Tread Depth Law and The
Effect of Tread Depth on Tyre Performance
Current tread depth legislation requires that car tyres must
have a minimum of 1.6mm of tread in a continuous band throughout
the central ¾ of the tread width and over the whole circumference
of the tyre.
However, despite the law, it is generally recognised in the
tyre industry that the legal limit is an extreme. Many tyre
manufacturers state that they design tyres to function as
well at 1.6mm as they do at 9mm (the accepted normal tread
depth when new). That is a surprising statement for any tyre
company to make, but some have said just that.
So, if a tyre performs as well at 1.6mm as it does at 9mm,
what happens at 1.5mm? Is there a sudden drop in performance?
Actually there isn't, because industry testing has shown that
when a tyre reaches around 3.5mm in tread depth, the level
of performance in the wet, in particular, starts to deteriorate,
as does its dry handling characteristics.

The recommended point for change is accepted Europe-wide
as being 3mm. So much so that ministerial cars in the UK have
their tyres changed at, you guessed it, 3mm.
Why
then the current legal limit of 1.6mm? There are several arguments
against the change, some of which you may question. One is
that the sudden change from 1.6mm to 3mm would have a serious
impact on the pockets of hundreds of thousands of motorists
who are already struggling to keep their cars on the road.
Another is that it would require changing all the tyre moulds
in use to increase the tyre wear indicator depth to 3mm. And
of course Europe plays a part, as there would not be universal
implementation of 3mm tread depth, requiring double standards
in production and possibly in policing.
The reality is that since tyres are now a global commodity
it would almost require a global adoption of 3mm as a minimum.
It doesn't take an Einstein to counter the arguments against
3mm, but until the legislation is in place you can make up
your own mind, scrape by on 1.6mm, or be safe on 3mm. Your
choice.
Incidentally - you may wonder why the normal new tread depth
is generally around 9mm. This is to do with the slip, ie distortion
in a tyre block, and its level of hysteresis. If you take
an eraser and holding it vertically, draw it across a desk,
you will see that it distorts before it loses grip - that
is what we call "slip". If you increase or decrease the length
of rubber you are flexing, the slip increases or decreases.
It becomes obvious that too much slip would make a vehicle
unstable. Remember that the compound is a compromise too and
the ratio of slip to tread block and the compound used is
all finely tuned. The industry norm is for car, van and SUV
treads, 9mm.
The Impact of Tread Depth on Tyre Safety
The braking and grip performance of tyres in wet weather
deteriorates considerably once the tread depth reduces below
3mm. This is because the main function of the tread pattern
of a tyre is to evacuate water. As the tread depth decreases
it gradually loses the ability to evacuate all water from
the road surface under the tyre and the car will eventually
aquaplane. According to Continental, tyres with only 1.6mm
tread depth are only performing at 55% efficiency compared
with 75% at 3mm and 85% at 4mm.
Many tyre tests have shown that the wet braking distances
of a new tyre compared with a tyre with only 1.6mm of tread
left on it are huge and can be the difference between life
and death. In fact, Continental point out that the increase
in wet stopping distance from 50 mph between a tyre at the
legal limit compared to a new tyre can be as much as 13 metres
(42 feet).
According to Continental, not only are drivers often unaware
of the dangers associated with tyres at 1.6mm, as many as
two-thirds of UK motorists admit to not knowing that this
is the legal limit. As many as 40% say they have never checked
the tread of their tyres and 20% say they don't even know
how to do it.

|