As October is approaching, temperatures are dropping, as are our air conditioning bills and need to water our lawns. Another thing that is dropping, by law, is the air pressure in your tires. I say by law, because the scientific principles that we use to calculate the relationship between volume temperature and pressure of a gas are called Gas Laws. Another thing that changes with your tires as the temperature drops is the hardness of the rubber. I want to talk a bit about taking care of your tires in the winter, namely about proper air pressure, when and if you might want to lower the air pressure, and whether or not you need to think about putting winter tires on.
Why Are My Tires Going Flat In The Fall?
If you are driving a modern car with a Temperature Pressure Monitoring System (TPMS), you may have noticed recently that when you started your car on a cold morning, your tires were showing a little bit flat. The main reason why you might be seeing tire pressure alerts on your dash in the fall is a principle that you learned about in High School science classes called The Combined Gas Law (pV=nRT). Simply put and in relation to the matter at hand, this law states that if the number of molecules of air in your tire stays the same (which it basically does), and the temperature goes down, then the volume of air in your tire goes down and so does the pressure, hence your tires are flat.
Should I Put More Air In My Tires In The Fall?
You are likely wondering now, if you should add air to your tires in the fall and winter if the pressure is just going to go back up when the tires warm up. It turns out that the temperature recommendations for your tires are based on the tire being cold. Your tire pressure recommendations take into account things like daily fluctuation of temperature, which can often have a difference of about 20-30 degrees F from the coldest to hottest temperature on a given day. So the expected time to check your tire pressure is in the morning before you drive the vehicle. If that pressure is below the recommended pressure for your vehicle, then you should feel confident in pumping up your tires to make sure that they are working properly for your drive that day.
Is There A Time To Lower My Tire Pressure?
If you are driving on naked asphalt, then there is no reason to reduce your tire pressure below your car’s recommended tire pressure. If you are driving on something like snow, ice, or sand, then there may be a reason to lower the pressure in your tires. To understand this, it is worth stepping back to understand what pressure is. Since we are talking about tires, lets stick to psi, which is defined as lbs per square inch, or a force spread across an area. If the same force is spread over a larger area, then you have less pressure, even though the force is the same. A knife cuts better than a spoon, because it is concentrating the same force on a much smaller area, so applying more pressure. Your car’s tires are doing exactly the same thing with the road, except you don’t want them to cut through the pavement.
Snow and sand are somewhat different from asphalt, because of what they do when pressure is applied to them. If you put more pressure into them, they tend to flow or move out of the way. It is important to understand that if you put more pressure in your tires, then they will push out harder, and the contact patch with the ground will be smaller. In fact, the area that your tires are in contact with the ground is roughly correlated with the weight of your vehicle and the pressure in your tires, e.g. If your car weighs 3500lbs and you have your tires filled to 35psi, then you should have a contact patch of about 100 sq. in. divided across all 4 or your tires, although this varies with the elastic force exerted by the strength of the rubber itself and the weight distribution of your car. If you lower the pressure in your tires to 20psi, using the numbers we used earlier, then your contact patch should go up to about 175 sq. in., or almost double. This can make you float higher in the snow or sand and maybe not get stuck. It is important to remember that you will need to pump your tires back up once you get off the snow or sand, because the extra deformation of your tires against the hard surface of the road will wear your tires much quicker than normal.
Do I Need Winter Tires?
The main reason to use winter tires comes from what happens to rubber when it gets cold. Any sort of rubber has a point called the glass transition point, below which temperature the rubber becomes hard and the coefficient of friction goes down. In tires, this point can be manipulated by using different rubber compounds. If the average temperature is below 40 degrees F, then your all season or summer rated tires will have likely undergone this transition and will have significantly reduced traction compared to normal temperatures even on dry pavement. You might chalk this up as marketing to get you to spend more money on tires, but in the long run, you might actually not spend that much more money on your tires for owning a set of winter tires. This is mostly because when your summer tires are not on your vehicle, you are not putting miles on them to ware them out. This will make your summer tires and your winter tires longer than if you only had one set of tires.
As to whether or not you need winter tires for your car, it is hard to say. Improved traction with the ground means that you are that much more likely to spend your time on the road instead of in the ditch. I personally carry a set of winter tires for my vehicle and my wife’s, and both of us have observed times when we saw lots of other people sliding around while we were able to maintain control to the point where we were wondering what the big deal was. I had doubted the difference until I tried it, and considering that it extends the life of both sets of tires to swap them out, it really isn’t that expensive, I definitely recommend getting a pair of winter tires if you spend time driving in the cold, and here in Utah, I usually have my winter tires on from about November to March.