[DeTomaso] Tire Pressure 101

[michael@michaelshortt.com]

Here's some information that I found to help answer questions better than
most of us can do.

Enjoy,

Michael in Savannah



At nearly every event, there is endless discussion about tire pressures.
Newcomers seem to know it's important, but few understand what to do, when,
and why. This article will focus on the practical aspects of adjusting tire
pressure to achieve maximum grip and handling balance.

Whether you're using race tires, high performance tires, or regular street
tires, the goal of adjusting tire pressures, as with all other adjustable
suspension components, is to maximize the contact patch, and thus the
handling grip of the car. Generally, adjustable shocks, anti-roll bars,
replaceable springs, wheel alignment, and other adjustable suspension
features are used to provide the majority of adjustment to the car's
handling balance and grip. Tire pressure is typically used as a fine tuning
adjustment.

However, the majority of weekend track drivers are using relatively stock
cars, and have only their tire pressures to adjust. Whether it's your only
tuning method, or the last refining step, you need to understand what is
happening to the tire during racing, and what the variables are to be
effective in adjusting tire pressure. We're going to present this in the
form of two phases in determining the correct tire pressures to race at.

First is the large scale setting of what the tire operates best at.
Considering the wide variety of tires and cars used at any weekend hobby
event, we can expect a wide range of optimum tire pressures to be used at
fully heated racing temperatures. Somewhere within the 30-40 psig range is
likely for most cars, but that's still a very broad range. If 40 psig is the
max, then 30-40 psig represents a 25% window, and nothing in racing is as
vague as a 25% window. So first, you have to determine where in this 10 psig
band your tires perform best.

The second phase is the fine tuning realm of within 2 psig of the optimum
setting. This small adjustable range can be played with based on track
conditions to tweak for the best performance at any given time on the track.

We want to first step you through a practical approach to your first few
race day's tire pressure settings, then step you through a system to
determine the general range of pressure the car handles best at, then
finally through some tips for fine tuning. First, however, there's some
basic background stuff to cover.

*The Tools*

First and foremost, you cannot possibly tune and optimize your tire settings
without having the proper tools.

At the very minimum you need an accurate and repeatable tire gauge. Don't
cheat yourself with a $3 pencil style gauge. Get a dial gauge, preferably an
oil-filled one, with a maximum reading of 50 to 60 psig. It should also have
a bleeder valve so you can bleed air from the tire while the gauge is still
connected. It's also worth getting the protective rubber case or shell.
Dropping a gauge can ruin its calibration, and the rubber shell can prevent
that.

To be most effective, you also need a tire pyrometer. You can do a lot with
just a pressure gauge, but you'll never be able to quickly and consistently
tune the tire pressures without temperature data. Many decent pyrometers are
available for a little more than $100. The probe types that penetrate the
tires are the preferred ones.
Taking Tire Temperature Measurements

The first rule in taking tire temperatures is that they must be taken when
the car comes in from a full race-speed lap. Taking temperatures after a
cool down lap is useless. If necessary, the driver should come in a lap
early (before the checkered flag) to ensure the tires are race hot. If the
driver waits for the checkered flag, chances are he'll get stuck behind
slower traffic. The track will be under a no passing rule during cool down,
and the tires will cool too much.

Tire temperatures are taken in three places on the tire. First on an outer
tread block on a street tire, or about 1" in from the outside edge of a
racing slick. Second, in the middle of the tire. Third, on an inner tread
block or 1" in from the inside edge of the tire. Often on high performance
treaded tires, you'll need to measure the second tread block in from the
edge. The outer shoulder block will not retain enough heat for a meaningful
reading.

The temperature probe should be inserted all the way in. Whenever taking
temps of a treaded tire, be sure to center the probe in the tread block. The
edges will cool faster, so it is important to have as much rubber
surrounding the probe as possible. Wiggle the probe a little when first
pressing it in to ensure it is well seated.

