Four-wheel steer­ing

Car Mechanics (UK) - - Contents -

It can be a chal­lenge try­ing to en­sure cars han­dle more se­curely at high speeds with­out af­fect­ing their agility.

Chris Ran­dall looks at one of the so­lu­tions.

Fit­ting a car with four-wheel steer­ing isn’t a new idea. Daim­ler-benz de­signed just such a sys­tem for the Ger­man Forestry Ser­vice in the 1930s. Back then, the aim was to help ve­hi­cles ne­go­ti­ate cor­ners on tight moun­tain roads, but it would be a few decades be­fore other car-mak­ers took ad­van­tage of the ben­e­fits for nor­mal road use.

Ja­panese man­u­fac­tur­ers were among the first to em­brace the tech­nol­ogy. Honda tested its own sys­tem in 1981, us­ing the front ends of two Ac­cords welded to­gether. Launched to the pub­lic in 1987 on the Honda Pre­lude, the sys­tem was en­tirely me­chan­i­cal at first (elec­tric mo­tor op­er­a­tion didn’t ar­rive un­til 1991) and used a shaft from the front steer­ing rack to trans­mit steer­ing an­gle to a plan­e­tary gear­box mounted at the rear axle; steer­ing arms then moved the rear wheels by a few de­grees.

Other man­u­fac­tur­ers soon joined in. Nis­san fit­ted some mod­els with its HICAS and Su­per HICAS sys­tems, which used com­puter-con­trolled hy­draulic ac­tu­a­tors to move the rear wheels.

Four-wheel steer­ing then seemed to fall out of favour as car-mak­ers con­cen­trated on other tech­nolo­gies to im­prove the han­dling of their cars, but the last few years has seen some­thing of a resur­gence of in­ter­est and there are now a va­ri­ety of mod­els of­fered with such an ar­range­ment. For some man­u­fac­tur­ers it gives the op­por­tu­nity to bring sharper, more dy­namic han­dling to the mix. For ex­am­ple, some ver­sions of the Porsche 911 em­ploy elec­tro-me­chan­i­cal ac­tu­a­tors to gen­er­ate rear steer­ing an­gles, while Re­nault’s Megane RS prom­ises the ul­ti­mate in agility thanks to its 4CONTROL sys­tem. The lat­ter can even be op­er­ated in the car’s Race mode, where the switch from op­po­site to same di­rec­tion rear steer­ing hap­pens at 62mph rather than 37mph – per­fect for set­ting fast times on a race track!

It’s not just high-per­for­mance ve­hi­cles that are ben­e­fit­ing. As mod­ern cars get larger with each gen­er­a­tion, one of the chal­lenges fac­ing car­mak­ers is not only en­sur­ing safe han­dling at high speeds – dur­ing emer­gency lane changes for ex­am­ple – but also in­creas­ing ma­noeu­vra­bil­ity in con­gested ur­ban streets be­cause four-wheel steer­ing ef­fec­tively short­ens the wheel­base.

This is par­tic­u­larly rel­e­vant given the rise in pop­u­lar­ity of large SUVS. One ex­am­ple is the 2018 VW Touareg. The first VW to be fit­ted with a steer­ing rear axle, it claims to re­duce the turn­ing cir­cle from 12.19 to 11.19 me­tres – a big im­prove­ment. Where mod­ern sys­tems dif­fer from Honda’s 1980s coupé is that they are es­sen­tially gov­erned by road speed rather than the steer­ing an­gle ap­plied to the front wheels. In the case of the Touareg, trav­el­ling at up to 37kph can see the rear wheels turn in the op­po­site di­rec­tion to the fronts by as much as 5°, while above this speed they turn in the same di­rec­tion.

Un­sur­pris­ingly, such sys­tems have be­come in­creas­ing com­plex as they make use of the vast com­put­ing power that mod­ern cars of­fer. Take Audi’s All­wheel Steer­ing, for ex­am­ple. Fit­ted to mod­els in­clud­ing the A8 lux­ury sa­loon and Q7 SUV, the sys­tem is man­aged by the Elec­tronic Chas­sis Plat­form (ECP), which utilises a cen­tral con­trol unit to an­a­lyse and record var­i­ous pa­ram­e­ters in­clud­ing road speed, roll and pitch move­ments, and yaw an­gles. Al­go­rithms process in­for­ma­tion ev­ery 1 to 25 mil­lisec­onds, con­trol­ling four­wheel drive and adap­tive sus­pen­sion sys­tems as well the rear-wheel steer­ing that op­er­ates via an elec­tric spin­dle drive and two track rods; by com­bin­ing op­er­a­tion of front and rear steer­ing, it can vary the over­all steer­ing ra­tio be­tween 9.5:1 and 17:1.

The Dy­namic All-wheel Steer­ing fit­ted to the lat­est Audi A6 is even more im­pres­sive, com­bin­ing rear­wheel steer­ing with what Audi re­fer to as ‘ac­tive su­per-po­si­tion steer­ing’ at the front wheels. This ar­range­ment min­imises the in­di­rect­ness in the steer­ing caused as the rear wheels turn in the same di­rec­tion as the fronts, and also re­duces the amount of lock needed to ne­go­ti­ate any given cor­ner. It works by mo­men­tar­ily de­cou­pling the steer­ing col­umn from the front wheels, al­low­ing the con­trol­ling elec­tron­ics to turn them in­de­pen­dently at a sharper an­gle than that ap­plied by the driver at the steer­ing wheel. The re­sult, Audi says, “com­bines di­rect, sporty steer­ing re­sponse with un­shake­able sta­bil­ity.”

What­ever the re­sults, it’s cer­tainly im­pres­sive tech­nol­ogy and is an­other ex­am­ple of how mod­ern car-mak­ers are striv­ing to make their ve­hi­cles safer and more en­ter­tain­ing to drive.

Audi A8 five-link rear sus­pen­sion with AI ac­tive sus­pen­sion, Qu­at­tro sport dif­fer­en­tial and all-wheel steer­ing. AI ac­tive sus­pen­sion ac­tu­a­tor – rear right. AI ac­tive sus­pen­sion ac­tu­a­tor – rear left. 48-volt power elec­tron­ics.

The 2018 Volkswagen Touareg – the first VW to be fit­ted with a steer­able rear axle.

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