Hi,
Always check your Tires when they are Cold. Your Pressures will be more consistent Tire-to-Tire, but this isn't why you check it
Cold. You check them Cold because the Car and Tire Manufacturer determined their recommendation under this condition.
Tire Pressure can vary widely based on both Ambient and Internal Temps. The Mathematical equation would be:
- p2/p1 = (t2 + 459.69)/(t1 + 459.69)
Where:
p2 = ending pressure in psi at temperature T2
p1 = starting pressure in psi at temperature T1
t2 = ending temperature in deg. F.
t1 = starting temperature in deg. F.
Note: the 459.69 factor converts the °F. to absolute temp in
° Rankine
Rearranging the equation:
p2 = p1 * (t2 + 459.69) / (t1 + 459.69)
This allows you to determine the approximate pressure resulting from a
temperature change.
Suppose you check your tires
cold after they've been sitting out overnight at 0°F. The Tire pressures all measure 30 psi. You then proceed to drive
spiritedly to a location, where the Ambient temperature is 100° F, the Air in your Tires has heated up an additional 100°F over the ambient due to the driving/speed frictional heat buildup, leaving the air temperature in the Tires at 200°F. You check the Tires immediately after stopping driving (Hot). Your Tire pressure would be 43 psi because:
p2 = 30 * (200 + 459.69)/(0 + 459.69) = 43 psi, or a rise of 13 psi.
If you check your pressure
Cold at, say 30°F and find them at 30 psi and, then again
Cold at 70°F, your pressure would be 32 psi because.:
p2 = 30 * (70 + 459.69) / (30 + 459.69) = 32 psi, or a rise of 2 psi
As a Rule, Overinflation is
better than Underinflation, though neither is ideal. Underinflated Tires allow a rapid build-up of heat which can cause the Tires to delaminate and their integrity to fail. Remember the Ford Explorer/B.F. Goodrich controversy? All the Rollovers were due to
Underinflated Tires. The Pressure stamped onto the Sidewall of the Tire is the Manufacturers MAXIMUM Tire Pressure for ANY condition and
should not be used.
So far as using Nitrogen, this is really unnecessary and DOES NOT eliminate Pressure changes with Temperature. This would violate a Basic Law of Physics – Charles’s Law. Charles's Law states that the volume of a given amount of Dry Gas is directly proportional to the Kelvin Temperature provided the amount of gas and the pressure remain fixed.
A good Rule of Thumb for setting your Tire Pressures was presented at a Tech Seminar I attended at a past Lotus National Convention by a Guest Speaker who was a Tire Engineer for BF Goodrich, Mr. Oscar Pereda.
He called it a
Realistic Starting Point, saying it has never been
just right, but is a good place to start. The rule is:
- (Vehicle Weight in lb/100) + 2 psi at the Heaviest end (Rear) + 2 psi all around if Suspension and Alignment are OEM Stock.
Example: Stock 986, 2,822 lb.
(2,822/100) = 28 psi
Add 2 psi all around = 30 psi
Add 2 psi to heavy end = 32 psi at rear
With modified suspension, the result is 28 psi Front, 30 psi Rear.
"There is no
Ideal tire pressure," Pereda said. The optimal setting depends on the individual Driver and his preferences.
For those inclined to find a setup for Track use, Pereda provided additional instruction. Drive in a circle, first one direction, then the other. Measure and Record Tire Temperature distributions with a probe-type Pyrometer that actually penetrates the rubber, not the
Aim and Click infrared type. You want bulk temperature, not surface temperature, because the surface cools rapidly while the inner temperature is more stable. When measuring Tire Temperatures, if the Center is hotter or cooler than Shoulders, there is too much or too little Pressure, respectively. But, remember, this is for a Track setting. Hope this helps…
Happy Motoring!... Jim’99