Ultra low-profile models suit the latest tractors
Tractor tyres are getting
bigger. And they need to if
they are to cope with the
contrasting demands of
higher horsepower, more
weight, longer wear, greater
comfort and reduced soil
compaction. Here we kick
off this Wheels and Tyres
Special, reviewing the use of
ultra low-profile designs
LOW-PROFILE high-performance tyres are no longer confined to the sportscar and GTi hatchback; that sleek, mean look is now available for tractors.
Not so much for cruising the high street, though, more for extracting greater performance from the sophisticated tractors made these days.
The challenge for tyre makers has been to produce tyres that meet often contrasting demands. They have to cope with more power, provide better traction and not slip on the rim, yet fitting and removing them must remain a practical proposition.
They must also cope with faster road speeds and heavier weights, yet must wear slower and have self-cleaning properties in the field.
The latest ultra-low profile tyres shows that the time manufacturers have spent with sophisticated computer design software and test rigs is paying off. These tyres may still be black and round but they are significantly more sophisticated in the subtleties of internal construction and materials used.
As a result, users can expect improved traction and longer wear, greater comfort and reduced soil compaction and, particularly on the road, less vibration and improved directional stability.
Trelleborgs introduction of the bias construction Twin in ultra low-profile designs has been the catalyst for change. Fitted to purpose-made rims, these big tyres demonstrated the potential for making big gains in traction and reduced ground pressure by providing a big tread area between tractor and soil.
The Swedish firms range has been extended further with two sizes to fit the rear wheels of tractors in the 120hp to 160hp range. The 850/50-38 (8-ply rating) is designed to replace a 20.8-38 tyre using replacement rims. A 50% aspect ratio sidewall means the tyre has much the same rolling circumference, so will fit beneath mudguards and will not affect gearbox ratios unduly.
A similarly low sidewall design is the new 750/50-30.5 (8-ply rating) to replace the conventional 16.9-30 commonly used on the front wheels of high hp tractors. Again, size-matching avoids four-wheel drive transmission problems.
The alternative to Trelleborgs specialist tyres is to switch from standard fitments to larger but lower profile sizes that will fit the same wheels. The 65-series (or 65% aspect ratio) designs are extending this concept.
Goodyears DT820 and the Michelin XM108 started the ball rolling and other firms have followed suit; this type of tyre is now available from Kleber (Super 11L), Taurus (Point 65), Alliance (R-1 from agent Kirkby Tyres) and Pirelli with its TM800.
Similar in look to other Pirelli designs, with a familiar lug shape, lug end pattern and soil-shedding "shelved" tread rubber between the lugs, the TM800 represents latest conventional thinking in low-profile tractor tyre design which aims to combine high traction and directional stability with a suppleness promoting superior road comfort.
The Italian manufacturer again emphasises the high road mileages that tractors clock up these days, and the faster speeds involved, in describing the design priorities. Sidewall lateral rigidity is needed to avoid sloppy handling and re-stabilisation of the tyre during and after a sharp turn at speed.
Large contact patch
But, at the same time, the sidewall must be supple to provide a flexible cushion of air and allow the tread to form a large contact patch with the soil or road surface.
Pirelli claims particular effort in designing the bead. This is a difficult area given that the tyre has to be fitted and removed from tractor wheels but have sufficient bead/rim grip once inflated to prevent slippage on the wheel under enormous torque loads. Most radical of the new ultra low-profile designs is the Vredestein Traxion+, which also puts particular emphasis on the fact that tractors spend more time covering greater distances on the road and at higher speeds.
The evidence of this is in the distinctive tread pattern, with lugs overlapping and positioned closer to each other than usual towards the centre of the tread. That places more rubber at the centre of the tyre to reduce both wear and vibration.
Lugs then curve outwards to an acute angle at the shoulders to give plenty of grip in the field. They are further apart than usual towards the shoulder to maintain soil penetration and self-cleaning characteristics. Traction improvements over the 80-series tyres the Traxion+ sizes replace is put at typically 14%, with a 10% cut in rolling resistance owing to extra flotation on soft ground. *
Trelleborg Twin 750/50-30.5 front tyres give a large footprint.
Left: Goodyear DT820 600/65R28 front tyres matched with 710/70R38 DT820s help transmit this New Holland 8870s 210hp rated output. Right: Distinctive tread pattern of Vredestein Traction+ puts lots of rubber at the centre for hard-wearing, low vibration road travel.