If you want to see tyres in all their glorious complexity, Clermont-Ferrand is the place to be. From racing bike tyres as thin as an earthworm to dumper truck versions that weigh as much an African elephant, Michelin makes them all.
But while many of these tyres no doubt have tough lives, there’s nothing quite like a tractor, sprayer or combine tyre to furrow the brows of a tyre engineer.
Here’s why. Thirty years ago, a Ford 5000’s tyres only had to support 2.7t and had a maximum speed on the road of 20mph. Now a Fendt 939 puts 16t on the ground and will zip along the tarmac at 60kph (40mph).
More frustrating than that, if you are a tyre engineer, is that you want low pressures to spread the weight when you are in the field, but higher pressures to reduce the rolling resistance on the tarmac. Michelin’s answer has been its Ultraflex tyres which, to a large extent, let you have your cake and eat it.
Trailer tyres get clever
Trailer tyres have traditionally been the poor relation of the tyre family, with farmers happy to use super-singles or cheap cross-plies at 4-5bar. But as trailer sizes got bigger, and worries about compaction increased, there has been a sharp move to wider radial tyres.
Michelin’s new CargoXBib tyres are the first ones to use its Ultraflex technology, though you won’t be able to buy them on the after-market until early next year. They can operate from just 0.8bar to 4bar, with a middle-of-the-road 2.5-3bar giving you a decent balance between road and field work. The footprint on the new tyre is also said to be 37% larger than super-single at 4bar.
A demo to show this was cancelled because of heavy rain the day before, but the deep ruts say it all. The 24t trailer that made them was run with one tyre at 2bar and the other at 4bar. The 2bar one made a modest rut 20cm deep, while the 4bar one gouged out a Grand Canyon-like 30cm one (see below).
Of course, the ideal pressure for field work would be 1.8bar, but if you’re doing much roadwork, 2.8bar is a better bet. Since only the truly saintly would regularly alter the pressure between field and road, the only answer is central tyre inflation. However, that’s still an expensive add-on.
Michelin has offered its SprayBib tyres since 2010, but plans to add bigger sizes – a VF 480/80 R42, VF 480/80 R46 and VF480/80 R50 – to accommodate ever-larger self-propelled sprayers.
It points out that with top-end sprayers now equipped with a 350hp engine, 6,000-litre tank and 54m boom (not to mention 60kph speeds in some countries), you need some serious rubber to keep it on the straight and narrow. The slalom demo (pictured top) of a 16t Deere sprayer with 2.2bar in the tyres and travelling at 35kph nicely illustrated the point that good tyres can give you surprisingly good handling on the road, too. Don’t try this at home, though.
Getting 350hp or 400hp from the tractor’s engine down to the ground requires some serious rubber. If you want to go down that road, Michelin’s AxioBib IF900/65R46 is the biggest ag tyre on the market. It was announced last autumn, is 2.32m tall and 900mm wide and will cost you about £7,000 as a replacement. Load capacity is 10,600kg/tyre and maximum speed on the road is 65kph.
Can tractor tyres get any bigger? Probably only if tractor makers move the cab forward. Finding a big enough tyre won’t be a problem; the Michelin range goes up to a 59/80R63 that weighs 5.3t and happily holds up a 600t dumper truck. That’s 10 times the weight of a fully loaded Claas Lexion combine.