The temperature reading will fluctuate for several seconds. Do not wait for
it to be perfectly stable. As soon as it settles to within a couple of
degrees F, take the reading and move on. You shouldn't have to wait more
than 5 seconds for each reading.

Always measure each tire in the same pattern. Whether it's inside, center,
outside, or the other way around doesn't matter. Just do it the same way
every time.

Be consistent in the tire sequence also. Generally, you'll want to measure
the hottest side first to be sure they're not overly heated. For example, if
the entry to the pits is after a series of long right turns, measure the
left tires first. Whatever the sequence, be sure to repeat it with every
reading.


Starting "Somewhere"

For your first racing experience, or your first experiences with a new tire,
you'll need to first determine the realm of the gross tire pressure setting.
Assuming there's no one to tell you how your tire and car combination are
best set up, you'll need to experiment to know whether the car handles
better in the 30-34 psig range, the 33-36 psig range, or the 36-40 psig
range. Once you have a feel for this, you can fine tune to within 1/2 to 1
psig for optimum handling balance and performance.

There are two points from which to set tire pressures. First, you have to
adjust the cold (ambient) pressures before the car takes to the track for
the first time, or after it has completely cooled off between sessions.
After the tire has reached full racing temperature, it is possible to fine
tune from the fully heated point, but it will also be important to know when
the tire has cooled back down what it's cold temperature is.

We know the tires are going to heat up when they're driven on. The friction
between the rubber and the road will generate heat. A lot of heat. This heat
is going to transfer to the gaseous air in the tire, and cause it to expand
which leads to an increase in the tire pressure. It happens to work out that
an increase of about 10 degrees F causes about 1 psig increased tire
pressure.

On the track, you're going to work the tires much more than is possible on
the street. Repetitive, high cornering forces and aggressive braking are
going to generate much more heat and therefore higher tire pressures.
Because of this, the typical pressures you run on the street are going to be
too high for the race track. During normal street driving, tire pressures
will increase over the recommended factory "cold" settings about 2-4 psig.
During racing, a cold tire pressure setting will increase anywhere from 6 to
10 psig. Tire pressures must be lowered from their usual street settings
before going onto the track.

As with just about everything, the "correct" starting pressure "depends."
There's a lot of variables. Still, you have to start somewhere, then you can
test various pressure settings to find the best balance in performance.
Remember, even the pros have to test several settings every race weekend to
find the best performing point. There is no formula, no "right" answer that
you can use every time.

So, if you're using street tires on the track, the generic, "start
somewhere" point is 5 psig lower than the car manufacturer's recommended
tire pressures. (Note: use the car manufacturer's recommended tire pressure
as the baseline. Many service and oil change stations with poorly trained
techs will pump tires up to pressure stated on the tire side wall. Hello!
That's the maximum setting, not the recommended setting. Watch out for those
guys!)

>From a cold starting point of 5 psig less than street pressures, you can
begin the trial and error cycle of making small adjustments and determining
the effect. To do this, you will need to run several laps to get the tires
hot to start with. Take it easy the first 3 laps or so until the tires come
up to temperature. The car will be quite sloppy with the initial low
pressures.

After 3 or 4 laps, the tires should feel consistent. Run several more laps
to get a feel of the overall grip level and the handling balance. If you're
held up by traffic, stay out until you get at least three maximum-effort
laps. Then come in to take the pressures and temperatures, and report on the
handling. Do not make a cool down lap before coming in as the tires will
cool down significantly making the tire temperature data useless (When you
park, be sure to not use the parking brake, or rest on the brake pedal as
you can easily warp the brake rotors).

If you have a temperature probe, you can use the tire profile reading as the
primary guide to tell you if the pressure should be increased, decreased, or
left as is.

   - if the centers of tires are 5 or more degrees hotter than the edges,
   then the tire pressure should be lowered. Try about 1 psi for each 4 to 5
   degrees the center is higher than the lowest edge temperature.
   - if the center temperature is more than a few degrees lower than the
   edge temperatures, then the tire pressure is too low. Try increasing it by 1
   psi for each 4 to 5 degrees the center is lower than the highest edge
   temperature.
   - use the guide below for more detailed tire temperature interpretation
   tips.

