Ac­tive rear steer­ing may be the 991’s party trick, but Porsche has been try­ing to con­trol rear toe for decades. To­tal 911 finds out how…

Total 911 - - Living The Legend – 911 Owner Reports -

While the 991 Turbo and GT3 may be the first Porsches to fea­ture ac­tive rear wheel steer­ing, nei­ther model is the first to try and con­trol the di­rec­tion of the rear axle un­der load.

Through­out the 1960s, 1970s and 1980s, most of the cars rolling off the Zuf­fen­hausen pro­duc­tion line fea­tured semi-trail­ing arm rear sus­pen­sion, whereby the two arms con­nected to each up­right are mounted to the chas­sis via rub­ber bushes (to re­duce vi­bra­tion and road noise).

While the de­sign is a cost-ef­fec­tive al­ter­na­tive to dou­ble wish­bones, dur­ing any de­cel­er­a­tion the car’s body tries to pull away from the rear sus­pen­sion. This causes the semi-trail­ing arm to de­flect thanks to the elas­tic­ity in the rub­ber bushes, push­ing the heav­ily loaded outer-rear wheels into toe-out.

If this de­cel­er­a­tion is caused by a mid-cor­ner throt­tle lift, the en­su­ing toe-out helps to desta­bilise the car, caus­ing a rapid snap into over­steer. To over­come this, Porsche de­vel­oped and patented the Weis­sach axle, a pas­sive rear-wheel steer­ing sys­tem that was fit­ted to the front-en­gined 928 grand tourer.

Com­pared to a stan­dard semi-trail­ing arm, the in­ner­most mount was moved fur­ther to­wards the rear, with a third, piv­oted link­age sit­ting be­tween the fore­most mount and the up­right. This pivot re­versed the di­rec­tion of the wheels’ travel un­der de­cel­er­a­tion, caus­ing toe-in that helped to pro­mote un­der­steer.

While the Weis­sach axle was never im­ple­mented in a 911, the 964 fea­tured a semi-trail­ing arm setup whereby the rear-most arm fea­tured a more flex­i­ble con­nec­tion than the for­ward arm. This made use of the lat­eral loads un­der cor­ner­ing to in­duce 928-like toe-in.

This ‘Weis­sach ef­fect’ was also cre­ated within the 993’s multi-link rear sus­pen­sion setup with the in­clu­sion of a par­tic­u­larly elas­tic fifth link, although plans to in­clude an elec­tro-me­chan­i­cal ac­tive steer­ing sys­tem were tested be­fore be­ing scrapped due to their com­plex­ity.

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