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Under
Pressure - Article by Jeff
Butcher, Courtesy of JOES Racing
Products
Emotional
pressure to find speed will be reduced
if you manage your tire pressure with a
plan. Every set up out there relies on
the contact patch and getting the
desired pressure at the point the rubber
meets the road. Several important
factors need to be considered for
optimal tire pressure.
To achieve precise tire pressure
readings you must have an accurate tire
pressure gauge. Starting with the right
gauge is paramount. Nearly all gauges
are more accurate in the center of the
range. For example; a 30 PSI gauge is
going to be most accurate between 10 and
20 PSI. It will still work at 5 or 25
PSI but the percentage of error
increases. For a 60 pound gauge you will
get good results between 20 and 40 PSI.
Just as before it will work fine at 15
or 45 PSI but the accuracy percentage
goes down.
Racing Tire Gauges with a Glow in
The Dark dial face are a big advantage
when it gets dark. There is a difference
in glow coatings and better quality
gauges have coatings that glow longer.
You can recharge a Glow in The Dark
Gauge quickly by placing it near any
light source.
Since
accuracy is best in the center of the
range, you should choose a gauge that
fits the pressure ranges for your type
of racing. Gauges found for $10.00
dollars at the local auto parts store
are designed for passenger cars and
their percentage of error is too high
for racing purposes. If your passenger
car has 30 PSI instead of 32 PSI it is
really not too big of a deal but on a
racecar 2 pounds would be the difference
between winning and kissing your sister.
Choose a quality gauge that has less
than a 2% accuracy tolerance. Obviously,
better accuracy allows your team to
slice information for repeatable
performance.
Larger
tire pressure gauges such as this 4"
gauge offer more accuracy. Larger gears
allow for precision machining and the
larger dial creates more resolution and
better viewing angle. Rubber bumpers
should always be included with racing
tire gauges.
Digital Gauges
A quality digital gauge can
give you better resolution yet you can
be fooled by the digital display on a
low quality version. Low quality digital
gauges may post a number on the display
but the accuracy must be supported by
quality sensors for the display reading
to be reliable. Digital gauges should
have a backlight for easy night time
viewing and the numbers should be large
and easy to read. Digital, by itself,
does not guarantee that the gauge will
be more accurate. You may find a
practical improvement as digital gauges
remove the variable of viewing angle.
Analog gauges can add to accuracy
percentage error due to the viewing
angle - the individual user’s
interpretation of the needle verses the
printed hash mark can cause a variation
in results. A quality analog gauge will
have minimal viewing angle error.
Digital Gauges can provide more accuracy
but digital alone isn't the only factor.
Quality racing gauges like the one shown
is very accurate. Gauges with sensors
designed for passenger car use are
inexpensive but their low quality
sensors are less than the standard
required for racing.
Paying
more for one digital gauge verses
another does not guarantee accuracy but
it can be a factor. There can be an
accuracy correlation to gauge head size.
If the display is very small then that
could be a clue that the electronics
contain low cost sensors. Digital
sensors intended for passenger car use
simply do not provide the resolution
needed for racing. If a digital gauge
rounds to half pound or even full pound
you most likely find improved accuracy
and quality with a gauge that reads in
1/4 pound or better digital increments.
Analog Gauges
For analog tire pressure
gauges you will find many options. Gauge
faces that have Glow in The Dark
coatings are a big help when it gets
dark. There is a difference in coating
quality so take note of the manufactures
that utilize longer lasting Glow
coatings.
Analog gauge head size is a factor
in assessing quality. In general, a 4”
gauge will be more accurate than a 2”
gauge. Larger mechanisms typically have
more precision as the larger gears have
the mass for easier machining. The
longer throw on a 4” pressure mechanism
offers smooth and steady needle movement
resulting in improved accuracy as
compared to a 2” version. With a larger
gauge face the needle is easier to read
and interpretation error is reduced as
the larger circumference provides an
expanded scale and improved visual
perception of the enhanced scale
graduations.
Liquid Filled Analog Gauges
There is a myth that liquid filled
gauges are better. The mere presence of
liquid does not ensure better quality.
Liquid filled gauges may or not be good
quality but the liquid alone is not the
factor that guarantees accuracy. Liquid
filled gauges work great to reduce
needle vibration. If a gauge were to be
mounted on a machine that vibrates then
the liquid would help to reduce needle
shake or bounce. Since tire pressure
gauges are not used in a setting where
vibration is an issue, the liquid serves
only as a gauge damper. The liquid does
absorb shock as the needle movement is
controlled during gauge inflation. This
dampening effect is desirable. Quality
analog gauges have internal dampening
systems without using liquid. Liquid is
one way to provide dampening but
dampening methods that do not utilize
liquid are equally if not more
effective.
Tire Gauge Care
Regardless of the type of gauge you
use, it pays to take proper care of your
precision tool. All racing gauges,
whether digital or analog, should have a
rubber bumper for protection. Ideally,
you would never drop a tire gauge.