  Description Temperatures
Left Front Tire Temperatures
Right Front Tire Interpretation Center is low by more than 5 degrees of the
highest reading 175 168 173       Tire pressure is too low. Increase 1 to 2
psig per 5 degrees difference Center is high by more than 5 degrees of the
lowest reading 168 176 169       Tire pressure is too high. Decrease 1 to 2
psig per 5 degrees difference Inside edges always hotter than center or
outer edges 160 162 170 172 165 162 Too much negative camber Outside edges
always hotter than center or inner edges 172 166 162 163 165 170 Not enough
negative camber, too much body roll during corners (causing positive
camber). Try stiffer springs or anti-roll bars. Left or right edges of tires
hotter than center and opposite edge 172 166 162 170 165 163 Probably the
last set of turns before exiting the track are long or hard rights. This
could be normal. Otherwise look for alignment errors.

If you do not have a temperature probe (get one!), then use this as general
guide.

   - Generally the tire pressures should be close to equal on a car with
   neutral handling. If the front or rear tire pressures are significantly
   different, you can adjust them to a mid point between the two readings. (If
   the fronts are 38 psig, and the rears are 32 psig, then try adjusting them
   while they're still hot to all be 35 psig). Try this setting for the next
   session.
   - If the tires felt very slippery, especially if they got worse on every
   lap, then the tire pressure is too high. Take each tire pressure and write
   it down. If only the front or rear tires felt slippery (the front was
   slippery if the car understeered and wouldn't respond to more steering
   input, the back was slippery if the car wanted to oversteer and spin), then
   lower the pressure of just that pair by 4 psig while they're still hot. Try
   this setting for the next session.
   - If the tires felt spongy, then the tire pressures are too low. When the
   pressures are too low, they will have low grip and still tend to slide like
   a tire that is too hard, but the feeling will be different. Instead of a
   smooth slippery slide, the tire will scrub and have more vibration in the
   slide. The difference is subtle, but you'll have feel for it in the steering
   wheel, and from the seat in your legs and back. In this case, increase the
   pressure while the tires are hot by 2 psig. Try this setting for the next
   session.

For the first few sessions you'll want to make large pressure adjustments of
2 to 4 psig. This way in about three sessions you can note whether the car
felt better in the low 30's, mid 30's, or high 30's psig range, Once you
feel the car behaves better in one of these ranges, you can move on to 1
psig incremental changes to close in on the optimum setting. If after 5 or 6
laps you know the car is just not right, don't force it. Come in and make an
adjustment. There's no point to driving a really poor handling car and
risking an off track excursion over it.

Once you have some experience with your car, you may find that the best
starting point is only 2 psig less, or as much as 6 or 8 psig less than the
street pressures you run at. There is a large difference in cars, tires, and
driving style that affect this. However, starting at 5 psig lower than
street pressures should prevent most tires from becoming excessively hard
and slippery, and from exceeding their maximum safe pressure.
Fine Tuning

Once you know the general tire pressure range you're aiming for, you can
make smaller increments to target the optimum setting.

If you're racing on a particular brand and model of tire for the first time,
or you've never taken tire data before, you have some trial and error work
to get through, and this will take some time. Depending on the track time
available to you, it may take more than your first event to close in on the
optimum pressure settings.

If you're running on a race track for the first time ever, this will
lengthen how long it takes to determine the best tire pressures as your own
inconsistent and incorrect driving will cloud the data you collect. However,
take the right data, (use the charts we've provided, and the information in
many of the recommended books), and you'll be able to quickly narrow in on
the best performing pressures.

Based on the car's handling and tire temperature readings, make adjustments
in small increments of 1/2 to 1 psig as needed, then run several more laps
to feel the difference. Repeat this cycle as many times as needed until the
best balance and maximum grip is achieved (as shown by your lap times). Use
the charts we've provided (above and the Tunding Guide) to help interpret
the feel and the tire temperature data into how to adjust the pressures.

