Getting a grip on the tyre tread depth debate |
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The subject of tyre tread depth is one that provokes almost as many heated discussions as the weather – at least within the tyre industry. This is because it has long been acknowledged that wet weather tyre performance deteriorates with diminishing tread depth. However, since the last scientific tests were conducted some 20 years ago – before the advent of technologies such as anti-lock braking and electronic traction control – the absence of corroborative data has meant that at best this has been little more than a commonly held belief. At worst, it is considered by some to be a cynical ploy by tyre manufacturers to prompt customers into replacing their tyres more frequently and thus sell more product. So, when the British Rubber Manufacturers’ Association began conducting independent tests into this very subject, the industry rightly held its breath and watched with a mixture of interest and trepidation – the resulting data proved conclusive.Before we go any further, it is worth reminding ourselves of the four primary functions of a pneumatic tyre. Its purpose is to transmit traction and braking forces, provide directional stability and supplement a vehicle’s suspension – tasks it performs by containing a volume of pressurised air. However, in order to deliver against this seemingly straightforward raison d’etre, all tyres must fulfil a raft of secondary functions. For the purposes of the 3mm debate, we will concentrate on the three that are most crucial to road safety:
Modern tyre manufacturing processes and materials have now evolved to such a degree that they ensure that the construction and compounds used to create products remain almost constant throughout the tyre’s life. Therefore the most influential factors that determine tyre performance are tread pattern design and tread pattern depth – and it is here that the 3mm debate begins. To prove or disprove the degree to which tyre performance is affected by tread depth, the British Rubber Manufacturers Association utilised the test facility at the Motor Industry Research Association (MIRA) near Nuneaton. It was here that a typical modern family hatchback with ABS was used for the test, representing the “average” driver’s daily transportation. This was used to illustrate the impact that tyre tread depth has upon safety across the broadest possible cross-section of motorists. The tyres fitted to the test cars were 195/65 R 15 H fitments of a proprietary premium brand – ranging in tread depth from 6.7mm to 0.9mm. A uniform water depth – between 0.5mm to 1.5mm – was then established to simulate moderately heavy rainfall. During these initial tests, the effects of tyre tread depth on straight line braking in wet conditions was duly proven – the difference in stopping distance of a car travelling at 50mph on tyres with just 1.6mm tread depth (the minimum legal requirement) increases by 37% – or 8.8 metres – compared to fairly new tyres with 6.7 mm of tread. To put this into perspective, a stopping distance of 8.8m is equivalent to more than two car lengths and could potentially make the critical difference between knocking over and killing a pedestrian or avoiding an accident altogether. So, while the results supported the commonly-held belief about tyre performance deteriorating with diminishing tread depth, what of the 3mm debate? Well, while the results showed little deterioration in performance between 6.7mm and 4mm in the wet braking tests, but there was a severe drop-off when tyre tread depth fell below 3.5mm. Commenting on the findings, Roger Sanders, General Manager Technical Services, Continental Tyres says: “As an industry, we acknowledge that tyre performance decreases in proportion to tread depth, but even we were surprised to what degree safety was affected. These initial results demonstrated that although tyre performance has continuously advanced over the years, the dangers of worn tyres remain very significant and are certainly far worse than most road users are aware of. “The decision of when to replace worn tyres remains the judgement of the driver and is generally based on factors of cost and safety. However, these initial findings demonstrated that serious consideration should be given to adopting a 3mm threshold. After all, a motorist would not be content with using brakes that were only 75% safe.” These staggering results were then presented to tyre, motor industry and safety professionals at Forbes House, Belgravia (home of the Society of Motor Manufacturers and Traders), who exchanged views on the data and decided how they might be applied in the future to the benefit of motorists. However, these initial results served only to stimulate debate and highlight the need for further research; research that would not only confirm the previous readings – thereby reinforcing their credibility – but also add fuel to the 3mm debate, drive change and strengthen any UK campaign to promote road safety. Against this backdrop, Continental Tyres commissioned further research designed to move the safety debate forward and prove that the 3mm tread depth safety threshold was not confined solely to the family hatchbacks and did indeed have implications for all motorists.
MIRA’s independent experts were again used for the tests and the same road conditions replicated to ensure like for like comparison of data. However, this time three different vehicle types were used: an executive saloon, a sports saloon and an MPV fitted with different tyre sizes from those tested initially (225/45 R17, 225/45 R18, 205/55 R 16 respectively). A variety of tread depths were again used during the tests, simulating the various stages of wear, and hot rolled asphalt – the most common type of road surface – was used during the testing phase. Each vehicle type then replicated the earlier tests conducted by the BRMA by accelerating to 50mph (85km/h) and braking to a standstill a minimum of six separate times to establish average performances and allow driver reaction times to be factored out. And the result? Well, motorists who, up until now, may have believed that up to the point at which their tyres reach a tread depth of 1.6mm, they are still delivering the safest possible performance have certainly been driving under a misapprehension, as the new figures demonstrate. The difference between a new tyre and a tyre at the legal minimum (1.6mm) under the same conditions again proven to be significant. The MPV, executive saloon and sports saloon all recorded significant increases in their stopping distance with 7.97m, 13.12m and 17.52m respectively. These figures represent a startling 30.9%, 48.8% and 64.5% increase in average stopping distances, but more startling still is what happens around that magical 3mm threshold. A significant drop-off is again evident – with 5m of the MPV’s 7.97 increase, 5m of the executive saloon’s 13.12 increase and 11m of the sports saloon’s 17.52m increase in stopping distance occurring once tyre tread drops below 3.5mm. This lastest research also showed that cars on wider tyres take
even longer to stop in the wet. Considering the growing trend of
vehicle manufacturers fitting wider tyres to cars, the 3mm debate
becomes even more important. Summarising the findings, Continental’s
Roger Sanders says: “Based on this evidence, there is a compelling
argument for recommending a 3mm tyre replacement threshold as the
optimum compromise between safety and economics.” And so
it seems that the weather – at least the wet variety – wields
an influence over yet another part of our lives. Discussing the
weather will undoubtedly remain our national pastime for the foreseeable
future, but the debate concerning tyre tread depth is only just
beginning to gather momentum, now with independent and statistically
valid research to add weight to its case. |
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