12 March 1999

Rubber tracks the way ahead? Well, it depends

On the face of it, rubber tracks appear to offer farmers a

carefree option for their spring and autumn cultivation

work. No more punctures, no pressures to adjust and no

duals to attach when the going gets tough. But are they

really the way forward for high horsepower tractors?

Geoff Ashcroft looks for some answers

LESS slip, more grip, no punctures and no pressure adjustment, at first glance rubber tracks appear to offer it all.

For those in the market for a cultivations tractor over 185hp, rubber tracks look to be an interesting option, but how do they really stack up against tyres?

Though still in their infancy compared with pneumatic tyres, rubber track development has been swift. Since 1986, the Mobil Trac rubber track system has been available in the UK on Caterpillars Challenger tractors, recognisable as Claas machines. Goodyear quickly followed suit with a system which is used by Case and John Deere, and other tractor makers follow the trend.

But do rubber tracks really hold the answers to farmers prayers?

"Not really," says Gordon Day, tractor specialist with John Deere. "It depends very much on how you intend to use them."

On loose soil, Mr Day reckons rubber tracks will outperform a similarly powered tractor – correctly tyred and ballasted – by a significant margin. And on wet or soft soils, tracks offer a floating characteristic where tyres can sink in and cause rutting. But on compacted soil, where grip is plentiful, Mr Day says the rubber track advantage almost disappears.

Mr Day is perhaps in a difficult position. His firm sells the 8000 series tractor range as wheeled and rubber tracked machines. And to favour one type over the other could appear to influence sales.

This aside, Mr Day has clear views on the efficiencies of both tracked and tyred tractors.

"The biggest problem with a rubber tracked machine is the rear three point linkage. If you are going to hang heavy, mounted equipment on a crawler, then efficiency takes a big dive," he says.

Maintaining contact

"It is all about maintaining track contact with the ground. If you carry a heavy implement on a tracked machines three point linkage, then the combined weight of the implement and tractor is transferred on to the rear drive sprocket and ground pressure increases tremendously.

"Deeres 8000T rubber tracked machines are balanced differently to the 8000 wheeled tractors, tracks require a 50:50 weight distribution to keep firm contact with the ground, whereas tyred machines need a nose heavy stance to make the front axle work as hard as the rear when draft is applied. For a crawler to work at its peak efficiency, it should only use the drawbar pin and pull in a straight line."

Andrew Rabett, Claass Challenger product specialist, has a much clearer view of the role of rubber tracks.

Taking slip as an example, Mr Rabett compares the rubber tracked Challengers 0-5% slip, to that of a conventionally tyred tractors 12-15%.

"With about 10% less slip, the rubber tracked Challenger offers a simple 10% increase in efficiency," he says. "You could translate this into travelling 10% faster, or working 10% wider, or saving 10% on fuel consumption. And the more horsepower you have got, the more efficient rubber tracks become, compared to tyres."

During last autumns wet conditions, Mr Rabett reports many Challengers were able to keep going due to their ability to float over wet, sticky soils.

"A rubber tracked machine spreads its load throughout the undercarriage, each set of wheels in the undercarriage acts as an axle, which helps to spread the load more evenly on the ground, putting hp where it is needed."

Put simply, he reckons a 10t tractor on two axles exerts about 5t an axle, whereas the 10t Challenger 55 has five axles in its undercarriage to carry the load more evenly.

"On 25in tracks, the 270hp Challenger 55 exerts a maximum of 5.9psi ground pressure," he says.

"Look at the extra rubber you would need to get a 270hp wheeled tractor down to the same level.

But the price premium persists for those who take the rubber track route. A Claas Challenger, for example, costs up to £30,000 more than a comparable wheeled tractor in the same power bracket. &#42