Each time the car comes in, use your log sheet (see the Practice Sessions
section for a downloadable log sheet) and immediately write down the tire
pressures and temperatures. Make notes as to the specific handling responses
in each corner of the track. Also, write down exactly the changes made on
each tire.

After several sessions, you'll get to know the range within a pound or so
for the tires that they perform best when hot. When they cool down, you can
make note of the cold pressures, and use those pressures the next time as a
starting point.

Once you have a feel for the target pressure when the tires are hot, there
is some math you can use to calculate a starting point that should be within
a pound or two of the best settings. How that works, requires some further
understanding of the ambient air temperature and it's effects on the tire's
pressure.
Adjusting For Ambient Temperature

We mentioned earlier that 10 degrees F changes the tire pressure about 1
psig. If a tire starts with a certain cold pressure in the morning, the
warming of the day is effectively adding pressure to the tire over the
course of that day. The temperature the tire runs at will be affected by the
combination of the ambient air temperature, the track surface temperature,
and the amount of friction introduced (how hard you drive).

If for your first session of the day, it's overcast, the air temp is 65 and
the track temp is 70, then in the afternoon the sky is clear, the air temp
is 85, and the track temp is 105, there is considerably more heat to
influence the temperature of the tire surface. If you drive just as hard,
the tire will be hotter, and the pressure will be higher. This change from
morning to afternoon is going to be much more pronounced in some climates
than others. The southwestern U.S. can see days with a 45 degree morning and
a 90 degree afternoon. This will increase the starting tire pressure by 4
psig by the end of the day. This will make a world of difference in the
handling of the car during each session.

To maintain the same racing pressure settings in the afternoon as achieved
in the morning, you'll have to compensate for the increased pressure due to
ambient temperature. While there is probably a formula to understand the
effect of the relationship of the ambient temperature and the track
temperature, it will be complicated by the aerodynamics around the tires,
and the heat generated by the brakes. You can generally use the ambient air
temperature and the 10 degrees per 1 psig relationship as a guide for
adjusting tire pressures throughout the day. Take ambient air temperature
readings at the start of each session, and use this to determine how to
adjust the tire pressures. As the day warms, you'll have to drop tire
pressures accordingly before each session.

One more item related to ambient temperature--you’ll find that if the car is
parked with one side of the car facing the sun, those two tires might be 10
to 20°F warmer than the shaded side (and therefore 1 to 2 psig higher in
pressure). You should cover those tires up with some simple plywood panels,
or at least know not to set them to the same pressure as the cooler side of
the car.
Optimizing Tire Performance

The above sections provided some practical guidance to settings tire
pressures. In these next sections, we'll take a little closer look at
exactly what it is we're trying to do.

What exactly is the goal of tweaking tire pressure? The bottom line is that
you're looking for the maximum traction possible over the largest portion of
the track possible.

With a given set of tires (which have a determined rubber compound and size
you can't change), the variables you can play with to achieve the maximum
traction is finding the tire's best operating temperature, and optimizing
the contact patch size. The discussion above in finding the best tire
pressure settings is really just an indirect method of tuning the contact
patch and having the tires operate at their peak temperature range.

The temperature of the tire is influenced by the ambient air temperature,
the surface temperature of the track, and your driving (how hard you push
the car). Because you won't have the luxury of choosing an optimum rubber
compound for the conditions like the pros, your main tool for achieving the
maximum grip temperature is driving the car at its maximum capacity without
exceeding it (and overheating the tires). This assumes the ambient
temperatures are warm enough to achieve the tire temperature needed. If it's
50 degrees outside, chances are, the tires will never get hot enough to
produce their maximum grip potential.

The contact patch of a given tire size is affected by the suspension
geometry, and by the tire pressure. In a stock street car, you don't have
any adjustability at the track in the suspension, so tire pressure is your
main tool.
Optimizing Tire Temperature

While we start with setting tire pressures in their cold state, we're really
interested in their pressures (and temperatures) when they're running hot.

