Tyre Air Pressure – What Every New Cyclist Should Know

 

The tyre needs air. It sounds perfectly common sense to most of us, whether a cyclist or not. But trust me, who has been in this industry for more years than my palm of five fingers can count, that I’ve been asked umpteen times if bicycle tyres require air. Duh. I presume most of the time the question actually means “how often (or much) do I need to inflate my tyre”. Which brings me to the following important points about tyre air pressure.

1.How much to inflate

Most car owners will be familiar with inflating a regular tyre of a passenger car to about 220kPa. Now, the common misconception is that a bicycle tyre will need a much lower pressure since it is much lighter than a car, right? Wrong! The narrower the tyre, the higher the pressure needed. Every good tyre will have a range of recommended pressure inscribed onto its side wall. Look closely under good lighting to see the numbers.

Maximum pressure stated on the side wall of a MTB tyre

Recommended pressure range inscribed on the side wall of a road tyre

2. How often to inflate

Air does not like to be trapped in an enclosed space. Like us, air likes to be free. Forced into a near air-tight tyre, air tries every means to break free. Though the rubber holding the air looks like a perfect seal, it is not. In short, air is going to have it it’s way and find a way out of the rubber. The higher the air pressure, the easier for air to squeeze its way out. So those of us riding road bikes with slim tyres of 23 or 25mm wide will be required to inflate their tyres once every few days. Those riding wider tyres like 28mm or more can make do with once a week. Mountain bike tires are much wider, typically in the range of 2 to 2.5 inches, and experiences much slower rate of pressure loss. These tyres can make do with once every two weeks intervention.

3. Air pressure is measured in different units

Much like the measurement of length in inch and centimetres, there is more than one unit of measurement of pressure. The  3 commonly used units of measurement for air pressure are: the bar, kPa and PSI. Even if you’re not sure if the number that you’ve been told (to inflate your tyre to) should mean bar, kPa or PSI, you will be able to guess the correct unit of measure by the magnitude of the numbers. If it is a single digit, then it will be the bar. If it is in the tens to single hundred, then that will be PSI. If it is in the 200-600s, then it will be the kPa. Most pumps are equipped with at least 2 units of measurement on the gauge’s dial, so you don’t really need to learn how to convert from one unit to the other.

Pressure gauge of a tyre pump displaying both bar & psi units

4. Two many valve types

There are 3 types of valves used for bicycle tyres (and that's two many). The most common ones found in Singapore are the presta (aka French valve or FV in short) and the schrader (aka American valve or AV in short). The less common one is the dunlop valve. Most good pumps that can be bought from your local bike shops should be able to support both the presta and schrader valves. Though making it compatible with either sometimes requires a laborious task for unscrewing a cap on the pump head and flipping an internal piece around. Some of the better pumps boosting of a “smart” head can accept both presta and schrader without having to do the flipping work. Saves you a lot of work, especially if you own bicycles with different valve types.

Schrader valve

Presta valve

5. About inflating tyres at the petrol kiosk

As if storing a bicycle pump takes up a large real estate, many new bicycle owners cringe at the thought of getting one. So if you haven't been inconvenienced by having to borrow a pump (from your neighbour or local bike shop) or pushing your bike to a petrol kiosk, then you need to know that the pumps in all petrol kiosks in Singapore support only the Schrader valve. If you own a bicycle with the presta valve, you’ll need an adapter.

Tyre with presta valve

Adapter on presta valve to pump at petrol kiosk

6. Pressure increases (tremendously) under heat

Since we’re in the topic of inflating your tyres at petrol kiosks, then I presume there’s a good chance the bicycle will be left in the car and the car left parked in the open, baked by the hot afternoon sun. A study* has shown that the temperature in a car parked under the sun on a summer day can rise from 80℉ (27͒℃) to 109℉ (43℃) within 20 minutes. And if left longer than an hour, the temperature can reach 123℉ (51℃). Here’s where the law of pressure vs temperature comes into play. The pressure law states that if the temperature doubles, the pressure will also double if the volume is held constant (as is the case of the air trapped in the tyre). So imagine the damage the increased air pressure can do to your tyre (or even the rims) if you’ve just inflated your road tyres to 100PSI and you've left it in a parked car on a hot day.

*Springer, Denize. (2010, August 17). Hyperthermia statistics heat up in 2010. Retrieved from http://www.sfsu.edu/news/2010/summer/12.html

7. Its the “tube” that holds the air, not the tyre.

After going through this entire article talking about tyres and air, the fact is, it is not the tyre that holds the air but a rubber lining we call a “tube” that is doing the job. Most of the time, when the tyre experience a sudden lost of air pressure, it is due to a puncture in the tube, not necessarily the tyre. And this can be fixed by patching the punctured tube or replacing it with a new one altogether.