uploaded 3/22 /2018
Verstappen in the Wet
by Paul Haney
This is another article in the spirit of correcting misconceptions
in the nature of the pneumatic tire, arguably the most complex
and useful device made on the planet. {The pneumatic tire allows
a vehicle to go around a corner with speed and control. Without
them we'd all have to live within walking distance of a rail
line. That would be a different world.}
What about computers you say. What about jet aircraft or
Formula 1 racecars? There are blueprints for those devices. And
manufacturing plants full of equipment. Their materials; steel,
aluminum, silicon are well characterized. But only a few organizations
are capable of the design, manufacture and development of tires.
Their manufacturing processes and equipment are complicated and
tightly hidden from outside eyes. Rubber changes with time, temperature,
and rate of load. Rubber generates friction forces in a complicated
array of processes.
Rain dominated the 2016 Brazilian GP Formula 1 race. During
a period of particularly hard downpour the stewards deemed the
conditions unsafe and sent out the safety car. Most of the drivers
followed the safety car on the usual racing line. Max Verstappen,
in contrast, spent those laps driving his car off line in the
corners and braking areas. He was looking for places where he
could feel more grip than on was available on the racing line.
When the race restarted Verstappen passed 12 cars in 15
minutes and earned for himself a podium finish. When asked why
he didn't just circulate behind the safety car like everyone
else he said, "I was just trying to find grip and alternative
lines."
But in this same interview the young Dutchman attributed
the lack of grip on the usual line as due to rubber and oil laid
down by the racecars. That's what the race announcers usually
say when they see a pass made off line in the rain. But that's
not correct. There is more grip off line but it's not due to
rubber and oil.
Track surface texture is the real reason for a decrease
in grip on the racing line and it's the same in the wet as on
a dry track: Rubber debris and oil would certainly decrease grip
but modern racecars don't spew oil and rubber gets blown off
the line quickly by water trying to get out of the way of the
tires.
I know drivers who, after hearing about the importance
of track texture, walked around tracks scuffing rubber-sole shoes
looking for areas in braking zones and corners where a little
extra grip might allow them to outbrake and outcorner their competitors.
They told me they found places that had more grip and then they
were able to pass more cars and even win races. It works. But
why?
Rubber generates friction forces in two major ways: adhesion
and deformation. Adhesion is generally thought to be the result
of momentary molecular bonding between two surfaces. Deformation
is akin to mechanical keying of road texture into the tread rubber.
Both of these processes are highly viscoelastic and increase
with applied load.
Adhesion friction forces depend on intimate contact between
the two surfaces. Any material that gets in the way of that contact
prevents the formation of forces due to adhesion. On a race track
that means any oil, water or dust prevents that intimate contact
leaving deformation friction to generate grip. So how does deformation
friction work?
This graphic shows a rubber block sliding on a textured
surface. You can see there is more force on the upstream side
of the texture than on the other side. The difference in those
two forces is the deformation friction force. More vertical load
means deeper penetration of the texture into the rubber and more
grip.
But differences in the shape of the texture matters also.
A little known characteristic of rubber is its ability
to polish materials as it slides on them. All those laps on the
racing line change the track texture on that line from tall,
sharp, pointy things to the likes of river pebbles. These graphics
show that worn aggregate is not only worn lower but the rounded
shape changes the contact angle of the rubber to the aggregate.
Both these wear characteristics reduce grip. The first lesson
drivers are taught is to follow the racing line so that path
gets well polished. Off the racing line the surface is apt to
contain many more tall, sharp, pointy things. Thus more grip.
Several years ago I was in a meeting that included an engineer
who worked with pavement and pavement materials. After the meeting
he drove me to a nearby two-lane road that had been freshly paved
with asphalt about six months earlier. He told me to start at
one side and scuff my rubber-soled shoes across to the other
side. After looking both ways and seeing no approaching vehicles
I did as he said. At road's edge there was a lot of grip and
it was difficult to advance in that fashion. Then I came to an
area where my shoes slid more easily. Then an area of more grip.
And again a lane of less grip before getting more grippy toward
the middle of the road. On the other side of the road I found
the same two lanes of low grip with areas in between them and
at the far edge where I had to work hard to scuff along.
It became clear to me that during six months of passing
cars their tires had polished two lanes on each side of the road
leaving grippier areas everywhere else. Ths was a clear, real-world
example of how tires polish a road surface drastically reducing
available traction.
In the dry the quickest way around the course is on "the
racing line" because it's the shortest path with the largest
radii around the turns. But even a dry track has areas where
the texture is less worn and a savvy driver can find some extra
grip. Rain tires, with softer tread rubber, can exaggerate the
benefits of the taller, sharper texture.
Max Verstappen is clearly an exceptional race driver even
as he's just turned 20 years old. But I don't understand why
he was the only driver at the 2016 Brazilian GP steering his
car off line looking for grip not found on the racing line.
How could all those other guys just drive around in line
while Max was using that time more productively? Surely they
want to win as much as Max. Are they overly confidant in some
inherent superiority of mind or body or machinery? Are they ignorant
of the variations in grip on a race course?
What about all those engineers and experts working for
the teams? Why didn't they instruct their drivers to explore
new lines and look for grip? Are all those on-board sensors and
digital processors and software just buzzing away looking in
other directions? What's more important than grip?
I don't think this is the last surprise we'll see from
the young Mr. Verstappen.
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