Every tire will have a temperature range where the rubber reaches its most
"sticky" point without becoming greasy, and without physically falling
apart. Race tire manufacturers often have this temperature identified, but
for a street tire, it's unlikely you'll find any spec sheet with this
temperature data identified. Finding the best temperature range of the tire
will come primarily from experience with driving on it, knowing how hard it
can be pushed before overheating causes it to become slippery. When you get
runs of 15 to 20 minutes or more that were fast (within a few tenths as fast
as you've been able to achieve), and the tire grip stayed consistent, you've
probably found the maximum grip operating temperature of the tire. Head back
to the pits, and take pressure and temperature readings, and use that data
as a target for that tire. For most street tires the temperature range can
be expected to be about 180 to 200 degrees F before they get slippery. You
may have sessions where faster laps are possible, but if, in attempting to
keep that pace, the tire progressively gets slipperier, then you're driving
the tires too hard and they're overheating.
Optimizing the Contact Patch

We know the tires are going to heat up when they're driven on. The friction
between the rubber and the road will generate heat. A lot of heat. This heat
is going to transfer to the gaseous air in the tire, and cause it to expand
which leads to an increase in the tire pressure. It happens to work out that
an increase of about 10 degrees F causes about 1 psig increased tire
pressure. During racing, a cold tire pressure setting will increase anywhere
from 4 to 10 psig.

The increased pressure has an effect on changing the tire shape, and the
resulting area in contact with the road. This "contact patch" is what we're
trying to optimize. In fact, we're trying to maximize it.

To understand the effect of pressure on the contact patch, let's first look
at a tire in a static load state (rolling straight or parked).

If the tire pressure starts out too high, the increased pressure from heat
buildup will make the tire shape somewhat convex. The contact patch will be
narrower with the outer edges of the tire not able to touch the road because
the rounded middle has lifted the edges. This tire is said to be too "hard."

If the starting pressure is too low, even after heating up, the tire shape
will tend towards being concave. The outer edges under the wheel rims will
have firm contact with the road, but the middle will deflect inwards. This
minimizes the contact patch in the middle. This tire is said to be too
"soft."

The perfect tire will have a contact patch the full width of the tire with
an even pressure across the width. This is the condition we're striving for.

We've just looked at the case of a static tire. However, when we're racing
on a road course, the tires are not always in such a static state. The
forces of cornering, braking, and accelerating will constantly be changing
the shape of the contact patch as the rubber twists from these forces.
Indeed, it is under cornering that we are typically looking for the highest
levels of grip possible. All else being equal, a car that can get through
corners the fastest, will be the quickest. Again, all else being equal, the
car with the optimized contact patches will have the most grip and be the
quickest through the corners.

While there are several suspension adjustments possible in race cars and
modified street cars, we may also need to "tweak" the air pressure on an
individual wheel basis to achieve the maximum contact patch under the
dynamic conditions of the road course. In fact, these will be different for
each track.

A harder tire may have a slight convex shape during a straight run, but may
be necessary to help keep the sidewall stiff enough during cornering to
prevent the tire from rolling under the wheel rim excessively. So, while in
a static state, the contact patch is smaller than optimally possible, this
actually increases the effective contact patch size during cornering.

These tire behaviors during cornering are what might lead us to having left
side tires, right side tires, or even a single tire at a different pressure
than the others. This would be done to fine tune the handling of the car
through certain corners.

The final goal is actually a compromise in looking for the maximum contact
patch during as much of the lap as possible. We may find that one settings
is optimum for corners 3 & 7, but that another is optimum for corners 2 & 4.
If corners 2 & 4 are faster than 3 & 7, then the emphasis must be placed on
corners 2 & 4 as there is more to gain from them being optimized. In the
end, the "correct" tire pressures are the ones that allow the fastest laps
in long runs.
Road Racing vs. AutoCrossing

Everything described above is aimed at long-run, road racing sessions. In a
2-3 lap qualifying run or single lap autocross racing, the tires will not
have a chance to heat up as much. Under these conditions, you will actually
have to tune the handling balance using "cold" tires and therefore the cold
pressures will be higher than for road racing practice or race sessions, and
closer to the car manufacturer's cold tire settings (assuming similar
tires).

