The test of a good tow­bar

Motor Equipment News - - NEWS -

The fail­ure of a tow­bar can have cat­a­strophic con­se­quences on the road, should a trailer break free and plough into other ve­hi­cles or even pedes­tri­ans, yet most people rarely give it a thought when pur­chas­ing one for their car, SUV or ute.

With the avail­abil­ity of low-cost tow­bars that are of­ten not built to New Zealand stan­dards, or even sec­ond-hand tow­bars taken from an­other ve­hi­cle, the chances of such a fail­ure have been in­creas­ing in re­cent years.

New Zealand’s leading tow­bar man­u­fac­turer, Auck­land-based

Best Bars, has doc­u­mented ev­i­dence of poor tow­bars be­ing sold in New Zealand to un­sus­pect­ing ve­hi­cle own­ers.

“It is wor­ry­ing, as we have com­plained for years about the NZ Stan­dard that was sup­posed to have been adopted into leg­is­la­tion for tow­bar de­sign, man­u­fac­ture and fit­ment, but there has been no move­ment from law­mak­ers to pro­tect the pub­lic by mak­ing this manda­tory” says John Frear of Best Bars.

Re­search car­ried out by Best Bars some years ago showed ev­i­dence of at least a cou­ple of thou­sand sub­stan­dard tow­bars be­ing fit­ted to New Zealand ve­hi­cles each year and Mr Frear says the sit­u­a­tion has not changed.

“That’s many thou­sands more ve­hi­cles that are us­ing our roads with po­ten­tially dan­ger­ous tow­bars, since we car­ried out that re­search.”

Com­mon prob­lems as­so­ci­ated with poor tow­bars range from the use of thin­ner metals, in­ad­e­quate fas­ten­ings, in­cor­rect fit­ting and poor de­sign. So what does make a good tow­bar? John says it starts on the draw­ing board, or in the case of tow­bars made by Best Bars, on the com­puter screen, with in­for­ma­tion drawn from three key sources: on-ve­hi­cle anal­y­sis & mea­sure­ment; scanned 3D im­ages of ve­hi­cles; and ve­hi­cle CAD data.

Once the ve­hi­cle has been rated by the man­u­fac­turer for tow­ing and rel­e­vant data col­lected, de­sign con­cepts for the tow­bar are then val­i­dated via Fi­nite El­e­ment Anal­y­sis (FEA), a soft­ware tool that quickly and ac­cu­rately eval­u­ates the stresses and strains the tow­bar will be placed un­der when in ac­tual use to de­ter­mine if the de­sign will pass the reg­u­la­tory tests.

This doesn’t hap­pen overnight. Best Bars has a large team of ex­pe­ri­enced en­gi­neers, project man­agers and tech­ni­cians who de­velop and re­view ev­ery tow­bar de­sign at each stage of the process.

Once a con­cept de­sign is ap­proved, an en­gi­neer­ing pro­to­type is man­u­fac­tured and the tow­bar is then put to work on Best Bars’ test­ing fa­cil­i­ties, where it un­der­goes a static and/or dy­namic test.

The static test is gen­er­ally per­formed with the tow­bar mounted on the ve­hi­cle, with force be­ing ap­plied through the tow ball in var­i­ous di­rec­tions to de­ter­mine the strength, per­for­mance and in­ter­ac­tion with the ve­hi­cle un­der dif­fer­ent loads.

The dy­namic test is of­ten per­formed with the tow­bar mounted on a spe­cially de­signed rig, and then sub­jected to con­tin­u­ous load­ing/un­load­ing of forces for up to 2-mil­lion cy­cles. It du­pli­cates the tremen­dous forces ex­erted on a tow­bar, not just in a nor­mal ve­hi­cle life cy­cle, but for far longer than most people will ever use it.

All tow­bars must pass the pre­scribed NZ Stan­dard (NZS5467), or the legislated Aus­tralian Stan­dard (AS4177 and ADR62), along with any mo­tor com­pany spe­cific tests that have been re­quested over and above this. Once a tow­bar de­sign has been val­i­dated, tested and com­plied, then man­u­fac­ture can be­gin.


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