Dropping tire gauges even one time can
cause accuracy error potentially voiding
the manufactures warranty. If you drop a
tire gauge and that does not a rubber
bumper then the shock is transmitted
directly through the analog mechanism or
digital sensor. Permanent damage can be
the result. Rubber bumpers on tire
gauges of any type are a must.
Using tire gauges properly is as
important as selecting the right gauge.
Over pressuring the gauge can and will
damage your gauge. Let’s say a racer
purchased a 30 PSI gauge to be in the
center of the range for racing tires. On
the way to the track the racer’s trailer
gets a flat and our racer uses their
racing gauge to check the trailer tires.
The subsequent 60 PSI pegs the needle.
In an instant it is more than likely
that permanent damage has occurred to
the racing gauge and the over inflation
has destroyed the calibration. Care must
be taken to not over load analog or
digital gauges even one time. Tire
gauges are precision instruments. Racing
tire pressure gauges lead a hard life
and the rough treatment introduces a
decline in accuracy. Storing gauges
safely during transport and on race
night will provide for better long term
performance.
Calibration
Many gauges have a fixed calibration
and can not be calibrated in the field.
Fixed calibration is held nicely for
long periods of time if gauges are not
dropped and are used within the required
range.
If you have a gauge that can be
calibrated you need to take it to a
certified testing house or send it back
to the factory for periodic calibration.
Neither of these options is ideal. For
racing purposes the most practical
method is to simply purchase a second
gauge and store it for the sole purpose
of checking the calibration of your main
gauge. Upon purchase – compare the two
new gauges on and verify that they
obtain the same reading on the same
tire. If there happens to be a small
difference simply record the difference
and periodically verify that the gauge
you use matches your master gauge and
the original comparison. Use the master
calibration gauge only for testing.
Keeping a test gauge as a master is the
most practical way to verify the
accuracy of the gauge you rely on. You
can compare your master gauge against
you track gauge weekly as part of your
set up routine.
If you use two gauges on one tire
and both gauges give you the same result
it is likely that they are accurate.
While possible, t is unlikely that two
gauges would be off by the same amount.
Adjustment
You can use tire pressure
and your accurate pressure readings to
adjust your car. For Bias Ply tires the
air pressure can be adjusted to help
your car get through the turns. Thinking
out the adjustment options can help you
maximize practice time or provide
handling adjustability during pit stop
races.
Using an accurate tire gauge will help
you match your contact patch for
adjustability. Thinking about how air
pressure affects contact patch size,
stagger and sway bar load will give you
more options to find the fastest set up.
It has
been my experience that you can not
stretch bias ply tires so it is
important to buy tires that are the
right size to begin with. Crews
sometimes over inflate tires in an
attempt stretch them. Measurements taken
right after a tire is over inflated can
show a larger circumference. The reality
is that as soon as the tire gets hot the
tire tends to return to the factory
size. Consistency in your pressures at
each corner on the car is critical too.
Adding 5 pounds above your standard
pressure to the RR in an effort to meet
your stagger numbers is not recommended.
With Bias Ply tires I recommend
staying within 2lbs of your standard per
tire to dial in stagger. If you can’t
get to your desired stagger within a 2
PSI window then your pressure and spring
rate changes will be so dramatic that
having the right stagger will not
overcome the pressure induced spring
rate changes. Over inflated tires create
too much heat and premature wear. Check
your tire sizes after mounting them and
if they are not the right size then ask
your supplier for another set. I am more
likely to drop the left side tire
pressures to help with stagger
adjustments verses over inflating the
right – it is a balance but over
inflated tires do heat up in the center.
Over inflation can cause over heating
and stagger variations so it should be
avoided.
To help your car handle here are
some Bias Ply pressure tips
Car is loose everywhere:
Add Pressure to the RF which loads
the sway bar for more cross weight.
Drop Pressure to the RR which
reduces stagger, adds cross weight, and
makes the RR footprint larger for more
grip.
Car is loose off:
Reduce pressure at the RR which
makes the RR footprint larger for more
grip, adds cross weight, and reduces
stagger.
Car is tight in the Center
and Loose off:
Drop the LR tire pressure.
Drop the LF Pressure. The added rear
stagger will help the car turn in the
center. The larger LR footprint will
help on exit. With both left side
pressures being lowered the cross weight
change will be minimal. The larger LF
footprint will create more grip in the
center helping the car to turn. Cars
that turn better in the center have a
better angle for the exit so often
curing the center automatically improves
exit issues.
Air pressure on bias ply tires is
fine tuning tool and the adjustments
work best when the car is already
handling well. Fine tuning can be
achieved and pit stop adjustments are
more beneficial if your tire pressure
gauge is accurate. Proper care and
selection of tire gauges is the key to
producing race winning accuracy.
Go Forward – Move Ahead
Jeff Butcher
Courtesy of JOES Racing Products
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