Tractor tyres put through their paces

Ever wondered how far you can push a set of tractor tyres? James Andrews visits Michelin’s test centre in France to find out

Roll the most expensive tractor tyre on the market alongside the cheapest and nastiest on offer and you’ll be hard pushed to spot the difference.

Both are round and black, both have a bead that fits them to the tractor’s wheel rim and both have a pretty similar looking set of rubber lugs around the edge. But push them to their limits and you’ll soon realise you’re dealing with two very different beasts.

French tyre maker Michelin is at the sharp end of agricultural tyre design and spends millions every year developing tyres. That means everything from the mix of exotic materials used in the sidewalls to the contours of the lug have been painstakingly engineered and tested.

Much of the maker’s R&D budget is spent on design and research, but a significant chunk is dedicated to grilling the tyres to make sure they perform when the going gets tough.

Philippe Planeix is one of Michelin’s seven-strong team of agricultural tyre testers. He works at Michelin’s 380ha Ladoux test facility, which is within spitting distance of the company’s headquarters in Clermont-Ferrand. The purpose-built centre has more than 41km of test tracks where you’ll find anything from a Bugatti Veyron to a John Deere 4650 being put through its paces with a set of the maker’s latest tyres on board.

It’s far from all high-speed, silky-smooth asphalt though – the tracks have been designed to simulate every type of European road a set of tyres might encounter. We also noticed plenty of rough, lumpy tarmac full of potholes that was clearly modelled on some of the UK’s finest roads.

For tractor and implement tyre testing there’s a large swath of cultivated land, too. And Michelin has a number of local arable farmers surrounding the site that allow tests to be carried out on their ground.

Michelin has dreamt up more than 50 gruelling tests for agricultural tyres and the test team clocks up more than 12,000 hours a year doing them.

We visited the site in early July and Mr Planeix showed Farmers Weekly three of the most important ones – the rough terrain comfort test, the compaction test and the high-speed manoeuvre test.

All of these are carried out using Michelin’s slightly-pokier-than-average Fendt 936. It’s been fitted with flexible sidewall Axiobib tyres and is weighted to give a hefty 3,600kg loading on the front tyres and 4,600kg on the rear.

Comfort testing

The comfort test is first on the agenda and ironically it’s by far the least comfortable of the three. The test involves driving around an unpleasantly bumpy circular track, increasing speed with each lap. By the time we reach 40kph it’s about as pleasant as perching on a jackhammer.

At the end of the ordeal, tyres are rated on a special comfort scale and if they don’t make the grade they’re packed back off to the drawing board.

To give Mr Planeix’s assessments a bit of scientific backup, the tractor is also fitted with a series of sensors and accelerometers. These measure the pitching and rolling motions in the cab as well as any vibrations.

Compaction test

The compaction test is next on the agenda and this is carried out in Michelin’s massive indoor sandpit. Buckets and spades cleared away, Mr Planeix backs the Fendt into the pit and stops. He then hops out and marks the front edge of the tyre’s footprint before driving away.

This is when the tricky part begins. He takes hold of the firm’s €70,000 3D laser and painstakingly maps the entire footprint. Cleverly, this transfers a 3D map of the footprint on to the computer screen.

This is analysed to see how much pressure the tyre puts on the ground and how deep it sinks in.

High-speed test

Finally we get to the most fun of the three tests – high-speed handling. Like most of the tests, it’s designed to push the tyres to a limit that even the most erratic of drivers will struggle to achieve.

First Mr Planeix cranks the modified 936 up to its hair-raising 67kph top speed and takes it for a spin round the racetrack with just 0.8 bar pressure in the tyres.

Several laps later, speed is dropped to a slightly more sedate 42kph for the swerve test. This is a tight chicane that requires him to crank the wheel from left to right as if to avoid hitting a wayward cyclist or inconsiderate rambler. The move is so aggressive the tyre lunges sideways leaving the rim grinding on the tarmac. Remarkably the tyre stays in place.

Not yet content, he charges toward one of the test-centre’s roundabouts and screeches round it at the same ridiculous speed.

We climb down and examine the brand new tyres. It’s only been a few hours of testing, but they already look like they’ve had a tough time of it.

They’re still in one piece, though, and they’ll be back for more tomorrow.

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