The principles of tuning are the same, though. Make a run, make notes as to
the handling performance, adjust the pressures in small increments, and
repeat the test runs until the fastest lap times point to the correct
pressure settings. Compensating for ambient temperature changes throughout
the day also applies.
Stock Front-Drive Cars and Tire Pressures

Something that we've noticed with many stock street cars is a significant
differential in the front and rear tire heat potential. In theory, all four
tires should be contributing as equally as possible to the overall traction
of the car. In theory, they should all use close to the same pressures, and
run close to the same temperatures. However, the typical stock street car,
especially front-drive cars, do not follow this in practice.

In a race car, the chassis (assuming adjustability of springs, shocks, sway
bar, etc.) has been set up to provide as close to neutral handling as
possible for most circumstances. This is the fastest setup, and it provides
equality among the four tires close to the theoretical perfect balance.
However, it takes an experienced driver to know how to handle the car at the
limit, or in emergency maneuvers. With many suspension adjustments to
achieve neutral handling in a race car, tire pressure is used for very fine
tuning of the setup.

Most street cars are not set up for neutral handling. A front drive car will
typically have significant understeer. While this is not conducive to fast
cornering, if someone is driving too fast around a corner, the front wheels
will slide and not turn the car as much as expected. The natural reaction of
the driver to this is to lift off the gas which is the right thing to do to
regain some traction. In a car that oversteers, lifting off the throttle
reduces traction on the rear tires and actually causes even more oversteer,
and will often induce a spin. Therefore, an understeering tendency becomes a
positive "safety feature" for a street car and the average driver.

Back to the theory that all four tires should contribute equally to the
cornering grip. There are also two other dynamics that work the tires and
generate heat: accelerating and braking. In a rear drive car, the rear
wheels have the workload of acceleration and the tire heat generated. The
front tires handle the majority of the braking load and the heat buildup.
While the two are likely not equal, it does help to distribute the workload
and keep the heat buildup shared. In a front drive car, the front tires have
the workload of doing both braking and accelerating. Furthermore, a factory
stock front drive car probably has a tendency towards understeering in the
corners, and that adds even more heat to the front tires from the excessive
cornering friction. This combination conspires to heat up the front tires a
great deal more than the rear tires.

During a driving session on the track, the front tires may increase as much
as 9 to 10 psig from the cold setting. The rear tires on that same car may
only come up 4 to 5 psig. This clearly points out that the car is far from
having the front and rear tires contribute equally to handling grip. This is
not optimum at all, and requires cold pressure settings at a seemingly odd
starting point.

If we look at the example situation above, we may find from testing that
when the tires are hot, a reasonably neutral handling balance occurs when
the front tires are at 34 psig and the rear tires are at 35 psig. If we know
that the front tires are going to increase in pressure by 9 psig from a cold
setting, then the appropriate cold setting is 34 minus 9, or 25 psig. The
rear tire cold setting would be 35 minus 4, or 31 psig. This may sound
completely counter intuitive considering that on the street, and when you
first go out on the track for the first few laps, these settings will feel
absolutely horrible (this setting will likely result in a lot of oversteer
and will feel very loose). However, on the track, on about the third lap,
the handling will settle down and should reach a neutral balance.

Remember that what matters is how the car handles in long runs, and it is
the hot pressures that are to be focused on. If this means starting out with
some seemingly strange cold settings, so be it.

As an aside, the best thing to cure significant understeer, and the first
money that should be spent on any factory car with excessive understeer is a
rear anti-roll bar. As a first modification, this will probably result in
more lap time improvement per dollar spent than anything you can have done
to the car.


-- 







Michael L. Shortt
Savannah, Georgia
www.michaelshortt.com
michael at michaelshortt.com
912-232-9390


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