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
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.