This excerpt is from Chapter 8 of The Racing & High-Performance
Tire. It appeared in the March 2004 issue of Sports
The Real World, Using Tires
People who are actually using tires-serious road racers, circle-track
racers, and autocrossers-have learned a lot about tires by trial
and error. Hopefully these excerpts from The Racing &
High-Performance Tire offer up some whys that will generate
more knowledge among these racers and make them more confident
when they're using their tires at the limit. Lower lap times
A racecar uses a set of tires a length of time, termed a tire
stint, mainly determined by the number of laps required to use
the fuel allowed by the rules. In most race series it takes more
time to refuel the car than it does to change tires, so no time
is saved by refueling only. In professional racing there is no
reason for the tires to have a useful life that is any longer
than a fuel stint.
For most tires the first lap or two are the fastest laps those
tires will ever produce. Recently Michelin returned to Formula
1 to compete with Bridgestone in a good old-fashioned tire war.
Some Michelin F1 tires appear to increase in performance after
five to seven laps and reach peak grip during laps 10-20 before
falling off slightly.
The Goodyear tires sold to NASCAR Winston Cup teams these
days "give up" at least a second in lap time from the
second through the tenth lap and continue to deteriorate until
the end of the stint. In fact you can see the tires give up during
the two-lap qualifying runs for the Winston [now Nextel] Cup
races. The first lap is almost always the quickest and many drivers
just run one lap and go back into the paddock. The amount of
give-up from the first to the second lap is 0.05 to 0.1 seconds
per mile of lap.
So why do tires give up? The reason is similar to that described
in the section on tire scrubbing. The rubber in tires, especially
tread rubber, undergoes cyclic stress at very high levels. The
work done by these rubbers result in heat generation. The combination
of high cyclic stress and high temperature is bound to generate
mechanical and chemical changes in the rubber.
Some elastomers, and rubber falls into that category, exhibit
stress softening and permanent set. Stress softening has been
attributed to displacement of polymer network junctions and entanglements
and/or the incomplete recovery to original positions of those
junctions and entanglements after stress deformation. The presence
of fillers [carbon black and silica] introduces possible additional
softening mechanisms, including breakage of rubber/filler attachments,
disruption of filler structure, or chain slippage at filler surfaces.
When intermolecular structures are irreversibly disrupted or
reform in new positions while the polymer is extended, the result
is permanent deformation. The rubber is said to have "taken
A rubber compound is a mix of many materials, including chemically
active agents. An excess of certain kinds of active agents can
provide an opportunity for a disrupted or broken bond to repair
itself by rebonding at the same location or in a new location.
Another mechanism for tire give-up is more simple. The tire
might wear to a tread thickness too thin to generate enough heat
and the tire temperature falls out of the range for max grip.
A thick tread, even a slick, deforms and the resulting hysteresis
generates heat. A thin tread deforms less, generating less heat.
Rubber is a complex substance, a mixture of materials and
chemicals manufactured with mechanical processes and various
heat and pressure cycles. In use, tread rubber sees mechanical
working and time at elevated temperatures very similar to the
processes it saw as it was manufactured. It makes sense that
more of the same processing would further change the rubber.
The material in a new race tire is semi-stable. If the tread
rubber had been totally cured it might be too hard to do its
job. So stress and heat can continue the curing process. Even
small amounts of energy from ultraviolet wavelengths in sunlight,
ozone in air, heat, or mechanical working can cause the rubber
in a tire to continue its vulcanization process or change in
The first heat cycle is called scrubbing and was described
earlier. Every heat cycle changes a tire to some degree, generally
in the direction of harder, less flexible, and less adhesive.
Race tires can loose effectiveness before the tread wears through
if they go through many heat cycles. For some tires three cycles
is too many, while others show a performance drop off initially
and then maintain a good level of performance until the tread
is worn off. Smart race organizers are incorporating long-lasting
tires into their rules so that "spec" tires can lower
the cost of racing. Some racers attempt to enhance the performance
of used tires with additives and treatments as described later
in this chapter.
