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The wheels rolling un­der­neath the 959 were as spe­cial as the car it­self, start­ing a trend among road-go­ing 911s still present on GT and Turbo cars

Total 911 - - Contents - Writ­ten by Chris Ran­dall Pho­tog­ra­phy by Porsche

The 959’s cen­tre-lock­ing wheel started a trend that’s come full cir­cle for to­day’s 911 GT and Turbo cars

The 959 was Porsche’s first use of a cen­tre-lock wheel on a road car and, al­though com­mon on 911s to­day, there’s rather more to th­ese rims than meets the eye. In fact, the in­no­va­tion be­hind their de­sign and man­u­fac­ture makes them more than wor­thy of closer at­ten­tion. The 17-inch rims were cast in mag­ne­sium and fea­tured hol­low spokes which formed a com­mon air space with the tyres. That was im­por­tant be­cause it al­lowed the tyre pres­sure mon­i­tor­ing sys­tem, which used a pair of pres­sure switches within each wheel, to warn not only of tyre de­fla­tion, but also loss of pres­sure due to a cracked spoke – a drop of around 3psi trig­gered a vis­ual and au­di­ble alert in the cabin. An­other unique fea­ture of the wheel was a rim shaped to ac­cept tyres man­u­fac­tured with the ‘Den­loc’ bead-lock­ing sys­tem that was in­tro­duced by Dun­lop in 1979. A de­vel­op­ment of their run-flat ‘Den­ovo’ sys­tem, it pre­vented a de­flated tyre from leav­ing the rim and al­lowed the 959 to travel at a max­i­mum speed of 50mph in the event of a punc­ture, as the 959 didn’t carry a spare wheel. Al­though Porsche had orig­i­nally planned on us­ing Dun­lop’s tyre, Bridge­stone had also de­vel­oped its own tyre with the Den­loc sys­tem, and it was Bridge­stone RE71 rub­ber that was ini­tially spec­i­fied for the pro­duc­tion cars.

The cen­tre-lock ar­range­ment was typ­i­cal of Porsche’s fas­tid­i­ous at­ten­tion to en­gi­neer­ing de­tail. The wheel bolt was hid­den by a plas­tic cover, and be­neath that was an insert – op­er­ated by the ig­ni­tion key – that acted as a lock, pre­vent­ing both theft and loos­en­ing of the bolt. The bolt it­self had a ro­tat­ing thrust ring that pressed the wheel against the hub, while a ta­pered ring on the wheel trans­ferred the clamp­ing force from bolt to wheel. Re­moval and re­fit­ting was a rather more in­volved task, too. Along with the wrench sup­plied with the car there was also a sep­a­rate re­duc­tion gear unit that needed to be used, its pur­pose be­ing to en­sure the per­son fit­ting the wheel could do up the bolt to the re­quired torque. The fig­ure quoted by Porsche was a mas­sive 850Nm, but us­ing the re­duc­tion gear meant that a more man­age­able 230Nm could be ex­erted on the wrench it­self. Also re­quired was the use of an as­sem­bly paste, ap­plied be­tween the bolt and wheel be­fore re­fit­ting, while Porsche also spec­i­fied an in­spec­tion ev­ery two years – it in­volved a check of the wrench and lu­bri­ca­tion of the re­duc­tion gear, bolt/thrust ring and the space be­tween the ta­pered ring and the wheel.

And, just to un­der­line the spe­cial na­ture of the wheels it had cho­sen, Porsche also warned that any dam­age to the pro­tec­tive coat­ing caused by clumsy tyre fit­ting would need ur­gent re­pair, as mag­ne­sium cor­roded quickly. We rarely give any thought to our wheels, but for own­ers of the 959 it was just an­other fas­ci­nat­ing as­pect of a very spe­cial car.

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