28 April 1995

perfect tyre

Standard moves closer to

The search for the "perfect" tractor tyre continues. Low ground pressure, good grip, long life, cheap prices – all desirable but a rare combination to be found in one tyre. Latest development which claims to have made significant advances in this direction comes from Michelin. Peter Hill reports

LESSONS learned from developing low-profile tyres have helped Michelin produce a "standard profile" radial traction tyre with improved levels of performance for every day use.

The companys design team claims improved traction and a longer effective working life for the Agribib, which has a more supple carcass and deeper lugs than the Bib XM18 tyre it will progressively replace.

It will also be cheaper than the XM18, despite these performance enhancements, and it introduces the tubeless concept into what is the largest sector of the agricultural tyre market.

"The Agribib will account for probably half our UK sales," claims Michelins Geoff Holmes. "Currently available in eight 24in, 28in and 38in sizes for front and rear driven wheels, there will be another four sizes by the end of the year."

The Agribib follows a succession of new tyre introductions by Michelin – the result of the company investing more than 5% of its turnover in research and development.

Design objectives have been to produce a tyre which improves traction – increasingly difficult as tractors become more powerful – reduces inflation pressures to minimise soil compaction and copes with greater road mileages.

"The 65-series XM108 has been very successful in achieving these aims and now accounts for around a quarter of our sales, despite a price premium of between 25% and 30% over a standard tyre," says Mr Holmes.

This low-profile tyre can be fitted to standard wheel rims to provide additional tread width and contact patch length through a combination of supple sidewalls and an ability to run at low inflation pressures; it is a combination which is also claimed to produce low vibration levels and good wear characteristics on Tarmac.

Many of the lessons learned in developing the tyre have been applied to the Agribib, says Michelin, with changes to the rubber compounds used, the internal structure and lug design, giving it a performance advantage over the Bib XM18 and similar tyres.

Longer footprint

Analysis of comparisons made at the companys Ladoux test centre near Clermont-Ferrand have prompted Michelin to claim a 5% traction advantage over the XM18. That, says the design team, comes from the longer footprint, created by the more flexible carcass, and the slightly longer and deeper lugs.

But more importantly, the Agribibs traction is maintained at a higher level than other tyres as it wears; with the tread 60% worn, the Agribibs performance advantage over the XM18 increases to around 12%, claims the manufacturer.

This is put down to the shape of the tyres lugs, which are slimmer at the base. Normally, lugs become wider at this point to prevent them being ripped from the carcass under high torque loads. As they wear, they become blunter, penetration becomes more difficult – particularly into hard soils – and traction deteriorates.

"The Agribibs lugs have almost vertical sides to help the tyre maintain penetration and traction as it wears," says Michelins UK technical manager, Ed Rainbow. "So, the tyre gives a higher level of performance over its life."

Michelin illustrates its claims with the results of tests which compared the Agribibs performance with that of a competitors "premium" tyre. They show that when 60% worn, the Agribib can generate as much tractive effort as the competitors when less than a third worn.

"Another way of looking at it is to find out how much wheelslip the tyres need to achieve a given tractive force," says Mr Rainbow. "With both tyres 60% worn, the Agribib needed just 15% slip – an acceptable level – when the competitors tyre needed 25%, which is more expensive in fuel and tyre wear."

Other design features of the new tyre include more rounded shoulders and buttresses to help support the outer ends of the lugs – as well as giving some protection to the sidewall. A raised step between each lug is claimed to improve self-cleaning by accentuating carcass flexing.

Reduced vibration

On-road vibration has been reduced across most speeds and the tyre should wear longer as a result of the different rubber compounds, carcass structure and slightly deeper lugs.

And, being a tubeless design – though it can be used with a tube – the tyre should be more puncture proof.

"When a tube is punctured you get rapid air loss; with a tubeless tyre, whatever punctures the casing often remains and seals the hole," explains Mr Rainbow.

Michelins Agribib radial carries as much as the XM18, despite being more flexible – note sidewall deflection on this 14.9 x 28 carrying 1350kg at 0.5 bar (7psi). Agribibs will progressively replace XM18s.

"Agribib will account for more than half of UK sales," predicts Michelins Geoff Holmes. Deep but trim lugs help maintain traction; buttresses support lug ends on round-shouldered carcass and help protect walls.