By Jim Kerr

Mix seventy-eight percent nitrogen, twenty-one percent oxygen, less than one-percent argon, a trace of other gases and what have you got? Air.

What we breathe is also used for many other purposes, including inflating our vehicle’s tires. Cushion tires, as the first inflatable tires were sometimes called, offer a comfortable ride, quiet operation and safety, but only if the air pressure inside your tires is correct.

Early pneumatic passenger car tires required inner tubes to contain the air, similar to most bicycle tires. Changing technology enabled tire manufacturers to add a sealing layer of rubber inside the tire casing, and the tubeless tire was developed. Even with the latest technologies available, checking tire pressures regularly is important for fuel economy, tire life and safety.

Temperature plays a huge factor in tire pressure. According to Bill Vandewater, Bridgestone’s Consumer Products Manager, for every 10-degree F drop in temperature, tire pressure will drop about one pound per square inch (PSI). With the wild swings in temperatures we see in our unpredictable climate, tire pressures could easily drop 5 PSI in a couple days just because outside temperatures drop.

It’s also important to check tire pressures when the tires are cool. Driving on an under-inflated tire generates heat in the tire and the air expands, increasing pressure. While the tire pressure may test correctly, internal tire friction is still generating heat that can
destroy the tire. Tire pressures should be checked after the vehicle has sat for several hours and before driving it more than a few blocks.

All tires leak air. The sealing membranes inside the tire slowly allow the air molecules to escape. On average, a tire loses about one PSI of air pressure every month. Forget to check your tires over the winter and they could be dangerously low on pressure by spring. When topping up tire pressure in the winter, water in the air line can enter the tire and cause the valve core to stick open and leak. After filling each tire, ensure there is no air seeping past it by pouring a minute amount of gas line antifreeze into the valve stem. Leaking air will force the liquid out, so you know you have a leak. If you add a little more pressure, the antifreeze will help prevent the valve core from sticking. Finally, keep a valve cap on each valve stem. The valve cap forms a primary seal so even if a little air leaks past the valve core, the cap will stop it.

Monitoring the tire tread wear can also indicate if the tire has been slightly over-inflated or under-inflated over the long term. If the tire tread is worn in the centre, then the tire has too much pressure. If the tire tread is worn more on both edges, then the tire is under-inflated. A tire with tread worn on only one edge indicates a wheel alignment problem, not incorrect pressures. Changing tire pressure only 1 or 2 PSI can affect how evenly your tires wear, but never inflate them less than the recommended pressures shown on the vehicle’s tire pressure label.

Filling tires with nitrogen gas is the latest trend for passenger tires because it has less pressure change with temperature variances. There is another advantage to using nitrogen. Oxygen atoms are smaller than nitrogen atoms so pure nitrogen leaks through the tire sealing membrane at a slower rate. Air, with it’s 21% oxygen, may leak one PSI per month, while nitrogen leaks about one PSI every 3 to 4 months. Nitrogen is good, but the cost for nitrogen tire fills in our area varies between $5 and $10 dollars each and if you top up tire pressure with regular air, the value of having the rest of the tire filled with nitrogen is minimal. While nitrogen may offer benefits, you can save money by filling with regular air and using that tire pressure gauge more often.

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