev­ery­one com­ing the other way.

And if your ve­hi­cle has an in­de­pen­dent rear end, like my Mit­subishi Pa­jero, then you’re go­ing to no­tice a change in the cam­ber of the rear wheels. That in­de­pen­dent rear end helps give the Paj great ride and han­dling char­ac­ter­is­tics on the high­way, but now all that weight is caus­ing the sus­pen­sion arms to ro­tate up­wards and forc­ing the wheels to roll on the in­ner edges of the tyres. This is called neg­a­tive cam­ber and will be scrub­bing the life out of those tyres.

So what, if any, is the so­lu­tion? One rel­a­tively easy an­swer is rear sus­pen­sion airbags. There are a num­ber of brands on the mar­ket but we went to see Polyair Springs, in the in­ner Sydney sub­urb of Sy­den­ham, and with out­lets right across Aus­tralia. Polyair has sold springs in

Aus­tralia for 35 years and has been on the mar­ket in the USA since 1949, so if it hasn’t got it right, no­body has.

Polyair pro­duces a range of airbag solutions for as­sist­ing the rear sus­pen­sions of ve­hi­cles, whether they be coil or leaf springs. Its Red Se­ries of airbags of­fers up to 450kg of load sup­port and is the an­swer for the av­er­age owner of a car­a­van or camper.

The kits come with the bags (prop­erly re­ferred to as air springs), installation valves, air­line, all fit­tings and mount­ing hard­ware and de­tailed installation in­struc­tions. If you are handy and used to work­ing on and un­der your ve­hi­cle, they can be in­stalled at home, but we fig­ured th­ese were too im­por­tant and chose to have Polyair in­stall them for us. With coil-sprung set­ups, the springs must be re­moved for installation so it be­comes a big­ger job than with leaf springs, although if there are bump stops, such as with a Land­Cruiser, then even that has to be re­moved.

Airbags are not de­signed to lift an al­ready sagged over­loaded sus­pen­sion. The idea is to pump up the pres­sure into the bags first, then ap­ply the load to pre­vent it sag­ging. Uniquely, the airbags don’t work by push­ing in a ver­ti­cal plane, but rather ap­ply their pres­sure out­wards, in all di­rec­tions, against the coils to limit their move­ment.

Once in­stalled and be­fore us­ing them for the first time, check the height from the rear wheel cen­tre to the top of the guard above then pump the bags up to 35psi be­fore hitch­ing up your van or camper. Start re­duc­ing the pres­sure, a lit­tle at a time, un­til you start to see the sus­pen­sion drop, mea­sured by the dis­tance be­tween the rear wheel cen­tre and the guard. That is the pres­sure you’ll need for your fu­ture tow­ing of that van and that setup.

With an in­de­pen­dent rear end, 35lb (15.9kg) is the max­i­mum rec­om­mended pres­sure for the airbags. The op­ti­mal weight of a live rear axle will be 5-10lb (2.6-4.5kg) lower be­cause of the more ef­fi­cient de­sign of the springs, which are mounted fur­ther out from the diff cen­tre, and be­cause of the big­ger springs. How­ever, the counter is that such set­ups can take a higher max­i­mum pres­sure (up to 45psi) and, there­fore, as­sist with greater weight.

On a long trip, mon­i­tor the airbag pres­sures at the end of each day as part of your post-drive checks. Heat from use can raise the bag pres­sure, so you may need to lower the pres­sure for the be­gin­ning of the day’s drive to al­low for this and pre­vent over-

in­fla­tion, es­pe­cially over cor­ru­ga­tions dur­ing high am­bi­ent tem­per­a­tures.

Dur­ing those times when you’re not tow­ing, you should keep a min­i­mum of 5-10lb (2.6-4.5kg) of pres­sure in the bags. At this level, you will not no­tice them there and your ve­hi­cle will still be com­fort­able to drive on the road, and as such, the bags will be re­duc­ing body roll, which is al­ways an is­sue with 4WDs.

If you have one of those 4WDs with a lot of add-ons, such as roof tray, spare wheel car­rier, tow bar, rear draw­ers or sec­ond fridge, then maybe a min­i­mum set­ting of 10lb (4.5kg) would be more suit­able.

When you’re chang­ing a tyre or re­mov­ing a wheel, en­sure the bags stay in the coil springs. This ought to hap­pen if you’re driv­ing of­froad as the dust tends to hold the bag in a rea­son­able shape within the coil. If you’ve been driv­ing around town you can help things out by putting 20psi pres­sure in the bags to hold them in place.

Keep in mind that airbags are not de­signed to re­place a good sus­pen­sion. They are in­tended to com­pen­sate for ball loads of 150-200kg, not for all those huge ex­tra loads of all those fa­mil­iar of­froad ex­tras plus a 3t van on top of that. If that’s your sit­u­a­tion, talk to someone about your en­tire sus­pen­sion pack­age.

How long will airbags last? Polyair claims to know of a pair of airbags that have been on the

road for 19 years and have cov­ered close to 800,000km, so its pos­si­ble they could out­last your car. All Polyair kits come with a two-year un­lim­ited war­ranty, and if you ever split a bag along the seam, no mat­ter how old it is, Polyair will re­place it.

The cost? We spent $345 on the ba­sic kit, $220 to have them fit­ted at Polyair’s de­pot and $132 for the rec­om­mended wheel align­ment. Some in­stal­la­tions will be cheaper, as there is no wheel align­ment re­quired for live axle in­stalls, and some ve­hi­cles have slightly cheaper kits.


When we had our Pa­jero fit­ted up we were go­ing sim­ply on rec­om­men­da­tion, so were keen to see just what we had ac­quired and how well it would work.

When empty, the Pa­jero had a dis­tance of 500mm from the wheel cen­tre to the guard above on the left and 495mm on the right, in­di­cat­ing a slightly sagged sus­pen­sion on that side, even af­ter only three years of use.

With the Polyairs in­stalled and in­flated to 35psi the ride height had in­creased to 511mm and 508mm re­spec­tively.

With­out the Polyair as­sis­tance, our Vista Cross­over XL, which has a Tare ball weight of 130kg, pushed the rear end down to 477mm and 475mm on the left and right, but at 35psi that went up to 490mm and 486mm. By care­ful ad­just­ment, we were able to drop that to 26psi be­fore the rear started to sag again. In prac­tice, I’d prob­a­bly run it at around 30psi, as I would nor­mally also be car­ry­ing camp­ing gear in the back of the ve­hi­cle that would add to the rear sus­pen­sion load.

Those fig­ures may not seem much, but when they were ac­com­pa­nied by an equiv­a­lent low­er­ing of the front end, the at­ti­tude of the ve­hi­cle changed and han­dling and brak­ing im­proved.

All around, it was an ex­er­cise that was well worth the money.

TOP: The Pa­jero was ripe for an up­grade. ABOVE: Polyair Red Se­ries airbags pro­vide up to 450kg load sup­port.

top: David brushes up on the finer points of airbag own­er­ship. above: If you’re not com­fort­able fit­ting the bags your­self, it’s worth paying for pro­fes­sional installation.

CloCk­wise from top left: The wheels were aligned af­ter the installation; The in­staller re­moves the wheels; Then checks the length of the air­line; And, fi­nally, pre­pares it to en­sure easy ac­cess to the valve.

CloCk­wise from top left: The in­staller starts to fit the air­line; The in­stall took a cou­ple of hours; The air bags work by push­ing against the coils; A base pro­tects the bags on con­tact; Polyair re­moved the coils to fit the air bags; The out­let valves for in­flat­ing the air bags are fit­ted next to each other.

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