Lower soil pressure? VF tires under trailers work, but mainly shallow

What is the effect of a VF tire under a trailer on the soil? Future farming compares the effect of VF tires with standard radial tires. – Photos: Michel Velderman

A VF tire under an agricultural trailer primarily results in lower soil pressure in the upper soil layer. The deeper you look into the ground, the smaller the difference compared to a standard radial tire. That is the conclusion after an extensive tire test in which the test team compares the effect of VF trailer tires with standard radial tires.

Suppose the tires of your agricultural trailer need replacing. Besides choosing between different tread patterns and details, you have to decide whether to opt for a relatively inexpensive option (standard radial tires) or dig deeper into your pockets and go for a set of VF trailer tires that are 65% more expensive. It is widely known that a tire with a VF (Very Flexible) designation has a flexible carcass, allowing you to drive with tire pressure up to 40% lower than standard radial tires. In other words, the carcass of the VF tire can better handle deformation, which is why you can drive with such low tire pressure. The question is: what does this do to the soil?

To find out, Future Farming conducted a series of measurements. We drove a loaded VGM 22-ton trailer over a buried mat with pressure sensors. Looking at the measurement results, you can draw the following conclusions:

At 8 centimeters below the ground surface, we measure a significant difference in soil pressure between the VF tire and the ‘regular’ tire.
When we measure deeper (25 centimeters deep), those differences are much smaller.
Investing in a VF trailer tire makes sense especially if you combine it with a tire pressure adjustment system.

How the Test was conducted

This tire test was performed with a VGM LK22 agricultural trailer (22-ton load capacity) equipped with Alliance Agriflex 389 VF 650/55 R26.5 tires. For the test, the trailer was loaded with soil up to a wheel load of 4,500 kg per tire, equivalent to a load of potatoes or onions. Tire service company Zuid West Banden in Klaaswaal (The Netherlands) provided a set of Galaxy Flotstar 650/55 R26.5 tires and supported the test with equipment, service, and expertise.

Tonnis van der Veen of tire manufacturer Bridgestone performed the measurements using a so-called Pressure Mat, a mat measuring 97 × 97 centimeters containing 22,000 pressure sensors. It measures the pressure distribution under the tire. The measurement was performed multiple times: with different tires, different tire pressures, and at various depths.

Specialists from tire manufacturer Alliance were present for advice and assistance. Our great thanks go to L. Kats Agroservice in Numansdorp (The Netherlands) for providing the location and materials.

Three tire pressures

The starting point of this test is the harvest of arable crops in the autumn. That means driving over loose soil — we did not test the effect of a grass sod in the top layer.

For this test, we used a VGM LK22 agricultural trailer with a 22-ton load capacity, loaded with soil to a wheel load of 4,500 kg per tire. We compared two types of trailer tires. As the standard radial tire, we used Galaxy Flotstar 650/55 R26.5 tires, which, assuming a road speed of 40 km/h, according to the inflation table, must be inflated to at least 2 bar. We compared these with Alliance Agriflex 389 VF 650/55 R26.5 tires, which, due to their VF carcass allowing more deformation, may be inflated to 1.2 bar according to the inflation table—even at 40 km/h.

Finally, we conducted measurements with the Alliance Agriflex 389 VF 650/55 R26.5 tire at a tire pressure of 0.8 bar. This is the pressure at which the tire may carry this wheel load up to 10 km/h — which you can do if you use the VF tire in combination with a tire pressure adjustment system. For indication: the set of four Alliance VF tires is, in this case, about €3,000 more expensive than the set of Galaxy standard radial tires. Because we focus on the effect on the soil, we used for each tire the minimum pressure that the manufacturer recommends in combination with the maximum allowable weight at that pressure. By the way, note that Galaxy is a sister brand of Alliance.

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For the test, we used a Bridgestone mat of about 1 by 1 meter, containing 22,000 sensors. This allows us to visualize the pressure distribution under the tire. We measured on hard surfaces and at depths of 8 and 25 centimeters.

Shallow depth: VF provides acceptable pressure

The measurements were conducted with a so-called Pressure Mat from Bridgestone. This is a mat roughly 1 by 1 meter containing 22,000 pressure sensors, allowing us to determine the pressure distribution under the tire. For the first measurement, we buried the mat at a depth of 7 to 8 centimeters and reversed the trailer over it. The Galaxy tire at 2 bar gave an average soil pressure of 1.3 bar at this depth. After changing the wheels — the wheel load remained exactly the same — we measured an average soil pressure of 0.9 bar with the Alliance VF tires at 1.2 bar. When we reduced the tire pressure to 0.8 bar (possible with a tire pressure adjustment system), we measured a pressure of 0.7 bar.

