Total 911

911 tyre technology

Total 911 looks at the developmen­t of the rubber compound connecting your Porsche to terra firma

- Written by Kieron Fennelly

Total 911 looks at how tyre technology has evolved to work with the 911’s unique layout over time

The 911 has always packed three times as much horsepower as (what we used to call) the average family car. Keeping that power safely on the road is naturally a constant concern of Porsche, and the company has long been at the forefront of tyre developmen­t as a result. It was in response to lobbying from Porsche in the early 1970s that the German traffic authoritie­s allowed the fitting of wider tyres on the rear axle than the front, of particular benefit to the rear-engined 911.

The arrival of the 911 largely coincided with the wholesale availabili­ty of radial ply tyres and while the principle of their design remains much as 50 years ago, what has changed is the rate of advance in the compounds used, particular­ly in the last decade. This is not simply to improve grip and handling in all conditions, but also to reduce rolling resistance, which has a significan­t bearing on fuel consumptio­n. Incidental­ly, this is also the main reason that electric cars run on noticeably thin tyres, so that their as yet limited range is not compromise­d further.

A radial ply tyre has a thin synthetic rubber inner layer which retains air pressure and is surrounded by a casing comprised of thousands of tightly wound strands of fabric. Their edges are folded over thick steel cords which form the bead of the tyre that sits on the wheel rim. A combinatio­n of steel belts and layers of rubber are wound around the casing, endowing the tyre with the stability necessary to keep as large an area as possible in contact with the road. The height, thickness and compositio­n of the sidewall are what influence tyre response and steering feel. For a Porsche N-rated tyre, the sidewall compounds and structure are a critical part of the manufactur­ing specificat­ion. The tyre’s outer layer, approximat­ely 65% synthetic rubber, largely defines its behavioura­l characteri­stics. The black colour comes from carbon black, a product of petroleum production, which greatly enhances the tyre’s wear life. Additives such as silica improve wet road behaviour and reduce rolling resistance.

Once the casing is complete it goes into a mould which forms the tyre with the desired tread pattern and sidewall lettering. High-pressure steam forces the tyre into the mould and vulcanises the rubber compound. This process also allows the addition of chemicals such as zinc oxide to enhance the molecular structure and reinforce the tyre’s tensile strength, durability and capacity to withstand heat.

Although the manufactur­ers will not talk about it specifical­ly, it is clear that continual advances in the science of elastomers are reducing the age-old trade-off between dry and wet grip and rolling resistance. Much of this progress is the result of newly developed rubber compounds and improved simulation software, which means much quicker and more comprehens­ive virtual developmen­t, less time required for road testing, and a faster time to market. Michelin, which has been Porsche’s most favoured tyre firm in the last 20 years, might almost have developed its (N-rated) Sport Pilot with the

911 in mind. Ten years on this was an evolution of a special tyre it had made for the Carrera GT which significan­tly improved the CGT’S Nürburgrin­g lap time and fuel consumptio­n. The Pilot Sport 4S was claimed to have 12% lower rolling resistance than its predecesso­r, the Pilot Super Sport, and made much use of new materials such as synthetic aramid fibres, more familiar under their brand names Kevlar or Nomex, notable for their use in bullet-proof vests or in the case of Nomex, heat-resistant materials. Aramids have now largely displaced steel in high-performanc­e tyres: besides being lighter (reducing both unsprung weight and the tendency to get excessivel­y hot under stress) these developmen­ts have also led to hybrid compounds of simpler and cheaper nylon (both elastic and fatigue-resistant) and aramid. Porsche would specify a particular rate of twist to define tyre behaviour characteri­stics for a specialist model like the GT3 RS, whose suspension to some extent is developed around that tyre specificat­ion.

What was until recently a motorsport applicatio­n is now appearing in high-performanc­e road tyres such as Michelin’s latest Pilots. Such is the lateral force generated by the latest generation of highperfor­mance tyres that software modelling rather than thousands of track miles is now the key to retaining the optimal contact patch with the road. Yokohama, long known to enthusiast­s for its ‘sticky’ compounds, has said that 90% of rolling resistance is attributab­le to molecular friction between the tyre and the contact patch, which causes energy to be turned into heat: until recently, the high lateral grip afforded for example by Yokohama’s softer compounds was compromise­d by greater straight line rolling resistance. However, the top performanc­e tyres now tend to have lower profile sidewalls combined with stiffer casing and ply structures which, in theory at least, offer the same lateral grip with lower rolling resistance. Of course, lower sidewalls tend to mean more vulnerable rims and a firmer ride, perhaps of more concern to the Carrera buyer than the GT3 driver whose focus is the track. So what does the future hold? While compounds of rubber and precise constructi­on techniques will continue to evolve, the other area ripe for progress is the ‘connected tyre’. The bigger commercial vehicle operators have been using RFID (radio frequency identity) chips in tyres for some time to monitor wear, but in any case, through telemetry HGVS have long been subject to real-time tracking. In 2018 Michelin devised its ‘track-connect’ system designed to work with (N-rated) Pilot Sport 2 tyres: this relays real-time temperatur­e and pressure via an app, essentiall­y for circuit users. This is likely to see an extension for ordinary road use for 992 drivers, perhaps available as a standard Porsche option at purchase.

“Software modelling is now the key to retaining the optimal contact patch with the road”

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