Goodyear is once again moving forward in the agricultural tyre world with the introduction of what is something of a departure for the brand, a range of wheel centres to fit its Low Sidewall (LSW) tyres.
As tractors get bigger, there is a growing paradox between the amount of power available and the ability to apply it to the ground, the two most important factors influencing the latter being the pressure applied to the soil and the contact area.
Increasing both the weight and the contact area has been the answer to increasing traction up.
From the introduction of rubber tyres in the 1930s, Harry Ferguson’s patented three-point linkage was the first major attempt to address this issue without attaching ballast onto a tractor.
Along with his engineer, Willie Sands, he developed a means of transferring the downward force of the plough to the rear axle of the tractor, greatly increasing traction without adding to the overall weight of the combination.
His system has since gone on to be the basic design concept around which all standard tractors are designed and built, but as well at it has served agriculture over the decades, it is now running up against the increasing weight of motive units and a growing awareness of the damage done to soils by compaction.
Power-to-weight ratio
The Ferguson TE20 of 1946 weighed 1,250kg, or 22hp/t, while a 200hp tractor of today will weigh up to 8t and give 25hp/t.
On the face of it, that is not much of an improvement in power-to-weight ratio over the intervening 75 years, yet with increasing power comes the need to apply it to the ground and so powered front axles are required, and they are not light.
Implements big and heavy enough to utilise that extra power have also been developed and these need a robust frame to be hung from and engines are no longer up to the job of holding a tractor together, a heavy duty chassis is now required which adds further weight.
All these factors have led to a considerable weight increase, a problem which has been addressed by reducing ground pressure in the field through increasing the contact area of the tyre and soil surface.
Pressing matters
Larger wheels and adjusting the tyre pressure have been the answer; tracked vehicles are also becoming more common but they are more expensive and not so well suited to road conditions.
Yet there is no such thing as a free lunch; big wheels and tyre pressure adjustment bring problems of their own. The bigger the wheel, the greater the tyre volume and so the longer it takes to adjust the pressure, as, unfortunately, the valve stem diameter has not altered with tyre size.
The ease of central tyre inflation systems has gone some way to answering this conundrum and when combined with low pressure and high flexion tyres, it is far more likely that the optimum air pressure and ground contact area will be set for the field conditions.
On the road
Other problems associated with large tyres is that when in road use they can cause handling problems, especially at speed.
The bouncing and swaying associated with large tractors running on tarmac is referred to as road lope and has several causes, although under-inflation is most often the problem.
It is worth remembering that HGV tyres will often run at 100psi or more, albeit with smaller wheels.
Another contributory factor is the rigidity of a tyre’s sidewalls; the more they are able to flex, the greater they are likely to cause road lope, and it is the very high flexion tyres that help with traction that have the most flexible sidewalls.
This also lies at the root of power hopping, which is the bouncing of a tractor when a load is suddenly applied to the hitch, the stretching and recoiling of the sidewall induces these shock movements as the lugs grip and then releases as the wheel turns.
Goodyear reengineers the tyre
To counter this, Goodyear introduced its LSW range of tyres that are, in effect, low profile tyres for tractors as they have a reduced aspect ratio.
A lower sidewall reduces the amount of rubber there is between the rim and the tread, helping to eliminate many of the problems associated with a tall sidewall as there is less material to flex and move as the tractor dynamics change.
This range of tyres has worked well and is a fixed part of the company’s range, but now they have been joined by purpose-built wheels to mount them on.
The company has chosen to offer them as waffle type wheels – these revert back to the idea of having separate rims and centres that can be affixed to each other in eight different configurations, providing eight different track widths.
Further details of the range will be given at Agritechnica where they are to be launched, but the company has already indicated that the design “allows tyres to seat correctly at lower pressures, improving safety”.
Rim slip
This counters another problem associated with running tyres at low pressure, and that is the tendency for the rims to slip within the tyre as there is not sufficient pressure to maintain the grip between the rim and tyre bead.
Off-roading enthusiasts have the option of clamping tyres to the rims with the use of beadblock wheels, but this, as yet, has not been offered as a solution on tractors.
The old trick of ‘welding’ tyres to a rim is fraught with safety issues and should not be attempted.
We will have to wait a couple of weeks before getting to see how Goodyear has approached the challenge of maintaining grip between the tyre and its rim at low inflation pressures, but the fact that a tyre company is adding wheel rims to its range may come as something of a surprise.
Goodyear agricultural tyres, however, are made under licence by the American Titan Tire Corporation of Illinois, a company that already makes wheel rims, so they are a product that already exists within the corporation.