Setting agricultural tyre pressure by axle load

Set agricultural pressure from the weighed load on each axle, at the speed the machine actually works at, read off the tyre manufacturer’s own load and inflation table. There is no single correct pressure for a tractor tyre, and the number on the sidewall is not it.

That is the whole method. The rest of this guide is how to get an axle weight you can trust, how mounted implements and ballast move it about, and what to do about a machine that has to work in a field and then drive 20 miles.

A load index is a single point on a surface: the maximum load at one reference inflation pressure and one reference speed. For a truck tyre that is a fair shorthand, because a truck tyre spends its life at one pressure doing motorway speeds. An agricultural tyre does not.

The same 710/70R42 might carry a heavy mounted plough at 8 km/h in the field, then a fraction of that load at 50 km/h on the road at more than twice the pressure. Its capacity in those two cases differs enormously, and the sidewall marking describes neither exactly. What describes them is the manufacturer’s table: rows of load against columns of pressure, for a stated speed — usually separate tables for 10, 30, 40 and 50 km/h, and separate tables again for cyclic field operation where a CFO marking applies.

A public weighbridge will do the job, and most merchants and grain stores have one. The sequence is worth doing in this order:

• Level ground, machine in working trim: full fuel and AdBlue, water ballast in, wheel weights on, front block fitted, operator in the seat.
• Front axle onto the plate alone, then the whole machine; the rear is the difference. Weigh the rear alone as a check — the totals should agree within the bridge’s tolerance.
• Do it twice for every implement: once in transport position and once lowered, where the distribution changes again.
• Weigh trailers, tankers and spreaders full and empty, and record the drawbar or hitch load separately. That load lands on the tractor’s rear axle, not on the trailer’s.

Corner weight pads tell you something a weighbridge cannot: the left-to-right split. An offset mower, a side tank or a plough carried out to one side puts considerably more on one tyre than on its partner, and pressures should be set from the heavier corner.

A three-point linkage is a lever. A rear-mounted implement hangs behind the rear axle, so the rear takes the implement’s weight plus the weight it levers off the front. A one-tonne mounted implement can add well over a tonne to the rear and take several hundred kilograms off the front. Front ballast puts that steering weight back, and lightens the rear a little in turn.

Consequences for pressure setting:

• Rear pressures set from an unladen weight will be far too low once the implement is on.
• Front pressures set with the front block fitted will be too high when it comes off for a lighter job.
• Liquid ballast is carried by the tyre, so it counts as load in the table lookup — and it cannot be taken out for a wet field in the way wheel weights can.

Ballast is a separate exercise, but the two interact. Aim for enough total weight to put the power down without carrying weight you do not need, then a sensible front-to-rear split — commonly quoted around 40:60 for heavy draft work with a rear-mounted implement, and nearer even for loader and transport work. Use the tractor maker’s own figure, then set pressures for whatever that ballasting produced.

The field check is wheel slip, which most tractors display. For heavy draft work in reasonable conditions the usual target is in the region of 10 to 15 per cent. Consistently below that and the machine is over-ballasted, burning fuel carrying iron. Consistently above 20 per cent and it is under-ballasted or the tyres are too hard, and a large share of the engine’s work is going into scrubbing lugs off the tread.

The two jobs pull in opposite directions. In the field you want the lowest pressure the load and speed allow: long footprint, low contact pressure, low slip, minimal surface compaction. On the road you want the pressure up — at 40 or 50 km/h the sidewall flexes many times a second and heat is what kills the casing, and higher pressure also stops the tyre squirming under braking with a loaded trailer behind it.

For the same tyre the field figure and the road figure can easily differ by a bar or more. That gap is the problem, because a contractor’s day is field, road, field, road.

Three practical ways out. If the machine is mostly field work, set it for the field and keep road moves short and slower — with the maker’s low-speed table in front of you, not a guess. If it does long road legs, carry a compressor and reset at the headland. If it does both all day, the case for VF casings or central tyre inflation gets serious, because a VF tyre narrows the gap between the two figures — see IF and VF agricultural tyres explained.

A central tyre inflation system does exactly what it claims: it lets the operator raise and lower pressure from the cab, in motion, so the machine can run genuinely low in the field and genuinely correct on the road. Where a high-hour machine does substantial road transport between fields — a slurry outfit, a contractor’s tractor, a self-propelled sprayer — the case is real, and it is made on fuel, on field capacity and on protecting the soil, not on tyre life. It is not a cheap fit: a compressor with enough duty to move real volume, plumbing to each wheel through a rotating union, valves and control, all of it living in mud. Re-inflating a big tyre takes minutes, so the system is used ahead of the headland rather than at it, and the rotating seals are a wear item. On a farm tractor that rarely leaves the yard the payback is poor, and a compressor and a decent gauge do the same job.

When the question is which tyre to fit rather than what pressure to run, start from the weighed axle load and work out the minimum load index. That is what this calculator is for.

On duals, remember the pair never shares load perfectly, so the dual figure is the one that applies. And if a size change is on the table, check the rolling circumference front to rear before you order anything — the rolling circumference calculator and the guide that goes with it cover the lead ratio problem.

• Check pressures cold. A tyre straight off a road run reads high, and setting it then leaves it under-inflated in the morning.
• Use a gauge with resolution at the bottom of the range. A truck gauge reading in half-bar steps is unsuitable when the target is 0.8 bar; a large-face low-pressure gauge with 0.1 bar divisions costs very little.
• Write the figures down — a card in the cab listing the pressure for each implement stops the setting drifting back to whatever it was last summer — and re-check after any change of implement, ballast or trailer.