This means that if you reduce the tire pressure by 40% (from 2 to 1.2 bar), you reduce the soil pressure at this depth by 30%. Additionally, you can state that with the VF tires at this wheel load and tire pressure, you have acceptable soil pressure, whereas with the standard radial tires, you do not. At least, if you use the old rule of thumb that a soil load of up to 1 kilogram per square centimeter can be remedied with mechanical soil tillage. Finally, if you have a tire pressure adjustment system, you can reduce the tire pressure by 60% (from 2 to 0.8 bar), thereby reducing the soil pressure by 46%.

As an experiment, we also measured the rut depth in the loose soil at the center of the tread. The Galaxy tires made a rut of 10 centimeters, the VF tires at 1.2 bar made a rut of 6.5 centimeters, and those same tires at 0.8 bar made a rut of 5.5 centimeters.

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Because we are particularly curious about what happens beneath the ground, we bury the mat with pressure sensors at different depths. We then cover the mat with a flat layer of soil, either 8 or 25 centimeters thick. Midway through the test, the wheels are swapped, ensuring the wheel load remains exactly the same.

VF Tires and Transport: choose a tire pressure Adjustment system

The main argument for choosing VF trailer tires is that you can drive in the field with low tire pressure. According to the inflation table, the VF tire at 1.2 bar with a 4,500 kg wheel load may be driven at 40 km/h on the road. During the harvest, you can, therefore, drive at 1.2 bar all the time to spare the soil. Note: that is the minimum required tire pressure at which the tire can tolerate its deformation—it is not a recommendation.

Be aware that with that low tire pressure, the contact area with the asphalt is 34% larger than with the Galaxy tires at 2 bar. This results in higher rolling resistance on the road, higher diesel consumption, and more tire wear. The four ‘soft’ tires also affect the vehicle combination’s handling.

In short, it is highly recommended to combine the VF trailer tires with a tire pressure adjustment system. Only then can you lower the tire pressure even to 0.8 bar and achieve a significant difference in soil pressure while increasing the pressure on the road for good transport properties.

Notably, we observed cracks forming in the ruts left by the Galaxy tires at 2 bar in the field. This indicates that the high tire pressure results in higher rolling resistance in the field. We did not see these cracks in the ruts of the VF tires at lower pressure.

Here, we measured the footprint on a hard surface. The tablet shows exactly how large the contact area with the asphalt is and where under the tire most of the weight rests. It’s important to realize that on the road, the footprint of the VF tire at 1.2 bar is 34% larger than that of the Galaxy tire at 2 bar. This is advantageous in the field but disadvantageous on the road.

Little difference in soil pressure at depth

When we repeat the measurement at the depth of the plow pan (25 centimeters), we see a different picture. At this depth, we measured a soil pressure of 0.6 bar with the Galaxy tires at 2 bar. With the VF tires at 1.2 bar, the measuring equipment indicated 0.5 bar, and when simulating the use of a tire pressure adjustment system (0.8 bar), we measured 0.4 bar. So you can say that if you reduce the tire pressure by 40% (from 2 to 1.2 bar), the soil pressure at this depth is reduced by 17%. If you reduce the tire pressure by 60% (from 2 to 0.8 bar), you reduce the soil pressure by 33%.

The conclusion is that the difference in soil pressure between a regular tire and a VF tire becomes smaller the deeper you look into the soil. The lower tire pressure and the accompanying greater deformation and thus contact area mainly affect the soil pressure in the upper layer. It is the high wheel load that causes compaction in the deeper soil layers. If you want to improve soil compaction in the deeper layers, there’s little else to do but ensure a lower wheel load—in other words, load the trailer less heavily.

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As an experiment, we tested the Galaxy tires (2 bar) before and after emptying the trailer. Remarkably, although the weight is almost 75% lower, the soil pressure is only 23% lower.

Empty trailer almost as bad for the soil

Anyone who thinks that you can drive over the land with an empty trailer without worrying about soil pressure is mistaken, as our measurements show. When we drive over a hard surface with a full trailer (4,500 kg wheel load), the pressure mat measures an average soil pressure under the footprint of 4.7 bar. The VGM trailer weighs 6,750 kg empty. You might think that if you tip the bed empty, the remaining soil pressure is negligible. But that’s not the case: the soil pressure with an empty bed (thus about 75% lighter) is only 23% lower (still 3.6 bar). This is because, along with the decrease in wheel load, the contact area of the tires also halves because the tires deform less. The soil pressure is determined by the wheel load divided by the contact area. In other words, if you want to prevent an empty trailer from causing relatively high soil pressure, you must reduce the tire pressure. This clearly proves that soil pressure, besides weight, is significantly influenced by air pressure.