uploaded 8/28/2001
Tire Basics: Part 2
Hysteresis
As mentioned before vertical load on a tire causes a small
deflection in the thickness of the tread rubber. Rubber is an
amazing material. You can formulate rubbers that bounce very
high when dropped. This type of rubber is very elastic and after
deflecting when it strikes the ground it pushes back and gives
up almost all the energy it absorbed as it rebounds.
Rubber is also viscoelastic and you can produce a rubber compound
that is slow to rebound after a deflection. The rubber absorbs
some of the energy during deflection. A ball made of this material,
called high-hysteresis rubber, hardly bounces at all. The energy
absorbed heats up the rubber.
Rubber generates grip by simple friction, thought to be caused
by momentary molecular bonding, and by mechanically keying into
road surface irregularities. High-hysteresis rubber keys into
road irregularities better. In the wet, when water lubricates
a road surface, mechanical keying is what you depend on for grip.
Needless to say racing tires have high hysteresis tread compounds.
Ditto for racing rain tires.
A rolling tire generates heat but usually the heat dissipates
into the air swirling around the tire. A sliding tire can generate
a large amount of frictional heat at the tire/road interface.
The rubber can get hot and loose its strength. If the whole tire
overheats, the materials holding it together can fail and the
tread separates from the carcass or, rarely, the tire bursts
from high internal pressure.
Heat, Pressure, SUVs
Several factors can contribute to heat build up in a tire.
Summer weather-hot roads, and high ambient temperatures-can heat
up a tire. High vehicle speeds, low internal pressure, and sliding
also add heat to a tire.
Each specific rubber compound has a temperature at which it
generates optimum grip. Above and below that temperature grip
falls off. For racing applications tire designers are very careful
to use a compound that reaches but does not exceed that optimum
temperature. Occasionally they get caught out when race-day weather
becomes extreme.
For street cars, tire designs are very conservative with cost
and safety the primary concerns. But tires are not zero-maintenance
products. The recent rash of SUV rollovers blamed on the tire
companies can also be attributed to people buying stylishly huge,
high-CG vehicles and neglecting the tires.
If I ever had a tread separation driving my Maxima I probably
be able to carefully head to the side of the road and put on
the spare. If I'm driving a Hugemobile SUV I could die in a rollover.
That's a big price to pay for having a higher, heavier car than
the guy next door. You can't buy cars like you buy shoes, strictly
on fashion.
Heads Will Roll at Ford
Ford made a lot of money by using existing truck platforms
for SUVs and not doing any suspension development. When they
saw they had a rollover problem due to a narrow track and high
CG, they lowered tire pressures so slower tire response made
a rollover less likely. Of course they wanted a super cheap tire
from their suppliers so they didn't change the tire specification.
Unfortunately that lower reccomended tire pressure added to customer's
bad habits-80-90 mph for hours in hot weather, loading the car
with whatever it will hold, ignoring checking tire pressures
at all-resulting in fatal accidents. When Ford tried to blame
Firestone the Akron company turned on them. Both are now hurting
financially and in the marketplace. The public doesn't know which
to believe so they mistrust both of them. Ford's decision to
blame Firestone has to be one of the most stupid decisions ever
made by a big company.
Ford has to take some blame because they built an unstable
car but the public should know better. The auto makers have preached
low maintenance for 20 years. Cars are better and require less
maintenance but tires, although better made from improved materials,
still need some attention. Normal air leakage and unpredictable
punctures cause lower internal pressure. The consumer absolutely
cannot ignore tires.
Trade-Offs
Tire performance can be very subjective no matter if the evaluator
is Amanda Soccermom or Michael Schumacher. An array of conflicting
priorities-grip/wear resistance, wet/dry performance, low cost/high
reliability, steering response/low rolling resistance-to name
a few, combined with the almost infinite mechanical and material
combinations inherent in an elastomer/textile composite product
results in a dizzying number of potential material formulations
and manufacturing processes each producing a tire with different
performance characteristics.
That's why race teams spend a lot of time tire testing. The
tire makers need the testing to try new combinations in on-track
conditions. The complexity of rubber compounds and interactions
in the tire construction precludes accurate simulation or performance
prediction prior to manufacturing a specific design. They just
have to make some and see how they work. They look at the lap
times and listen to the drivers' feedback and decide what tires
to bring to future races.
Tire Testing Pays Off
Grip is not the only goal in a racing tire. Safety-freedom
from catastrophic failure-is extremely important. Durability
matters even if the tire only has to last the number of laps
between pit stops for fuel. But driver feel, confidence, and
control are also important and these are extremely subjective
and widely varying among drivers. And the only way a tire manufacturer
can find out how their new tires feel to a driver is to have
them drive on those tires at a real race track.
You want to know how important tire testing is to a race team?
In 1997 I travelled with the PacWest Champ Car team and wrote
a book called Inside Racing. (I still have copies, look on the
home page for the link.) For some reason PacWest did not participate
in the Firestone tire test at Mid-Ohio. A few weeks later at
the Mid-Ohio race they were totally out to lunch. It was a horrible
weekend for the team. Alex Zanardi, driving for Ganassi Racing
dominated the weekend and won the race. He also won the CART
championship that year.
Just before that race at Mid-Ohio PacWest driver Mauricio
Gugelmin had driven in a Firestone tire test at Sebring. The
Firestone guys told me Mauricio gave them good feedback and they
liked working with him.
I asked Mauricio about the tire test, how important that was,
and when he might see some benefit, "Here at Mid-Ohio they're
using the tire that Zanardi liked at the test and you can see
how good he's doing. The tire we tested at Sebring was a street-course
tire and our next street race is Vancouver."
Mauricio dominated at Vancouver and won the race! That's how
important tire testing is, especially when there is competition
between tire manufacturers. At that time Goodyear and Firestone
supplied tires to CART teams. Now only Firestone makes CART tires
and testing is less important. Current CART tires are conservative
designs using relatively hard compounds.
Formula 1 Tire War
But the current tire war between Bridgestone and Michelin
in Formula 1 is fierce. The manufacturers can't please all the
drivers and teams so they have to pick an organization to concentrate
on. You can bet that Bridgestone is making tires that Michael
Schumacher likes because he is far and away the best driver.
I'd bet Williams gets the majority of attention from the Michelin
guys. At first it was probably the BMW engine they liked but
now Ralf Schumacher and Juan Montoya are showing they have the
ability to win races.
The Formula 1 race teams are extremely competent technical
organizations peopled by hundreds of PhD. engineers and software
jocks. They own wind tunnels and run them almost 24/7 generating
more downforce and less drag. The engine manufacturers are spending
billions a year developing more powerful, lighter, lower CG powerplants.
At their best all this money and talent gains a few tenths of
a second better lap times A YEAR!!!
But here comes a tire war and lap times drop 2 SECONDS at
each race. More grip and control means higher corner speeds,
quicker acceleration, and better braking.
Tires are a big deal. We need to respect them and use them
intelligently.
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