The weather is freezing, you can see your breath and engines are roaring. We are in Ivalo, testing the winter performance of tyres on heavy machines such as tractors.
Product Manager Matti Serenius of VTT Expert Services Ltd, who is supervising the tests, is happy with the conditions. "We usually test tyres for winter grip here, in Finnish Lapland. Here we have plenty of ice and compressed snow tracks for tyre testing, as well as guaranteed winter conditions."
Traction and lateral grip are the most commonly measured properties of work machine tyres. In addition, we occasionally drive along a so-called 'handling track', where we assess the driving and traction properties of the tyres in practical situations.
"Customers of our testing activities include both Finnish and overseas tyre manufacturers. The range of tyres tested covers everything from tractors and loader tyres to those of excavators. Testing is performed during product development, when makers want to be sure about the traction properties of a new tyre model," says Serenius.
For example, tractors and loaders are often used to maintain streets. This has increased the need to develop the winter properties of tyres. It is not unusual, nowadays, for contractors to change their tractor tyres in winter to ones specially made with winter maintenance in mind.
Measuring tyre traction in practice
Measuring traction involves measuring the wheel slip and tractive force of a tyre. Measurement is performed by fitting the test tyres onto a tractor weighted for axle masses in accordance with the tyre's load capacity, and a truck is used to brake the tractor's motion.
Measuring the lateral traction of tyres in practice
This measurement evaluates the lateral traction of the tyre, i.e., the steerability of the tyre and how easily the machine slides sideways. Measurement involves driving the tractor and a truck alongside each other at the same speed, with a winch cable and force sensor between them. As the tractor is steered away from the driving direction of the truck, the tractive force grows, generating a force curve as the function of the steering angle.
Testing also performed in summer conditions
Tractions are also measured in summer conditions. The performance of a tractor tyre on fields or that of an excavator in soft sand create challenges in practice; tyre manufacturers are seeking answers to these issues based on the related measurements.
In addition to traction, a large number of measurements of properties such as vibration, fuel consumption and vibration damping are performed.