Tires, Legislation and Safety



David Shaw
08/01/2014

A couple of news stories this month set me thinking about the question of how we legislate – or not – to maintain tire safety. One was fairly high-profile, the other less so.

The high-profile story concerned the United States where the NHTSA has been pondering the subject of tire ageing and whether the US Government should introduce legislation to prevent older tires being fitted to vehicles.

The other was a brief discussion at the UK TyreSafe meeting about tread depth.

Tread depth is an emotive subject. I think most people in the tire industry understand that tire grip and especially wet grip tends to diminish as the tread depth wears down from around 4mm to the legal limit of 1.6mm.

The trouble is that you can't say that.

The engineer in me has seen endless charts which demonstrate that wet braking performance diminishes from the moment the tire is fitted until tread depth reaches a legal minimum and beyond. The degradation in performance starts out slowly, so that there is very little difference between a 7mm tread depth and a 5mm tread.

As the tyre approaches 3mm, the curve starts to change slope, and the degradation in performance accelerates. Different tires have different shaped curves, so there is no easy rule of thumb to predict how the performance will change with tire wear.

Those in the standards and legal business, however, say that there is no reliable evidence to suggest that tires with a tread 1.7 mm deep are any less safe than those with a 5mm tread. That's mostly because the data has not been collected and analysed properly. Deliberately.

Imagine the outcry if a senior tire industry engineer from a reputable company said that tires with 1.7mm tread depth were less safe than those with 5mm tread.

Since the law says that a tread depth of 1.6mm is the legal minimum, the dogma we have to repeat is that all tires with a tread depth of 1.61mm are safe, while those with a tread of 1.59mm are unsafe.

To an engineer, this is patently ridiculous. Nevertheless, anyone in the standards and legislation end of the tire industry has to adopt that as the legal position. Incidentally, despite calls from the Emergency Services in the UK and elsewhere, there is no prospect of the UK or any EU member State changing the tread depth requirements on summer tires any time soon.

The other issue in this area is the NHTSA view that they will not seek legislation on tire aging.

Just as most of us know that performance diminishes as tires wear, we also know that exposure to high levels of UV light, ozone and high temperatures will cause tires to age more quickly than those stored in cool, dark conditions.

In the southern US states of Florida, Texas, Arizona and New Mexico, the summer climate is harsh. Sunlight is strong in those near-tropical latitudes; temperatures can easily climb above 40°C. Meanwhile the thin atmosphere does little to block UV rays.

Tires stored in less than ideal conditions in that part of the world will suffer premature damage. Storage conditions vary from the open air to simple barns with corrugated iron roofs.

Spare tires stored in the trunk of a car or underneath a pick-up truck suffer just as badly.

If you put these tires on a heavily-laden vehicle, allow them to lose a bit of inflation pressure and then drive for long distances at high speeds, there will be failures.

NHTSA did not challenge this. Instead they picked up on one aspect of the equation. Tire pressure monitoring systems have led to a reduction in driving under-inflated and besides, the use of different tire compounds and constructions since 2008 has meant that tire failures are less common than they were prior to 2008.

This seems an odd conclusion. I tend to put it in the same category as the dogmatic stance that tires with tread depth above the legal minimum are safe, while those below are unsafe.

The law recognises clear, quantified limits. It does not recognise progressive degradation.

In both of these cases, the degradation takes place over a period of months or years. It is all-but impossible to identify a moment when the tire changes from mostly safe to mostly unsafe.

The engineer in me says that the probability of a catastrophic failure in grip or in pressure-retention increases as the degradation continues. But the lawyer in me wonders how we can codify the moment a tire becomes unsafe.

In the case of tread depth, the legislators have chosen a more or less arbitrary number – 1.6mm.

In the case of tire age, it looks like the legislators in the US are ducking that decision.

I think that is mostly because the selection of an arbitrary time limit for tire age is so difficult due to the large number of factors which affect tire degradation over time.

I suspect another factor is pressure from the tire makers to avoid suggesting that tire age might be a factor which makes them liable for someone else's mistakes or deliberate actions.

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