The optimal tyre pressure plays an important role in getting the best out of heavy machinery.
For the same machine to perform at its best, one 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.
Nokian Heavy Tyres technical manual includes a chart for speed, pressure and load-bearing capacity for each heavy tyre. It shows that as the speed increases, the same tyre pressure carries less load. So, as a very practical example, working on the field at slow speeds calls for much lower tyre pressure than doing a high-speed road transit. 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.
When the machine is operated in an uneven terrain such as forest, the tyre pressure becomes an increasingly complex issue. Take a forestry forwarder for an example: 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 tons of timber. As the tyre pressure is the only thing carrying the weight in the tyre-wheel combination, the forestry tyres should be filled to their maximal operating pressure.
Nokian often witness that forestry machine contractors are not aware of the correct tyre pressure under full load. 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.
Regular tyre pressure monitoring is the key when you want to get the best service life out of your tyre investment. 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.
When a tyre pressure monitor is in use, the machine operator has more 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 – a few hundred euros compared to a few thousand. The outer tyre will live on and last for its intended service life.
Besides load, speed and operating surface, there is yet another variable: the tyre temperature. As a rule, the tyre pressure should always be checked when the tyre is cold. This applies to cars as well as to heavy equipment. How the tyre temperature affects its 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). Source