12 March 1999

Row-crop demands pile on the pressure

Over-worked and under

inflated. That sums up the

lot of the farm row-crop tyre

ROW-CROP tyres are kept on board for applications that do not suit them, and often run on insufficient air for the weight they are expected to carry.

The technical requirements and limitations of these narrow covers are often neglected to a greater extent than those for conventional traction and low ground pressure flotation tyres.

Yet they perform a role of equal importance that is twice as demanding. Whether used for crop spraying, fertiliser spreading or harvesting potatoes and sugar beet, row-crop tyres are expected to perform the near-impossible task of carrying a lot of weight on a narrow footprint without causing undue damage to the soil beneath.

The introduction of radial construction, tougher carcass designs and larger diameters has, to some extent, helped keep pace with the growing power and weight of tractors and other vehicles on which they are used.

Most makers produce sizes with a choice of standard or heavy duty construction which are commonly indicated by two or four stars rather than ply ratings. Differences in load capacity between the two go from 500kg to nearer 900kg a tyre, which can make all the difference with some tractor and implement combinations.

As for sizes, tyres catering for wheel diameters up to 48in feature in the Alliance Row Crop, Continental Contract AC90, Kleber Super 3, Michelin Narrow XM25 and Taurus Rowcrop catalogues, while Goodyear pips the field with a 50in partner for its 320/90R46 (12.4R46) DT800 row-crop tyre. All cater for big tractors or provide extra ground clearance on mid-size models and self-propelled sprayers.

Increased tyre diameter not only helps ground clearance but extends the tyre/soil contact patch to a degree. More importantly, it increases the volume of the tyre, and the more air that can be crammed into a tyre, the heavier load it can carry. Or, conversely, the less pressure it needs to carry a fixed load.

Inflation pressure is one area where row-crop tyre users do go awry, it seems. That is the impression of Julie Leighton of Sam Moreton & Sons and Maureen Giddons of Standen Engineering, both involved in the day-to-day supply of row-crop wheel and tyre assemblies.

Farmers often forget just what high pressures these tyres need to carry the weight of a tractor and sprayer, they say. For a road speed of up to 25mph, most tyre manufacturers work on a maximum pressure of 3.6 bar (52psi), although for low speed working with heavy loads, Kleber and Michelin load/pressure tables extend to 4.4 bar (65psi) with a 6mph speed restriction.

Such pressures are not always needed, it all depends on the load and application. But pressures in row-crop tyres will always be considerably higher than with larger volume conventional sizes, which are normally at the 1.7 to 2 bar (25 to 30psi) mark or, preferably, less.

Speed is an issue, though, since drivers sitting comfortably in a quiet cab are inevitably inclined to go for the quickest journey time home rather than worry about the tyres ability to cope.

It is significant, for example, that row-crop wheels for Fastrac and similar tractors are built to a heavier duty specification than their conventional tractor counterparts, even though the tyres have no special speed rating.

Most popular size on this tractor is a 270/95 (11.2in) radial in 36in and 42in diameters, or 44in at a pinch if the extra clearance is important and the poorer turning circle can be tolerated.

Fastracs can go taller, though, with the A M Phillips stretch conversion and mudguard modification which allows 48in wheels and tyres to be used. Alliance agent Kirkby Tyres offers all these sizes, with Continental, Kleber, Michelin and Taurus having one or more in their respective ranges.

On conventional tractors, a 230/95R48 (9.5R48) gives the narrow width needed to run down potato rows, with an 270/95R48 (11.2R48) making a good general-purpose choice or 13.6R48 where operators are after a little extra height and there is sufficient clearance beneath mudguards.

For the most part, operators settle for wheel and tyre sets with the same overall diameter as the original fitments so that transmission gearing remains the same. A crucial aspect of the selection process for four-wheel drive tractors, though, is to ensure that front and rear combinations keep the tractors front drive axle lead within acceptable limits. That is best left to the experienced row-crop wheel and tyre supplier.

Operators also tend to leave tyre choice to suppliers, with Alliance and Taurus taking the lions share where buyers are happy to go for the cheapest price. Where a lot of road work or arduous applications are likely to be involved, then Kleber, Continental and Michelin offerings are more likely to be specified. &#42