Fleet and safety: Much more to safety than just tread depth

Michelin reveals there is much more to safety than just tread depth after claims that worn tyres offer greater wet performance than new ones, Andrew Ryan discovers. 

The sophistication of the latest vehicle technology is astounding. Some cars are able to warn each other about upcoming hazards, while manufacturers are also developing artificial intelligence systems to monitor driver well-being.

However, the role of the relatively humble tyre in vehicle safety should not be underestimated.

As the only parts of a vehicle which are in contact with the road, the best braking or avoidance systems are ultimately only as good as the grip they provide.

New tyres typically come with 8mm of tread and it is illegal to use them when this wears to less than 1.6mm.

To ensure compliance and increase safety, many fleets and leasing companies change them when the tread reaches 2mm.

Some organisations, such as safety charity Royal Society for the Prevention of Accidents (RoSPA), are calling for tyres to be changed at 3mm.

However, Michelin is challenging this line of thinking. It says that instead of just tread depth, the performance of a tyre is affected by many individual characteristics, such as casing design, materials used, rubber compounds, tread designs and shape of grooves.

In other words, the technology used to design and manufacture them.

To demonstrate this, earlier this year it held an event called ‘The Truth About Worn Tyres’ at its Ladoux Research and Development Centre in central France.

Fleet News was one of a handful of UK media representatives invited to attend.

“All tyres are not equal when they are new, and our tests at Ladoux have shown us that tyre performance is even less equal when worn,” says Terry Gettys, executive vice-president of research and development at Michelin Group.

“The truth is that every tyre, once fitted on a car and driven, starts to wear, and the more it wears, the more the specific characteristics of a tyre change.

“Quite surprisingly, we have discovered that some tyres worn to the legal limit have a wet braking distance virtually the same as some new tyres, and this is one of our corporate messages: tread depth is not a good indicator of wet braking performance.”

In forming this view, Michelin tested 24 new and worn tyres currently on sale, classifying them as budget, mid-range or premium products.

‘New’ tyres had the full 8mm of tread; ‘worn’ had the legal minimum 1.6mm.

While the tests were for demonstration purposes only and the results cannot be treated with the same credibility as controlled scientific experiments, they did clearly show that, as Michelin claims, there is much more to tyre safety than just tread depth.

At the moment, new tyres are rated under the EU tyre labelling scheme for their performance in three areas – wet braking, fuel efficiency and external noise.

Michelin is calling for tests to also be carried out on worn tyres to give fleets and drivers an idea of how they will perform over their whole lifecycle.

“Today’s tyre technology makes it possible to have high levels of grip right down to the last millimetres of tread,” says Gettys.

“We want to raise awareness of this and we believe that all organisations and consumers should consider both new and worn performance of tyres before purchase.”

Michelin allowed journalists to either witness or take part in the demonstrations at the event.

Test one: Dry braking

This involved drivers braking to a halt from 100kph (63mph) on a dry road. Over two runs, the car fitted with new tyres stopped in an average of 37.6m, while the vehicle with worn tyres came to a complete standstill in 35.2m – 2.4m less.

While this may seem the wrong way round, it is the reason racing cars use slick tyres in the dry as they give maximum traction.

“Dry braking gets better for worn tyres, and rolling resistance also gets better, so fuel consumption decreases,” says Francois Fink, research and development director specialist in dry performance at Michelin.

Test two: Wet braking

In this test, Michelin compared how quickly cars fitted with premium new, premium worn, budget new and budget worn tyres could stop from 60kph (37mph) in standing water.

The test found stopping distances were: 

•  Premium new: 19.2m
•  Premium worn: 22.1m
•  Budget new: 22.7m
•  Budget worn: 27.6m

“As we can see, the premium worn tyre is as good or slightly better than the budget new tyre,” says Cyrille Roget, group technical and scientific communication director at Michelin.

“This shows that it is really important not only to look at the tread depth, but also to look at the true performance of the tyre when worn.”

In a separate test, Michelin compared the wet braking performance of a car with worn premium tyres and one with new budget tyres in conditions which mirrored the test used in the EU tyre labelling scheme.

Driving at 80kph (50mph) on a road with 1mm of standing water, a car fitted with worn premium tyres stopped after 41.6m.

In comparison, a car with new budget tyres came to a standstill after 45.3m, meaning the worn tyres’ stopping distance was 3.6m shorter.

Test three: Wet lateral grip

In this test, cars fitted with mid-range worn and budget new tyres had to follow a white line within a 42m radius circle, with the drivers gradually increasing the speed until losing traction in wet conditions.

After 12 attempts in both cars, the budget new tyre lost traction at an average 35.7mph, 0.5mph less than the vehicle fitted with mid-range worn tyres.

The handling of the car fitted with the budget new tyre was also more predictable when it lost grip.

Michelin says the lateral wet grip testing is a good representation of the wet cornering stability of a tyre and this demonstration shows that lateral wet grip and wet braking are well correlated.

It is the same quality being tested, only the direction of the tyre travel that changes.

Don’t be too quick to change

Michelin says there are two other significant reasons why fleets and drivers should not change their tyres too soon: cost and the environment.

“If tyres are changed early, before the legal limit, this reduces the useful life of the product and consumers would make unnecessary purchases,” says executive vice-president of research and development Terry Gettys.

Tyres can account for up to one-third of a fleet’s maintenance budget so changing them more often can have a big cost implication.

Research carried out by Michelin shows that changing a tyre with 3mm or 4mm or tread remaining – instead of 1.6mm – equates on average to an extra tyre per car every two years.

The company says the rolling resistance of tyres also needs to be considered.

This is responsible for 20% of a car’s fuel consumption, which means that one tank in five is used to overcome the tyres’ rolling resistance.

When the tread falls to 1.6mm, the rolling resistance is 80% of that of a new tyre.

Therefore, keeping a tyre on the vehicle until it reaches the legal wear limit increases the time when it is in its most fuel-efficient state.

Research for Michelin by Ernst & Young shows that, across the EU, drivers are paying an extra €6.9 billion (£6bn) a year in purchase costs and fuel than if they delayed tyre purchases until their treads were worn to 1.6mm.

Michelin adds that changing tyres too early would result in 128 million additional tyres being used a year in Europe, which is equivalent to nine million tonnes of CO2 emissions.