One of the easiest, yet most often overlooked, vehicle maintenance items is tire pressure. It is an incredibly critical component to vehicle dynamics. Tire pressure has an effect on a whole host of topics: handling, wet weather performance, grip, tire wear, tire temps, braking distance, acceleration rates, cold weather grip and off-road grip. It cannot be overstated how critical tire pressure really is.
So, what’s the ideal tire pressure for your vehicle? First and foremost is the manufacturer's recommended tire pressure. This number is found on the inside of most vehicles' door jambs (called a door card). The manufacturer’s air pressure recommendation is based on the vehicle load, or weight while the tires are cool. If you buy an aftermarket tire, its specs must be equal to or greater than the specs on the door card (see your tire dealer for more information prior to purchasing tires). The manufacturer's recommended pressure is what they test the vehicle's performance with and is the minimum for the original equipment's baseline safety and performance.
The second detail to look for is the tire manufacturer's recommended maximum pressure. This is found on the sidewall of the tire. This pressure is based on the tested load on the tire for that specific size, load and speed rating. However, there is more to it than that. This number changes depending on vehicle weight and usage speed. Each tire manufacturer has a chart for what the optimal pressure should be according to usage. The tire's maximum pressure is usually higher than the rating on the vehicle itself.
For normal street use, it is recommended to stay at or above the PSI on the vehicle's door card and below the maximum posted on the tire's sidewall (providing you have chosen tires within the recommendations of the vehicle's manufacturer). Maintaining this pressure range will increase the life of the tire, keep the tire wear within reason and keep the vehicle's handling predictable and safe. It also ensures that the tire's contact patch with the road is evenly spread across the entire width of the tire (read: maximum traction).
Underinflated
What happens when tire pressure is below the recommended level for vehicle load? A lot, actually. When a tire is below the recommended level, the tread surface cups inward toward the rim. This means that the corners of the tire are carrying the load of the car. This makes the tires wear excessively on the shoulders, and they usually hum or sound louder than normal. Low tire pressure is inherently dangerous because tire sidewalls flex more, creating inconsistent and unpredictable handling. Underinflated tires decrease vehicle stopping distance and increase wear. It also makes wet weather driving very treacherous. When a tire is low on pressure, the tire tread can’t evacuate water out of the contact patch and instead traps it, increasing the chances of hydroplaning and losing traction. Plus, underinflation dramatically reduces fuel mileage.
Overinflated
Overinflated tires have their own share of issues. Overinflating narrows the contact patch, which decreases grip. It also makes the vehicle tend to understeer more (not turning into a corner properly). Overinflation makes the center of the tire wear quicker because it makes the relatively flat surface of the tread face bulge outward in the center. This shape change reduces the tire’s contact patch and increases the chances of hydroplaning in wet weather because the tire tread is not completely engaging the road surface. Too much air pressure also makes the vehicle ride stiffer, as the tire cannot flex as much and won’t absorb bumps as easily.
Camber
Camber can greatly influence tire temps and pressures. Neutral camber (+/- .5 degrees) keeps an even contact patch, which keeps temps even across the surface of the tread. On track cars, camber is negatively increased, especially on the front, because the contact patch moves towards the outside during cornering. Driving while excessively stanced puts the weight of the car on the inner sidewalls of the tires. This creates a disproportionate amount of heat on the inner corner of the tires that increases pressure. Eventually it will cause the sidewalls to fail and shred the tires. This happens regardless of if the tire pressures are properly set.
Exceptions to Overinflating and Underinflating
If you watch any sort of car racing you will hear the announcers or teams talk about adding or taking out air pressure from the tires. There are a few reasons for this. The first is that as a tire heats up (as a tire rolls and flexes it heats up the air inside, which then expands), the handling characteristics change because the tire physically changes shape. For every 10 degree increase in temperature, the pressure goes up about one pound. The other change is vehicle dynamics, which can make steering lighter or heavier. Simply put, it is all about maintaining the maximum contact patch of the tires and keeping the setup consistent. In autocross it is common to see people taking out air pressure as their tires heat up so that they keep the same shape and consistency.
There are also circumstances where it may be beneficial to run lower air pressures. Winter weather in the snow belt is a prime example of when to run lower air pressure. Doing so allows the tires to mold over the ice and snow on the roads for better mechanical grip. The same is true for off-road conditions. But realize that the trade-off is poorer pavement performance and wear, so don’t forget to air-up when the snow melts or when you return to the pavement.
There are a few times where it is beneficial to overinflate the tires. When towing or carrying a heavy load, higher air pressure prevents the tire tread from cupping due to the increased load. This scenario is usually discussed at length in the vehicle's owner manual.
Nitrogen vs. Air
Nitrogen vs. air is a common debate. On some vehicles you might see valve stems with green caps. The green caps denote that the tires are inflated with a nitrogen mix. The reason for this is that a nitrogen mix is not as temperature sensitive as normal air is because the molecules are larger and thus, also less prone to leaks. This keeps the tire pressures more consistent and less reactive to heat buildup. The downside is that it is harder to come by aside from dealerships or tire shops. Nitrogen also tends to have less water vapor buildup than normal air, as water vapor tends to corrode Tire Pressure Monitor Systems (TPMS) sensors.
It is recommended that you check your tire pressure at every oil change and prior to any long road trips. Keeping your tires properly inflated is critical for safety and handling. It also has a direct effect on tire wear and fuel efficiency.
Now that you know about tire pressure, click here to learn about the Holy Trinity of Better Braking.