Witnessed a recent debate about what is the appropriate temperature to check tire pressure at. The debate centered around warm or cold, there wasn't an exact temperature given by any of the parties.
What is the appropriate time to take your tire pressure and fill your tires?
When they are warm or when they are cold?
Why should tire pressure be taken at that particular time of warm or cold?
Cold. All OEM's door placards specify the tire pressure be set when the tires are "cold". But what is cold? From a random Honda owners manual:
Measure the air pressure when tires are cold. This means the vehicle has been parked for at least three hours, or driven less than 1 mile (1.6 km). If necessary, add or release air until the specified pressure is reached. If checked when hot, tire pressure can be as much as 4–6 psi (30–40 kPa, 0.3–0.4 kgf/cm2) higher than if checked when cold.
Note: This means ambient outside air temp, not inside a warm garage.
Low tire pressures carries far higher risk than high pressure. Since low pressure causes tire heating which leads to rapid deterioration and possible explosion. Low tires also cause poor handling due to higher flexibility in the sidewalls.
High pressure carries a shorter list of problems; The smaller and misshaped contact patch increase centerline wear and can reduce road friction.
The OEM's want the pressure on the higher end of the allowed range because of the lower risk and the pressure is going to drop naturally.
Concerning hydroplaning (aquaplaning), higher than recommended tire pressures, and braking/steering "grip", let´s remove unnecessary complication/confusion. The factors at hand are #1 Contact surface of the tire with the pavement, #2 Tire capability to pump water through the groves, #3 Suspension behavior. The more contact surface... the better the braking/steering. You want friction.That simple!! Overinflation reduces contact surface. With proper remaining grooves, the tire will pump out water "adequately", this meaning that any and all tires will start hydroplaning to some degree after a given speed (Square root of tire pressure in Lb. x 9 = Initial hydroplaning speed MPH) is a close enough calculation, let´s be practical and not "millimetric"). Obviously, if you are driving in 4 inches of water no tire in the world is going to pump all that out. High tire pressures induce a harder and longer suspension travel and, especially with old or oil shock absorbers that loose performance as they heat up and part of the oil becomes "bubbly" due heat, there is a segment of time with no tire contact at all with the road when the tire is between the max up travel from a given "bump" on the road and rebound. The engineering departments of the manufacturers of our vehicles have tested behavior in just about every possible driving condition, from the brrrrrr of Sweeden to the Saudi deserts (the Europen ones do) so... just follow the Vehicle Manual in ALL aspects. If you want to go to extremes... under certain conditions is possible to get hydroplaning... in a completely dry road, providing air humidity and temperature are high enough. In aviation all cockpit flight crews kept that in mind. I hope I helped a bit.