Abrasion and Graining
The abrasion patterns shown in the photos in Fig. 8.6 were
produced by abrading two different rubber samples on different
road surfaces. The abrasion profiles compare natural rubber abraded
on silicon carbide cloth (the top row of photos) with a worn
tire (the bottom row).
What you see here is called graining. The cause, typically,
is overly aggressive driving with a tire that is too soft and
grippy for the conditions or the driver has overworked the tires
before getting them up to a working temperature. Abrasion patterns
are not necessarily caused by gross sliding. A requirement for
the development of an abrasion pattern is "unidirectional
sliding." Sliding in random directions does not produce
these patterns. The orientation of the pattern is important because
it indicates the direction of relative motion between the tread
and the road.
Here's how that pattern gets worked into the rubber. When
a new rubber sample is continuously abraded in the same direction,
the rubber develops an array of nearly parallel ridges at right
angles to the abrasion direction. The shape of the ridges in
cross section, seen in Fig. 8.7, is saw toothed, with the teeth
pointed against the direction of abrasion. During sliding, deflection
waves in the rubber turn into peaks which are bent over, exposing
the upstream side to abrasion. The peaks wear thinner into teeth
and the tips are eventually torn off.
Sliding on smooth tracks does not always produce abrasion.
Abrasion is generally initiated by local stress concentrations
at the contact between track asperities and rubber. Abrasion
intensity depends on shape rather than size of the asperities.
Experiments have shown that road surfaces exhibit wide variations
in abrasion characteristics.
Lab experiments with abrasion in an inert atmosphere-nitrogen-show
less abrasion than the same process in air. This leads to speculations
that antioxidants in a rubber compound can make it more resistant
Unfortunately, once a graining pattern is worn into the surface
of a tire it's difficult to wear the pattern away. The ridges
tend to perpetuate as wear continues. Even worse, since the tread
is not evenly loaded after it has been grained, the tire loses
grip. It's just another way for a driver to mess up. That's why
experienced drivers are so valuable.
Graining on Pavement
These photos, taken by Ted James, show graining on tires used
in club racing on a pavement road course. The tire in Fig. 8.15
shows a front tire with even wear, no graining.
The next image, Fig. 8.16, is a rear tire with graining on
the inside edge, perhaps due to excessive camber. The graining
is not present across the entire tread, suggesting slippage during
acceleration. More camber would create more graining and the
pattern would appear over a wider area of the tread.
In the chapter on compounding we described the vulcanization
process and mentioned that if the process goes too far, the rubber
can "revert" or return to its uncured state. Reversion
can also happen when race tires get way too hot. When you see
a driver lock up a tire the smoke tells you something bad is
happening. The resulting flat spot is caused by frictional heating
of the tread rubber causing reversion. The overheated rubber
softens and is scrubbed off the tire.
Another, less severe, form of reversion is blistering. The
photo in Fig. 8.17 shows a right-rear tire with a line of blisters
around its circumference. Too high cold inflation pressure or
too much pressure build-up during use caused the tread to overheat.
The heat is generated at the interface between the belt plies
and the tread and the rubber melts, causing a local blister.
Mark Blundell won the Fontana CART race in 1997 for the PacWest
team but he barely dodged the blister bullet. It was a very fast
race on a very hot day. The Firestone guys saw the problem after
early pit stops and told the teams to use lower cold pressures
that would decrease loading in the middle of the contact patch.
But a few teams still had problems.
Some tire companies have developed anti-reversion compounds
that resist blistering. That's all I know about it because of
course they won't talk about it. And I can see why this would
work. If you can run your tire a little bit hotter without blistering
that would be more forgiving for the racers using the tire and
maybe the tire would give a little more grip at the higher temperature.
That's assuming the anti-reversion compound has no negative trade-offs.
If you learned from this excerpt, you should buy the