Nokian Tyres: Why is tyre pressure so important on heavy machinery?

With 28 years’ experience at Nokian Tyres, Ari Törmä knows much more than the shape and colour of the average heavy off-highway tyre (round and black, for those of you joining us for the first time). In his time with the Finnish manufacturer, the technical customer service manager has made tyres, tested their limits in an R&D capacity, and kept tabs on the health of customers’ tyres. He says this has taught him much about how tyres work structurally and what typically gives in. One of the most crucial points he emphasises is the importance of correct tyre pressures.

“The optimal tyre pressure plays an important role in getting the best out of heavy machinery,” states Törmä. “For the same machine to perform at its best, you may need to adjust the tyre pressures many times during the working day, depending on the work at hand, the weight of the work implements, the machine load and speed – just to give some examples. And of course, unlike cars, heavy machinery often has many axles and a weight distribution far from an even 50/50. Each axle has its own optimal pressure in different situations.”

Speed, pressure & load

The Nokian Heavy Tyres technical manual includes a chart for each heavy tyre’s speed, pressure and load-bearing capacity. It shows that as speed increases, the same tyre pressure carries less load. Ari Törmä notes, for example, that working a field at slow speeds requires much lower tyre pressure than travelling at higher speeds on the road. “When the tyre pressure is set optimally according to the speed and load, the tyre is safe, has the best grip, it does not warm up too much and it wears more evenly. On the other hand, just a single long road transport with a wrong tyre pressure can cause irreparable damage.”

One of the customers that Törmä visits is a Finnish farmer who sows crops very early in spring when the soil is vulnerable after winter. The sowing machine runs at just 5 km/h (3 mph) on the field, a speed that allows tyre pressure to drop as low as 0.4 bar (5.8 psi). “It is surprising how the effect to the soil is drastically smaller when compared to 0.8 bar (11.6 psi), which leaves much deeper tyre grooves to the soft field,” observes the technical customer service manager. He adds that while the farmer is very pleased with the reduced soil compaction, he does not drive on the road with that pressure under any circumstance.

Working a field at slow speeds requires much lower tyre pressure than travelling at higher speeds on the road (Photo: Nokian Tyres)

Off-road a different game

Tyre pressure becomes an “increasingly complex issue” when machinery operates on uneven terrain such as the forest floor, and to prove the point Törmä points to work performed using forestry forwarders: “On a level surface it is easy to define the optimal tyre pressure for each axle according to the evenly distributed load, but on the highly uneven forest floor the two rightmost tyres can be up in the air, leaving the two leftmost tyres to carry the entire load momentarily. These load peaks can leave just one tyre to carry tens of tonnes of timber.” As the pressure within the tyre is the only thing carrying the weight in the tyre-wheel combination, Törmä stresses that forestry tyres used in applications such as these should be filled to their maximal operating pressure.

“In my work, I often witness forestry machine contractors are not aware of the correct tyre pressure under full load,” Törmä continues. “The load peaks in the forest lead to the tyre sidewall flexing too much, causing sidewall damage. If the tyre pressure is, say, half of the maximal operating pressure, the damage is considered accidental and is not covered by the tyre warranty.”

The importance of monitoring pressure

Ari Törmä describes regular tyre pressure monitoring as the key to getting the best service life out of a tyre investment. Not keeping an eye on pressure levels can be costly: “It is often the case that a forwarder operator is picking up the timber left by a harvester, and suddenly one tyre starts to lose pressure. Unfortunately, the operator cannot see it happening or feel it in any way until the track falls off from the bogie. This can mean that a forestry tyre is damaged beyond repair. And what’s more, you still need to limp home from the forest. After that, nine out of ten times the tyre must be replaced.”

Using a tyre pressure monitor gives the operator additional options. They can unload the forwarder in time and drive home before the damage gets too severe. “In that case, they often get away with just replacing the tube” at a cost of just a few hundred pounds instead of several thousand. “The outer tyre will live on and last for its intended service life.”

Check pressure at the right temperature

In addition to load, speed and operating surface, tyre temperature is a variable that needs to be looked at. Ari Törmä shares that, as a rule, tyre pressure should always be checked when the tyre is cold – something that applies to cars as well as to heavy equipment. He adds that the affect tyre temperature has upon tyre pressure depends on the size of the tyre’s airspace.

“In a forestry tyre, the pressure rises approximately 0.17 bar (2.4 psi) for every 10 degrees Celsius (50° F). In harbour tyres, the pressure rises about 0.35 bar (5 psi). Only a few moments of working can lift the inner temperature of a harbour tyre from 0 to 80 degrees Celsius (32° to 176° F). This causes false results when checking the pressure – 10 bar (145 psi) appearing as 13 bar (188 psi).”

The technical customer service manager relates the experiences of machinery operators in a Dutch terminal. The pressure of their reach stacker tyres was monitored every Monday after the night shift, when the tyres were still warm. This gave too high a tyre pressure reading, and the compressor would not fill them beyond 10 bar (145 psi). The maintenance manager changed the routine and started checking tyre pressures first thing Monday morning, when the tyres were properly cold. It turned out the tyre pressures were typically 8.5 bar (123 psi) instead of the recommended 10 bar (145 psi). “This was enough to lower the tyre load-bearing capacity and service life in the long run. Just a simple change can make a big difference to the total cost of ownership of the machine.”

Ari Törmä, technical customer service manager (Photo: Nokian Tyres)

It’s the air that matters

In container handling, excavating work, forestry work, backhoe loaders and many others, it is important to have a stable foundation for your work. Besides tyre structure, the correct tyre pressure plays a significant role here. Törmä notes that only a tyre that has optimal operating pressure can absorb the shocks and maximise working accuracy.

“In conclusion, it always pays off to know what is going on inside your tyre. The airspace inside is the only thing carrying sometimes tens of tons of weight, so you should check and adjust the tyre pressure according to your work – at the right time, on regular intervals and when the tire is cold. It will reward you with safer and more efficient working, lower tyre costs, lower fuel consumption, less unexpected downtime… and the list goes on.”