Introduction: Tread shape, inflation pressure, and contact area decide whether a rough terrain forklift floats across soft ground or digs itself in.
A loaded forklift crossing a rain-softened yard tells the story faster than any spec sheet. The engine, the mast, and the hydraulics all push in the same direction, but the only parts touching the ground are four tires, and loose soil responds to them in ways pavement never does. Mud, sand, and wet topsoil deform under load, so the machine either spreads its weight and keeps rolling or cuts a rut and starts sinking. Tire shape, tread pattern, inflation, and contact pressure are the variables that decide which one happens, and they also explain why a bigger tire is not automatically the better tire.
How Tire Shape and Tread Pattern Interact with Soft Ground
A tire working on loose soil is really a pressure-spreading device. Soil has a limited bearing strength, and when average pressure under the tread climbs past that limit, the ground fails: the tire sinks, soil rises around it, and the machine starts fighting its own ruts. Every part of tire geometry — sidewall height, section width, lug depth, lug spacing — is a way of managing that pressure and keeping whatever grip is available. The 16/70-20 tire fitted to the 4-ton class Telstone T40 is a useful example to read closely: 16 inches of section width, a 70 percent aspect ratio, and a 20-inch rim, which adds up to a tall, wide, air-filled tire with room to deform.
1. Pneumatic Tires Deflect to Create a Longer Contact Patch
The reason a pneumatic tire beats a solid one on soft ground is deflection. Air carries the load, the sidewall flexes, and the tread flattens against the surface. The contact patch stops looking like a small oval and stretches into a longer footprint. Since ground pressure is load divided by contact area, that longer patch lowers the average pressure the tire applies — the same 4,000 kg spread over more soil. A tire at lower pressure deflects more and spreads further, which is why soft-ground operators often run reduced pressure. The trade-off is that a very soft tire squirms more, heats up faster, and rolls less precisely on firm ground, so the setting is always a compromise between floatation and stability.
2. Large Tread Lugs Clear Mud Before Grip Disappears
Tread lugs on soft ground do two jobs. They bite into the surface and shear it, which is where forward force comes from, and the gaps between them give loose material somewhere to go. A shallow, closely spaced pattern fills with wet clay quickly, and once the tread is packed smooth the tire behaves like a slick, spinning on top of the surface. Deep, widely spaced lugs stay open longer and eject mud as the tire rotates. That aggressiveness has a cost, because on hard or paved surfaces a coarse lug pattern touches the ground at fewer points, which raises local pressure, increases vibration, and wears the tread faster. Since most outdoor yards mix firm tracks with soft patches, the pattern gets chosen for the worst ground the machine will regularly meet.
What Rolling Resistance Tells You About Loose Soil Travel
Rolling resistance is the force needed to keep a wheel rolling at a steady speed, and on soft ground it is dominated by the soil rather than the tire. On concrete, most of the loss comes from the tire's own internal friction as rubber flexes and recovers. On mud, sand, or fresh topsoil, the tire has to push a small wave of material ahead of it, press the surface down, and then climb out of the depression it just made. Published rolling-resistance coefficients for rubber tires on hard surfaces sit around a hundredth of the vertical load, while loose sand and mud push the same coefficient an order of magnitude higher or more. That difference changes how a forklift feels and works. Higher rolling resistance means more engine power goes into making ruts instead of moving loads, so fuel consumption rises and travel speed drops on the same machine, with the same operator, carrying the same pallet. Heat also builds in the tires during long hauls. FAO soil and water guidance on agricultural traction makes the same point from the farming side: on soft soils, a large pneumatic tire at a sensible pressure limits soil deformation and improves tractive efficiency, because less of the engine's output disappears into compacting the ground. Repeated passes make it worse, since every pass deepens the rut and the next tire has to climb a steeper wall of soil.
Why Tire Size Alone Does Not Guarantee Better Traction
Bigger tires are often treated as a cure-all, but size only sets the potential. Actual ground pressure is the real load divided by the real contact area, so inflation pressure and axle load decide just as much as diameter. Two machines can run the same 16/70-20 tire and behave completely differently: one lightly loaded at low pressure floats, while the other at full rated capacity with hard tires cuts straight through the surface layer. Occupational guidance on industrial lift trucks treats ground condition as a selection factor for exactly this reason, because the soil under the wheels is part of the machine's working envelope. Floatation and traction are also different goals. A tire that spreads its load well on wet clay may still spin, because clay shears easily and there is little for the tread to grip. On thin soft soil over a firm base, a narrower and more aggressive tire often pulls better than a wide floatation tire, which sits on top of a weak layer without reaching anything solid. Where sinkage is the limit, size and pressure help most. Where soil shear strength or slope is the limit, tread bite and weight distribution matter more. A larger tire also raises the axle and changes the gearing effect at the ground, so more engine effort is needed for the same pull. A rough terrain forklift manufacturer sets the starting point with tire size and tread for a rated capacity and an expected ground type, and the machine's real soft-ground behavior comes from how pressure and load are managed afterwards. Soil moisture, load, inflation pressure, and tread wear shift performance from one day to the next, which is why tire choice is a match to a site's worst condition rather than a fixed rule.
Conclusion
Tires are where engine output finally becomes motion, and on loose soil they are doing two jobs at once: spreading load and finding grip. Deflection and sidewall height decide how large the contact patch grows and how low ground pressure drops. Tread lugs decide whether the tire keeps biting or packs itself smooth. Rolling resistance decides how much of the engine's power is left for the load after the soil has taken its share. Size helps mainly when sinkage is the limiting factor, and it stops helping when the problem is weak soil or a slippery base layer. Readers who want to see these ideas on a real machine can look over the published T40 specifications, where tire size, ground clearance, and rated load sit side by side.
FAQ
Q:Why do rough terrain forklift tires matter on loose soil?
A:Loose soil fails when pressure under the tread exceeds what the ground can bear. Tires are the only contact point between the machine and that surface, so their width, sidewall height, tread pattern, and inflation set the real contact area and the pressure applied to the soil. Get those wrong and a 4-ton machine sinks, cuts ruts, and loses forward force even with a strong engine.
Q:What does rolling resistance mean for a forklift on mud?
A:It is the force needed to keep the wheels turning at a steady speed. On mud and soft sand, most of that force goes into deforming the ground, pushing a wave of soil ahead of each tire and compacting a rut behind it. The practical result is higher fuel use, lower travel speed, more tire heat, and less pulling power left for the load.
Q:Are larger tires always better for rough terrain forklifts?
A:No. Large tires give more room to deflect and spread load, which helps when the ground fails by sinking. But on thin soft soil over a firm base, or on wet clay that shears easily, a wide floatation tire can spin while a narrower, more aggressive tread pulls better. Inflation pressure, axle load, soil moisture, and tread wear all change the outcome.
Sources / References
4. Aquifer recharge with wastewater
General guide for industrial lift trucks | Safe Work Australia