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| John Dorken of the British Rubber Manufacturers'
Association examines the tyre tread depth debate |
Tread depth is the one area
of tyres that is specifically governed by law |
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| “11% of cars and vans checked
had at least one tyre below the legal minimum” |
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In the arcane world of tyre technologies, tread depth has always
been a hot topic for debate. This is hardly surprising. It is, after
all, what in the wet could be the difference between a safe glide
to a halt and a terrifying skid. While the motorist has a general
duty to ensure that his or her tyres are in a safe condition, tread
depth is the one characteristic of tyres which is very specifically
governed by law. That said, it is amazing – and alarming to
say the least – how little attention motorists pays to this
facet of their vehicle.
Surveys have shown that 80% of motorists are unaware of the minimum
tread depth – 1. 6mm. Latest figures from the tyre check programmes
undertaken by the industry’s safety body – the Tyre Industry
Council (TIC) – showed that 11% of cars and vans checked had
at least one tyre below the legal minimum, with a further 11% with
at least one tyre below 2mm. It is clear that this is an issue that
needs to be seriously addressed. A key question that has long been
in contention is whether the legal minimum should remain at 1. 6mm.
This is the level set by a European Directive. However this does not
mean that it is cast in stone.
It has long been argued in many quarters that the minimum should be
raised, on either a statutory basis or as a voluntary standard, and
a number of players in the tyre business have adopted practices that
suggest that minimum is indeed set far too low. Whatever the merits
of going for a particular figure, it is clear that the motorist needs
to be educated better on the importance of running with tyres with
a tread depth considerably higher than the legal minimum. This has
long been the tyre manufacturers’ focus, with the campaigns
that they undertake through the TIC. But they are aware that to be
thoroughly convincing there has to be sound evidence on which such
a drive is based.
A couple of years ago, the British Rubber Manufacturers’ Association
(BRMA), which represents the principal manufacturers operating in
the UK, looked around to see what recent work had been done in this
area and found that there was very little and, while it was fairly
sure that the basic message would still hold good, decided that it
would be well worth undertaking some new research to demonstrate that
this was the case. BRMA accordingly commissioned MIRA, the highly-respected
motor industry research body, to carry out some objective testing
to determine the variation in wet grip with tread depth. The work
was done last summer.
Motorists should seriously
consider replacing worn tyres with tread depths well above the
legal minimum |
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| "Cornering performance of
tyres began to reduce rapidly for tread depth values below
3.5mm" |
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Stopping distance performance with straight line braking was measured
on five sets of tyres representing typical car tyres currently on
the road, all on two different test surfaces, with a water depth of
0. 5-1. 5mm. Five tread depths were chosen – 6. 7mm (a typical
depth for new tyres), 4. 1mm, 2. 6 mm, 1. 6mm and 0. 9mm, with the
6. 7mm depth taken as the reference point. The results showed that
there was little deterioration in performance between 6. 7mm and 4mm,
but a severe drop-off when tyre tread depth fell below 3. 5mm. At
tread depths of 2mm the stopping distance increased by 30% or more
in comparison with the reference tyre, while tread depths of 1mm required
55% or more than the reference. In addition, cornering tests were
also undertaken on the same set of five tyres. The results showed
that, in a similar manner to the straight line braking test data,
the cornering performance of the tyres began to reduce rapidly for
tread depth values below 3. 5mm. The industry had always been aware
that tyre performance decreased with tread depth but the results show
that, although there have been dramatic advances in tyre and vehicle
technology in recent years, there remain some very real risks in worn
tyres. The findings reinforce the TIC’s call to motorists to
check tread depth and tyres regularly – at least once a month
– and while it is up to them to judge when to replace worn tyres,
they should seriously consider doing so at tread depths well above
the legal minimum.
Some fleets, for example, have a policy of changing tyres around the
2. 5mm mark and some police forces do so at 3 or even 4mm. The Department
for Transport, however, has remained unconvinced that any change in
the law is justified and rightly or wrongly challenged the proponents
of change to come up with evidence that the change is justified in
terms of reducing accidents and injuries. Since there is no routine
collection of data by the police linking accidents to the condition
of tyres that is an impossible mission, at least at present, so we
shall have to rest for some time on the minimum being where it is.
However, that is far from the end of the story. There would always
be an argument as to the level that should be set if the current minimum
was raised. There is, of course, no tread depth level above which
one can say a tyre is safe and below which it is unsafe. Any particular
figure is bound to be somewhat arbitrary. If the present minimum is
being ignored, how much value is there in increasing it? That said
a particular figure is more likely to stick in the driver’s
mind. With the MIRA report published and welcomed as most helpful
in demonstrating the importance of tread depth, those who want to
deliver the safety message now have a quarry of up-to-date evidence
to draw on to demonstrate the case.
The MIRA report, An Investigation into the Effects of Tyre Tread Depth
on Wet Road Braking and Cornering Performance” – MIRA-1002250
– is available. |
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