All-season tyre test puts ‘seemingly outrageous’ 4mm tread recommendation under the spotlight

With the introduction of the CrossClimate + earlier this year, Michelin made the claim that the snow traction its tyre delivered with a tread depth of only 1.6mm was equivalent to that provided by competitor tyres with tread depths of 4mm. Furthermore, Michelin stressed that its tyres are made to be used down to the legal tread depth limit of 1.6mm. And why not? Changing earlier is simply throwing away good tread rubber.

Michelin questions why a number of manufacturers, including the makers of some premium brands, recommend tyres used in winter be switched at 4mm. And it’s not alone in wondering this – when presenting its 2017 all-season tyre test, German car magazine Auto Bild asked if it was really necessary to swap a tyre when half its tread still remains.

“If the tyre makers had their way, it’d be close of play at half time,” observed testers Dierk Möller and Henning Klipp in issue 47/2017 of Auto Bild. Quitting the pitch and going home after just three quarters of an hour is unimaginable, however the pair note that this is exactly what the “rubber princes” recommend when motorists buy new tyres. “What lies behind this seemingly outrageous recommendation from the tyre industry to use tyres only to a minimum tread depth of 4mm?” the testers ask.

To find the answer to this question, Auto Bild didn’t just test new all-season tyres in this year’s test, it also tested the products when their tread depths had reached 4mm and 2mm. As testing at three various tread depths trebled the number of tyres involved, Auto Bild restricted its evaluation to six size 185/65 R15 tyre models – the Goodyear Vector 4Seasons Gen-2, Hankook Kinergy 4S, Michelin CrossClimate +, Nokian Weatherproof, Pirelli Cinturato All Season and Vredestein Quatrac 5. Continental’s new all-season tyre, the AllSeasonContact, was not tested; Dierk Möller says its absence can be explained by the tyre’s relatively recent arrival in the market. Auto Bild began evaluating at various tread depths back in February and conducted its snow testing in New Zealand during the European summer, too early in the year for the Continental tyre to participate.

A buffing machine was used to bring the 4mm and 2mm examples to exactly the right tread depth and the tyres were then driven for 1,000 kilometres on open roads prior to testing. All tyres were subject to wet, dry and snow tests, with four disciplines examined in each category. Cost considerations – total mileage and cost per mile, as well as rolling resistance – were also taken into account.

Only 2 tyres safe down to the limit

What the Auto Bild testers found was that with four of the six candidates, it was “on grounds of driving safety” advisable to replace the tyres when the tread reached the half-way mark. Only two products demonstrated safe driving at a low tread depth. These were the CrossClimate + from Michelin – vindicating the claims made by the ‘Long-Lasting Performance’ campaign – and the Goodyear Vector 4Seasons Gen-2.

The Michelin tyre was crowned the test winner. Given a rating of ‘exemplary’ for its performance when new, the CrossClimate + delivered sufficient performance to warrant an overall rating of ‘good’ when factoring in the results for the 4mm and 2mm tread depth tyres. This is a better result than four of its rivals achieved with factory-fresh tread levels. The loss of traction experienced with the CrossClimate + at worn tread depths was much lower than with some of the test candidates, and in various other disciplines the 2mm tread depth Michelin tyre delivered performance levels similar to those of the other candidates at 4mm. While all used tyres experienced a drop in performance in most areas (the exceptions being dry handling and braking, and rolling resistance), Auto Bild found that when it came to performance degradation, the CrossClimate + suffered less than the others. The publication considered it an “exemplary all-round talent with well-balanced driving performance and the lowest loss of performance over an entire lifetime.” The tyre showed some weakness in aquaplaning at 2mm, and its high purchase price was counterbalanced by good mileage.

The other model deemed safe when approaching the legal tread depth limit was the Vector 4Seasons Gen-2. With a rating of ‘good’ in new condition and ‘satisfactory’ overall, the Goodyear tyre was ranked second in the test and was described as an “all-rounder with the best driving performance on wet and snowy surfaces” when new and at a tread depth of 4mm. Grip levels in the dry, which Auto Bild considered “modest” to begin with in the new tyre, improved when the Vector 4Seasons Gen-2 was tested at 4mm and even at 2mm. The testers spoke of value for money thanks to “good mileage and a moderate price.”

Recommendation only conditional when worn

That was the good news. What followed suggests that some manufacturers realise their own products’ limitations when recommending a changeover at 4mm, and by giving this counsel they effectively expect consumers to compensate for their own shortcomings in research and development.

“For four of the six candidates, it is advisable on grounds of driving safety to swap as early as half tread depth,” wrote Möller and Klipp about the remaining tyres in the test. When taking performance at all tread depths into account, the testers were unable to grant the four tyres anything above a ‘conditionally recommended’ rating. All four were considered ‘satisfactory’ when tested with fresh tread.

According to Auto Bild, it was “only in new condition” that third-placed tyre the Pirelli Cinturato All Season delivered “decent performance on snow and good grip in the wet.” The tyre’s winter qualities were said to “slip into the ‘red zone’ at a remaining tread depth of 4mm.” The Pirelli tyre was also marked down for its “low mileage and high rolling resistance.”

The Hankook Kinergy 4S was credited with safe driving characteristics on snowy and wet surfaces, albeit only with full tread. “At reduced tread depths the characteristics improve on dry road surfaces, but performance levels on snow and in the wet sink,” reported the testers, who also noted disappointing mileage from the Hankook tyre.

It was a similar story with the Vredestein Quatrac 5. Its performance when new was considered well balanced, however winter and wet performance was strongly reduced at 4mm and 2mm tread depths. Safety reserves during aquaplaning were also found to be low, however the Quatrac 5 was able to achieve a midfield result for wear.

The wooden spoon in the comparative test went to the Nokian Weatherproof for its strong drop in performance at lower tread depths. In new condition, the Weatherproof “convinced on snow and in the wet, yet was weak when braking on dry surfaces,” the testers shared. “With 2mm remaining tread (it was) abysmal in the wet and convincing on dry surfaces.” Mileage was also found to be poor.