Pressure and the Tire

One of the common strengths found in most curriculum vitae (CV) and resumes is that of a team player and of course the ability to work under pressure. But in real life, we have something whose job is to work every day under pressure. We call this thing the tire. It is that round rubber that moves our vehicles from point A to B, help us land and take off in aeroplanes and found in a majority of many machines that run on the ground.

Pressure is the force experienced in a defined unit of area, mathematically defined as

P= F/A

F= force measured in Newtons
A= area of contact measure in square meter (m²).

Looking at the definition of pressure, you will notice the relationship between pressure and area. It is an inverse relationship, i.e. if one increases the other reduces. For example, an increase in an area without a subsequent increase in force reduces the value of the pressure.

There are various units of pressure which include pascal (Pa) or kilopascal (KPa), bar, atmosphere (atm), Torr, and pounds per square inch (psi).

The typical unit of pressure measurement in tires is the pound per square inch. It is pounds of force per square inch of area.

1Psi= 0.06890 Bar= 0.070 Atmosphers= 51.715 Torr= 6894 Pascals

Growing up in Nigeria, I thought the holy grail of tire inflation is 50 psi. Indeed, riding on most commercial vehicles, it is common to hear the driver tell the street side vulcaniser "50", a psi the vulcanisers are always happy to inflate to.

It was later I got to know the 50 stands for 50 psi, and also I got to know that the value varies by vehicle type and type of tire.

Which pressure is good for my tires?

If the vehicle still has the stock tire and ream, the specified tire information and pressure is usually located at the driver's side door pillar. Do not use as a guide the maximum inflatable pressure written on the sidewall of a tire. Just like the name goes, it is the maximum pressure the tire could take. For instance, if the maximum inflatable pressure of a tire is 60 Psi and the car's recommended pressure is 80 Psi, then that tire is not suitable for the vehicle.

But what happens if the tire I bought is different from the recommended tire? How do I get the right pressure for the car I am using? This question could be handled without hitches if you purchased from a reputable tire dealer.

But you can help by running a little calculation yourself. Remember how I said pressure P is inversely proportional to the area at the beginning of the article? Ok. That implies if the tire has a bigger sidewall, as seen in most tires for jeeps and trucks, it will require a lower air pressure due to the bigger area (volume).

For instance, if your vehicle tire pressure information has it, you need 35Psi and tire has a maximum load capacity of 3500 LBS or 1588kg at 44Psi.

To get the load per Psi, we divide the total load by the maximum Psi= 3500/44= 79.5 lbs per Psi.

Total load at 35Psi= 35 x 79.5 = 2784 lbs

The new tire is rated at 4200 lbs at 44psi which gives us 4200/44= 95.45lb per psi.

Therefore the air pressure needed on this new tire from this car rated 35Psi with tire capable of handling 3500Ibs at the maximum of 44Psi =

2784/95.45 = 29Psi

Notice this is less by 6Psi due to the bigger sidewall of the tires which requires a lower pressure.

Why should I care if the tire pressure is right?

There are many reasons why it is critical that a tire is inflated to the right pressure, chief of which is blowouts. A blowout occurs due to a rapid decrease in pressure.

If we assume things like the weather, tire and road conditions are ok, one of the major causes of most pileup we see on the highway is as a result of a blowout. One of the major culprits in a blowout is pressure: both overinflated and under-inflated tires. In under deflation, the tires are unable to support the weight of the vehicle. Just like a wire bent beyond its elastic limit will snap, a tire which is made to flex beyond its limit will fail and often with disastrous consequences.

Also tires at low pressure due to underinflation makes for a high-risk driving experience as it increases the stopping distance, a loss of control while negotiating a bend. It increases cost due to more wear in the tires through excessive heat generated by the abnormal contact on the ground and is not fuel efficient due to more friction due to resistance in rolling. Most people complain of fuel but may unwittingly be wasting the one they had by the wrong inflation of their tires. You can save up to 3% in fuel consumption by keeping tires at the appropriate pressure.

Most new and inexperienced drivers are warned not to speed during the rain as it increases the chances of hydroplaning. Hydroplaning is when a vehicle skids or slips as a result of tires inability to scatter the rain. This is one of the reasons the treads in tires are there to help channel the water while leaving the tire to make contact with the ground.

If you look up ways of avoiding it, you are very likely to find where inflation/pressure of the tire plays a major role in prevention.

Going by the formula the National Highway Traffic Safety Administration, assuming the water depth exceeds the tire's treads,

Hydroplaning speed= 10.35 x √inflation pressure

The result of the pressure difference and speed at which hydroplaning will occur as tabulated below.'

Tire Pressure (Psi)	Speed at which hydroplaning could happen (mph)
30	56.7
25	51.8
20	46.3

Notice how the speed at which hydroplaning may occur increases with increase in tire pressure.

Overinflation also comes at a cost. First, is that vehicle the vehicle will make an uncomfortable driving experience since the tires act as a shock absorber. If overinflated the sidewalls become too stiff making for a bumpy ride.

There is the uneven wear, which is noticeable when the tire starts wearing from the centre. Blowout is possible too especially in older tires due to the high pressure at the sidewalls.

How do I check if my tire is running at the right pressure?

The best way is to get tire gauge and check the pressure of the tires as against the recommended inflating pressure. Experts recommend you do this in the morning as the tire will not be hot. High temperature affects pressure. Warm air expands, similarly cold air shirinks. Sometimes during a cold chilly morning, you may experience the tire pressure monitoring system (TPMS) give a flase low tire pressure warning.

There is also the "chalk and drive" method. This system involves the following: inflate the tires, draw a line with chalk across the tire, load up the vehicle as you normally use it. Drive about a half a mile. Stop and examine the chalk for wear pattern. If the only centre wears off, the tire is overinflated, if sides then its underinflated. If the wear is even, you are good.

Others opine that another way to check the pressure is through the use of a laser or infrared thermometer. Check the temperature across the tire after driving on a hot/warm day. If the temperature across the surface area of the tires is not the same, it implies the tires are underinflated (if the edges have higher temperatures) or overinflated (if the centre is higher). You can then adjust the Psi until the surface area temperatures are the same.

Conclusion

The tire pressure is something we should check, at least once a month, to ensure the safety of us and other road users.

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^REFERENCES

• ^Pressure
• ^{How do 30 pounds of air in your tires hold up 2 tons of car?}
• ^{Hydroplaning Basics: Why it Occurs and How You Can Avoid it}
• ^{Skidding and/or loss of control from hydroplaning}
• ^{I've overinflated my car tyres. What's the worst that can happen?}

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