Thursday, August 27, 2009

Car tyre pressure for long drives

I recently went for a long drive (450kms) at a single stretch. I had the usual question of 'how much air pressure do I inflate in my car's tyres?'. This is the first time I was going all alone for such a long distance, so I decided to understand a bit more about air pressure and do the right thing.

In the Internet, there was no good summary of what is the right thing. I read a number of forums and articles, before I believed I understood. Let me explain a few basics of air pressure so you understand better. It is a well known fact these two external factors affect the tyre pressure:

1. Car's running time: If the car is on the move, the air pressure increases (possibly due to the collision between the air molecules, as they spin at a good speed). So it is generally advised not to fill up air after driving for quite some distance (>2km?) -- because by the time one reaches the petrol bunk the air pressure would have gone up by few psi's (a unit of measurement of tyre's air pressure). If there is no other means, then it is advised to leave the car at rest for appropriate amount of time before filling up air (this is mostly impractical). OR fill up few psi's (2?) more than what you intend, to account for the expansion.

2. Ambient temperature: This is straight forward. Air expands on heating -- thus the pressure inside the tyre is proportional to the temperature. So it is advised to fill up air in the morning or in the evening when the temperature has cooled down a bit. This is the right thing because, the recommended air pressure is always the "minimum" air pressure that is recommended for the tyres for that load. This is why the values change from car to car even if the tyre properties are the same. The maximum pressure a tyre can withstand is usually embossed on the tyre itself (usually in the range of 44 psi, in India).

Based on all these facts, during a long drive, it makes sense to expect the tyre pressure to increase heavily. As a result a common misconception is to fill up few psi's less than the recommended. Unfortunately, there is a logical explanation that supports the common misconception -- I had a similar opinion earlier. However it turns out that this is "wrong". At reduced air pressure, the area of the tread that is in contact with the road increases -- this gives better comfort, but poorer handling of the vehicle. Due to the increase in the area of contact, the heat generated at the tread increases -- at a long run, this leads to a faster wear and tear of the tyre tread and poor control. An already worn out tyre might even burst at high speeds -- not to mention what happens to the driver.

To add to it, when I reached home (after 450kms) and measured my tyre pressure again (if you don't have a tool, get one for long drives), 2 psi had vanished from all my tyres!!! Now, this also means that on a long drive, due to the pressure on tyre (bumps and jumps), the air had also leaked gradually (all 4 tyres of mine are brand new and also have nozzle caps, nothing to suspect on the tyres). Watch out, so you don't go below the recommended pressure mid way of your drive.

Usually the recommended air pressure is much lesser than the max pressure the tyre can withstand (for eg., for my car, the max tyre pressure is 44 psi, and the recommended is around 30 psi) -- so on a long drive it is advised to inflate the tyre to a few psi's more than the recommended, for the reasons mentioned above. I had inflated to 34 psi for this drive.

Understand, inflate and have a safe drive!!

Disclaimer: That said, I am not responsible if there is any unexpected event due to the increased pressure. Use your own conscience to validate the info above.

17 comments:

  1. Thanks for getting knowledge of that.

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  2. Thanks for analysis. Maruti SX4 recomended pressure is 30 PSI and in owners manuel it says "If continuous high speed driving is required, increase Tyre pressure by 5 PSI over recommended inflation pressure"

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  4. Proper tire pressure means a lot for your car to move smoothly. Both less and more air pressure may affect the tires. Geraid, actually you have understood right. The tire pressure depends on the car's running time. Well, it's great that you shared this info which would definitely help other readers to understand it straight without any confusion. If you're making a long drive, then just not the tires but you need to check your car overall and make sure it's ready for the trip. Hybrid Maintenance Petaluma, CA.

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