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Lower profile tyres bring higher yields

The tyres fitted to quarry plant have a huge bearing on the efficiency of the whole production process. David Taylor reports on some of the latest technical developments

THE IMPORTANCE of tyres in the development of new earthmoving products is easily overlooked - yet tyre selection and performance is as crucial for earthmovers as it is for Formula 1 racing cars.

Tyres are far and away the most expensive consumable component on virtually all wheeled earthmoving equipment - and especially large plant used in quarrying and mining applications.

Tyres are expensive to make; their design and manufacture demand very exacting tolerances and the cost of making the moulds and tooling up the production line is also high.

The cost of the biggest tyres is further magnified by the fact that they are never produced in large quantities, simply because there are relatively few very big wheeled loaders and dump trucks about.

But, despite the small market, the main producers still devote considerable time and money to research and development. Consequently, there has been steady progress in the design of tyres for earthmovers in recent years, particularly in exploiting the benefits of radial-ply tyres.

The most recent step has been the introduction of wide-base, or low-profile, tyres for wheeled loaders and dump trucks.

Michelin, with its XLD 70 range of tyres for medium-size wheeled loaders (100-250 PS), claims to have pioneered this type of tyre, ahead of main rivals Bridgestone and Goodyear. The range arrived on the scene almost two years ago.

The '70' in the product designation indicates the aspect ratio (tyre section height/width) of the tyre. Hence, the cross-section height of this tyre is 70 per cent of the tyre's width.

'Aspect ratios haven't changed for years,' confesses John Ballinger, Michelin's earthmover marketing manager. 'They've typically stayed at about 80 per cent until we introduced our 70 per cent aspect ratio tyres.'

A large part of the work of medium-size loaders is in stockpiling, rehandling and truck loading, where productivity is one of the major considerations.

Having a smaller rolling radius than equivalent 'normal-sized' tyres, the XLD 70 has increased rimpull - by as much as 10 per cent, which improves the traction and penetration capability for better and quicker bucket- filling. Adhesion is also improved because of the larger ground contact area.

Another benefit readily apparent to the driver is the more comfortable ride. The lower centre of gravity (from the smaller diameter of the tyre) reduces pitch and roll and generally provides a higher level of stability.

In addition, the four-block tread pattern is effective at providing tread flexibility for easy manoeuvrability, and at the same time filters out vibrations and absorbs shocks - therefore protecting the transmission and drive-line.

Michelin has now applied similar thinking to a range of tyres designed for articulated dump trucks - the XAD 65 series, with an aspect ratio of 65 per cent.

Mr Ballinger explains: 'Mobility is the key word for ADTs. Because they are so widely used and have to cope with such a variety of conditions - from hard rock to soft ground, steep grades and high-speed hauls - customers expect to achieve the best of traction, flotation, puncture resistance and durability. With these new wide-base tyres, we have aimed at producing a good all-round tyre, with enhanced stability and flotation.'

By putting more of the tyre in contact with the ground, it can transmit more power, while maintaining flotation, which means the trucks continue to work effectively in the most adverse conditions.

Although Michelin, which has a reputation for its R&D capability, claims to have pioneered the latest low-profile tyres, other manufacturers have not been far behind.

Indeed, Mark Butcher, Bridgestone's earthmover area sales manager for the South-East, points out that the idea is hardly new; exactly the same trend is well established in a more familiar sector of the industry: 'Car tyres have steadily become lower and wider for the same reasons.'

Bridgestone's latest low-profile earthmover tyre is the 750/65x25, a 65 per cent aspect ratio tyre for ADTs. These have proved highly popular since their introduction almost two years ago, says Mr Butcher, and can be found on most Volvo and Terex machines these days as both factory-fit Bridgestone tyres.

Mr Ballinger says the first Michelin low-profile tyres were developed principally as a result of consultation with end-users.

'Because of the cost, people who operate earthmovers take more notice of the kind of tyres they fit than most other customers,' he says.

One of the first customers to take delivery of ADTs with the new Michelin XAD tyres was Patersons of Greenoakhill, near Glasgow, which has just added two new Cat D300Es to its mixed dump-truck fleet on the Greenoakhill landfill site. Fitted with large, 50 cu m dump bodies, the trucks are operating continuously on the the landfill rather than haulroads, so the conditions are always relatively soft.

'The biggest difference we have noticed,' says Patersons' Willie Orr, 'is that with the larger 'footprint' area and lower ground- bearing pressure, the new trucks can travel about on the landfill without rutting the surface. This not only improves their speed and stability but, because of the reduced penetration, there is less side-wall damage and they have fewer punctures.'

Patersons reports that there is also less damage beneath the vehicle - to axles and brake-pipes for instance - when machines are fitted with low-profile tyres. This is because wide tyres are less likely to sink into soft ground and, although the machine is lower, ground clearance is maintained.

Both Michelin and Bridgestone are understood to be about to launch new earthmover tyre ranges. Naturally, they are playing their cards close to their chests. Bridgestone's Mark Butcher would say only we can expect to see more low-profile tyres in future.