Archive Article: 1997/09/06
AS TRACTOR manufacturers develop more powerful tractors, tyre makers are being challenged to produce tyres capable of more efficiently transforming that power into useful work.
Its a challenge which they have risen to; tractor tyre technology has advanced over the past decade more than at any time since the first radial traction tyre.
Trelleborgs introduction of the bias construction Twin in ultra low-profile designs made a big impact on the scene. The wide tyres, fitted to purpose-made rims, offers big gains in traction and low ground pressure by providing a big tread area between tractor and soil.
The latest trend, however, is towards ultra low profile radial designs that can be fitted as original equipment, or which enables tractors to be upgraded with wider tyres on original rims.
Many tractor operators have come to recognise the benefits of switching from standard 80-series to the wider 70-series tyres. And now, lower profile 65-series designs are extending the concept.
Michelins supple XM108 and the Goodyear DT820 started the ball rolling, a comprehensive choice of sizes is now available in these two designs, for both front and rear fitment on four-wheel drive tractors.
Smaller sizes are also suitable for fitment to other vehicles where premium traction can make all the difference to performance. Self-propelled sprayers and telescopic handlers are obvious examples.
Now, other manufacturers are offering similarly high performance tyres – the Kléber Super 11L, Taurus Point 65 and Vredestein Traxion+ are now available in a growing number of sizes, with the Alliance R-1 also coming in for consideration.
Fitting bigger tyres offers two principle benefits:
• better use of available power and higher work rates through increased traction
• reduced soil compaction by spreading the weight of the tractor and implement over a larger tyre/soil contact area
In addition, the fact that larger tyres can be operated at lower pressures gives a comfortable ride.
The raw figures in the table (left) shows the increase in tread width gained from up-grading tyres, which appears modest. Bear in mind though, that the increase in width is double per axle, and at a lower inflation pressure a longer and possibly broader contact patch is created, so the increase in grip-providing tread area takes on more significance.
A like-for-like comparison of a 600/65R38 Kléber Super 11L with the 18.4R38 Super 9 it can replace, emphasises the point.
Fitted on a typical 130hp tractor carrying a five-furrow mounted plough, the smaller tyre needs to be inflated to 1.65 bar (24psi) to carry the 3,380kg (7451lb) per tyre load. The larger tyre, in contrast, needs only 1.17 bar (17psi). This is partly because of the low-profile tyre structure but more especially because of its larger volume – the 600/65R38 contains 34% more air than the smaller tyre, and it is air more than anything else that supports the tractors weight.
This degree of difference in inflation pressure and consequent tyre stiffness is immediately noticeable in the field. The bigger tyre/lower pressure combination does not transform the ride comfort of a rigid axle tractor but it does produce a noticeably less jarring ride.
The likely improvement in traction and flotation from the upgrade is illustrated by a comparison of the contact patch areas of the two tyres – 3,355sq cm (520sq in) for the 65-series tyre against 2,632sq cm (408sq in) for the smaller 80-series – an increase of almost 27.5%.
Alternatively, the larger tyres greater load carrying can be used to carry heavier implements without extra burden on the soil.
Further advantages become apparent when a lot of road travel is involved; again, the larger tyre/tarmac contact area that comes from a wider tread and lower inflation pressure can significantly extend tyre life – by up to 50% according to some studies.
Vredestein puts particular emphasis on this aspect given the increased distances – at faster speeds – that tractors are these days expected to cover on the road.
In addition to its low-profile design, the Traxion+ has a distinctive tread pattern which places more rubber at the centre of the tyre. The tread lugs overlap at the centre and are positioned close to each other. Reduced vibration as well as low wear are key aims of the design.
The Traxion+ lugs then curves outwards to an acute angle at the shoulders to give plenty of grip in the field. Here, the lugs are a little further apart than usual to encourage penetration and good self-cleaning characteristics.
Traction improvements over the 80-series tyre that the Traxion+ can replace is put at typically 14%, with a 10% cut in rolling resistance owing to extra flotation on soft ground.
Hungarian manufacturer Taurus – now part of the Michelin group – has benefited from recent tyre supply shortages from the major manufacturers. The companys standard and 70-series designs have become a more common sight on new tractors as a result and the Taurus business hopes to capitalise on that with the Point 65 ultra-low profile radial.
As with other tyres of this type, the Point 65 can be fitted in place of equivalent 80-series tyres without going to the added expense of replacing wheels. Sizes are relatively limited as the company builds production but will progressively increase.
Kirkby Tyres also has a limited choice of 65-series R-1 farm tyres from Israeli manufacturer Alliance. But it includes two sizes at the top-end for high horsepower tractors.
The Kléber Super 11L range is expanding with sizes for front and rear wheel mounting on four wheel drive tractors up to 180hp. Compared with existing designs, the new design has more squared shoulders, and curved lugs set at a slightly shallower angle.
The distinctive inter lugs are retained – they help with self-cleaning as well as affording tread protection – while block mouldings provide sidewall protection without detracting from the flexibility that enables the tyre as big a contact patch as possible under load.