Archive Article: 1997/10/18

18 October 1997

STANDING still there isnt much to beat a rubber-tracked tractor for low ground pressure. Fire it up and compaction may be greater, and traction less than some wheeled equivalents.

So says Dr Hartmut Döll, of the Martin Luther University at Wittenberg, Germany, who has compared compaction, slip and traction from a rubber-tracked Caterpillar tractor with that of tractors fitted with single and dual wheels.

Dr Döll also told a seminar organised by the Swiss dual wheel specialists Schaad Brothers that he had monitored the use of dual wheels only on 220hp tractors being used on a 3,200ha (7,900 acres) farm in eastern Germany for nearly 10 years. In each field 2ha (5 acres) had been worked by tractors fitted only with single wheels to provide a comparison. Some small plots were also used as a back-up for the comparisons.

Yield checks showed there was consistently 10 to 11% extra yield of spring barley from using dual wheels for all field traffic, and 5% or more extra yield from winter wheat. "The differences might not matter for a small farm but they do for a large farm," said Dr Döll.

The ability to get on early in wet conditions could be important for spring cereals which get an establishment boost from the available moisture, particularly on soils which later dry out. He saw no reason why the benefits should not apply on heavy soils as well as lighter ones.

Compaction was the main drawback to the use of standard single wheels in poor conditions. Measurement of cereal plants showed that where compaction had occurred the root mass was just as great as in non-compacted ground. However, the roots were much shallower and did not develop fully the fine root hairs which were essential to taking up nutrients and moisture from the soil.

The farm was also being ploughed every year, said Dr Döll. If the compaction occurred in the plough pan and was made worse by wheel slip or a layer of straw, then sugar beet would react like cereals by sending roots sideways.

Some wheel tracks could reduce the drainage capability of compacted soil by half. Either wide wheels running at less than 0.8 bar in the field, or dual wheels, protected the ground better than single wheels. However, running wide wheels at low pressure to boost traction could put a strain on the tyre sidewalls.

Dual wheels spread the compacting pressure of the tractor but also helped keep up traction, particularly at medium to high working speeds where slip could be reduced by up to a third.

Low tyre pressures could increase traction by up to 30% compared with normal running rates. However, he was sceptical of there being any advantages for using rubber tracks, even when compared with single wheel tractors with tyres at 0.8bar.

The effect of the rollers operating inside the rubber tracks was to create peaks of greater pressure on the ground once the machine was rolling than some wheeled equivalents. "There is no reason to say rubber tracks are a solution to make wheels disappear," said Dr Döll.

Dual wheels, which could make use of existing wheels and tyres on the farm, provided a flexible solution for various farm tasks from ploughing and cultivation through to farm transport, he added.

During the monitoring carried out on the university farm, the effort required in ploughing was considerably reduced on ground where measures against compaction were taken.

Rubber tracks, wide tyres or dual wheels? David Millar reports on German research which casts surprising light on which systems tread the gentlest.

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