4WD MODIFICATIONS - TYRES & WHEELS
We’ve been hoping for years that the airless, puncture-proof tyre would save us from plugging flats in mulga-root country, but it seems that our salvation is some years away. OTA brings you up to date with non-pneumatic-tyre (NPT) development around the world.
The blindingly-obvious attraction of an airless tyre is its puncture-proof design: no air means no punctures. Also, there’s no need for pressure maintenance.
Michelin’s Tweel NPT range is the best known airless tyre initiative in the off-road-equipment market. The Tweel has radially-disposed blade-shaped ‘spokes’ in pairs, but some other brands have various geometric shapes to connect the hub with the circumferential tread band.
All major tyre companies are heavily invested in NPT technology and have tyres at various stages of development. To date, the most common applications are in low-speed vehicles, from bicycles to earth-movers.
At this year’s Brisbane Truck Show, Bridgestone showed its NPT truck concept tyre, but was quite close-mouthed about it and the display tyre was fitted with a red band that effectively blocked any view of its structure. However, we managed to find a photo of the interior that shows a resemblance to the Michelin ‘spoked’ design.
The earliest patent we can find for the NPT dates back to 1938, when one J V Martin of the United States invented a safety tire with hoops of hickory encased in rubber and fitted with criss-cross spokes of ribbed rubber. It could drive over 100mm blocks when tested in a springless test car.
The next recorded NPT R&D came much later, in 2005, when Michelin started developing an integrated tire and wheel combination that the company called ‘Tweel’ – a name derived from ‘tyre and ‘wheel’ – that were fused into a unit assembly that operated entirely without air.
However, it took Michelin nearly seven years to sort out its NPT baby and even then it has remained confined to fitment on low-speed, on- and off-road equipment.
At the same time there was competition in the form of Resilient Technologies’ Terrainarmor tyres that were available on some Polaris all-terrain-vehicles (ATVs). Polaris even bought-out Resilient Technologies in the hope of making that tyre an in-house exclusive.
Resilient Technologies’ airless tires were tested in 2011 by the U S Army, on military ATVs and Humvees, but the tests proved unsuccessful.
For some time, Polaris offered its in-house resilient Technologies’ NPTs as optional fitments on some ATV models, but for the last few years the company has switched to Michelin Tweels. A set of four is available for around US$3000.
In 2019, Michelin and General Motors presented a new generation of airless wheel technology for passenger vehicles – the Michelin Uptis Prototype – Unique Puncture-proof Tyre System – at the USA’s Movin’On Summit for sustainable mobility.
The two companies began testing the Uptis Prototype on a fleet of Chevrolet Bolt EVs on public roads in Michigan in 2020, with the intention of offering airless tyres to passenger car buyers by 2024.
In 2021 the US Army resumed NPT testing, fitting Tweels to the Polaris MRZR, a military version of the popular off-road vehicle that American forces have used in places like Afghanistan in recent years. The current testing regime is being carried out by the Tropic Regions Test Center (TRTC), which conducts realistic evaluations under tropical conditions.
In an industry first, in July 2021, Goodyear released an NPT assembly for fitment to ‘Olli’ autonomous public transport vehicles, operated by the Jacksonville Transportation Authority (JTA) in Florida, USA.
Olli is produced by Local Motors and when launched was the world’s first co-created, autonomous, electric vehicle and is manufactured using 3D-printing technology.
For the past three years, Goodyear and Local Motors have tested an Olli shuttle, fitted with Goodyear’s NPTs. Now the project is moving into daily use by commuters, in a Test & Learn phase.
NPT tyres are different from ‘run-flat’ tyres that are favoured by military and security services around the world. These tyres have an inner structure that gives the tyre some mobility even when the air-filled section of the tyre is punctured. Many of today’s cars and SUVs are fitted with ‘mild’ run-flat tyres We’ve covered the run-flat situation here.
NPTs have no air chamber at all and rely on spoked or honeycomb structures to connect the wheel nave to the rim. This structure is designed to provide flexibility and innate ‘springing’.
Michelin claims its Tweel has load carrying, shock absorbing and handling characteristics that compare favourably to conventional pneumatic tyres, but current models develop severe vibration when driving over 80km/h. Therefore the Tweel is currently commercially available only for golf carts, ATVs and skid-steer vehicles.
Tweels are also designed to power through serious damage for far longer than even run-flat pneumatic tires. Using drill bits,TRTC testers damaged the Tweels to simulate as if they had been shot prior to some of the evaluations.
In typical low-speed, off-road applications Tweels conform to multiple different terrain types and can last three times as long as standard tyres, but the high-speed dynamics required for road use are far more difficult to engineer than the high radial-load applications typically encountered in off-highway applications.
At this stage of development it would seem that NPTs for heavy 4WDs travelling at road speeds are some years away, but you never know. We’ll keep up to date with NPT issues.