Commercial vehicle tyres and the label

The history of the EU tyre label is closely connected with the passenger car tyre segment. The criteria that led to standardised test procedures for fuel efficiency/rolling resistance, wet grip and external rolling noise primarily focused on the passenger car segment, and summer passenger car tyres in particular. Despite this, the label doesn’t just apply to C1, or passenger car tyres that are produced from 1 July 2012 (DOT 2712), but also to light commercial vehicle tyres (C2) and truck tyres (C3). Retreaded tyres, which account for a large percentage of Europe’s truck tyre market, are one of a number of exemptions to the labelling legislation.

While passenger cars are typically fitted with identical tyres on each wheel, the situation in the truck tyre segment is more complex. A tractor unit is generally fitted with steer and drive axle tyres whose tread designs significantly differ from another. The tread design on a steer axle tyre is characterised by grooves while blocks are the rule on a drive/traction axle tyre. It should come as no surprise that a tread pattern dominated by blocks will be inferior in terms of rolling noise and rolling resistance compared to a pattern made up of grooves, and this fact has been taken into account by the setting of higher limits for drive axle tyres. Continental’s description of its test procedure for the three label criteria, with additional explanations, is summarised below:

“Rolling Resistance

The legal test for rolling resistance is an indoor drum test in accordance with ISO (28580): Rolling resistance force divided by the tyre load [kg/t]. A test drum is driven by a motor at a constant speed. The freely rolling tyre is accelerated by the test drum. The power loss to keep drum and tyre running is determined by the engine. Configuration:

– 85% load applied
– 100% air pressure applied
– 2m drum diameter”

Only a few years ago the criterion predominantly influencing truck tyres was mileage. This factor is of course still extremely important for fleets and is meticulously documented by operators. But with the enormous increase in fuel prices the tide has turned and rolling resistance has also become a decisive factor in truck tyre sales; thus so far it seems ideal as a label criterion. But before jumping to any conclusion, remember that the truck sector contains numerous sub-segments, and within these the spectrum of demands placed upon tyres can quickly and completely change. For construction vehicles, operators certainly view aspects such as durability and puncture resistance as priorities. In regional applications rolling resistance accounts for about 25 per cent of total vehicle resistance, a far smaller proportion than for long distance transport, where it is closer to 40 per cent (and, incidentally, is harder to measure). Buses in urban service require kerbing-resistant sidewalls. The list goes on, but is incomplete without mentioning two fundamental qualities of premium truck tyres, namely retreadability and safety.

That said, most tyre developers express confidence that much potential exists to further optimise truck tyre rolling resistance. This is expected to occur through significant progress in research into rubber compounds. Whereas synthetic rubber has played a significant role in the production of passenger car tyres for a number of years, natural rubber is still king when it comes to commercial vehicle tyres.

So far industrial chemists haven’t cracked the secrets of natural rubber’s polymer chains and how they can be applied to the needs of truck tyres. The dominant point of view, however, is that the development of any advanced rubber compound will be mainly driven by rolling resistance. For material suppliers like Lanxess, a sizable field lies fallow. And in recent years premium tyre makers such as Continental have also notched up significant achievements in the area of compounding and invested heavily in this, for example in production processes for distributing fillers throughout the compound as evenly as possible (a process that is both time and energy intensive, and therefore one that increases production costs).

It should also be noted that when referring to passenger car tyres we talk about “full silica” products, while the silica content even in premium truck tyres is still only ten or 15 per cent. It is safe to say that enormous latent potential remains.

“Wet Grip

Wet performance is measured as braking distance ratio to the standard reference test tyre (SRTT) expressed as G (wet grip index). G is calculated for truck tyres as the average deceleration from 60 km/h down to 20 km/h of the candidate tyre in ratio to the SRTT. Continental’s main test facility is the Contidrom, an ISO-certified in-house test track.”

No tyre maker need worry about its products ending up in the worst category – G – printed on the label. This category is not used at all for commercial vehicle tyres. And when it comes to wet grip, another letter is also missing. Continental notes that for the foreseeable future no truck tyre capable of simultaneously achieving an A rating in both the wet braking and energy categories will be produced. The reason for this is the conflict of objectives between minimal rolling resistance and maximum grip in wet conditions. Both cancel each other out.

This is a classic conflict of aims that vexes tyre developers. Progress in compounding may be well suited to pushing the balance of all performances to a higher level, however the conflict remains both unsolved and, furthermore, its relationship to other qualities such as mileage is very complicated and developers – as is the case at Continental – must not just keep wet braking performance in focus, but also steering and handling performance, in order to produce a balanced and safe tyre.

“External rolling noise

The legal test for measuring external rolling noise requires an ISO-certified test track (ISO 10844). Continental’s test track at the Contidrom, the EU pass-by track, meets this ISO-standard. The testing procedure for noise consists of a set of runs. For example a truck must accelerate to 70km//h, then the engine is switched off and the vehicle coasts by a microphone at a distance of 7.5m from the tyre. The label classes differ by 3 dB(A). 3 dB(A) is about the minimum delta a human ear can distinguish when listening to sound examples one after another.”

Within the whole tyre labelling discussion, it seems interest is lowest for the third criterion, external rolling noise. This may have something to do with the fact that legislation pertaining to this was passed some time ago, independent of the label. Thus the classification of external rolling noise in the new EU tyre label requires the decibel limitation set out according to EU Directive 2001/43/EC as well as the coming standard in Regulation (EC) No 661/2009, which as of this year is mandatory in the original equipment segment and will come into effect in the replacement market in 2016.

The differences between passenger car and truck tyre products become apparent in light of the label criteria: At constant speed tyre road noise becomes the dominant source of noise from about 60km/h onwards, while this is not the case for trucks and buses until they reach about 90km/h. And even within the truck tyre segment itself the tread-related differences are obvious – block treads are inherently noisier than grooved treads. These differences do not, however, excuse tyre developers from their responsibility to minimise rolling noise levels by the use of specific tread designs. But at this point an editor could raise a point a tyre engineer would rather keep quiet about talking of rolling noise with certain vehicle operators, for example those working in construction sites, is quite ridiculous.