Tech Tidbit: Tire pressure and you

We all know that we need to inflate our tires to prevent flats and to make riding easier. Do you know why? Why does a fully inflated tire resist flats more than a soft one, shouldn’t it want to pop? Bicycle tires are a wonderful thing and we don’t realize how much they affect our riding experience. You don’t have to be a seasoned racer to appreciate what a difference proper tire inflation can have.

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A brief note on tires, wheels, and tubes:

A wheel is made of metal (on the right of the photo above). It has a hub in the center, spokes, and a metal rim. A tire contacts the road and has some sort of tread (on the left). A tube, or inner tube, lives inside the tire and holds air (in the middle). It is important to know the difference between these three things.

Regularly pump up your tires. A perfectly functioning tire will loose air over time.

For starters, you should know that a normal, brand-new tire and tube will loose air over time. Air can migrate through the rubber and even tiny passages in the valve given enough time. As a guideline, a typical skinny road bike tire (700x23c) can lose half of its pressure in two days. A mountain bike tire (26×2.0) will fair better due to the larger air volume, it might last a week before you notice a difference in pressure. This means you should get into the habit of checking your tires frequently. It also means that if you store your bike in the winter (or any long period), expect those tires to be dead flat when you return. Save yourself time and money by pumping up your tires to see if they hold air before heading to the bike shop for repair.

Why is low pressure bad? Two things: first, it can make riding more difficult by increasing resistance. Second, it can allow pinch flats. Pinch flats are the denizen of city riders; they are caused by hitting large bumps, curbs, or potholes. If the tire isn’t fully inflated, hitting a bump can squeeze the inner tube all the way down against the metal rim of the wheel, causing a pinch flat. If it’s bad enough, you can also damage your wheel.

The maximum inflation pressure listed on your tires isn’t always the right pressure for you.

That seems straightforward, so let’s pump those tires all the way up? This also isn’t ideal. The manufacturer will list the maximum safe inflation figure on the sidewall of the tire. This is the MAXIMUM pressure you can use. For many riders it is not the ideal pressure. All modern bicycle tires are pneumatic: they are full of air. You have no idea how great that is. It gives every single bike cheap, lightweight suspension. Without it, you could roll over a quarter and know if it was heads or tails.

manufacturer’s maximum inflation pressure

Because the tires are the suspension, they support the load (rider and bike) and filter out some shock and vibration. The great thing about tires as suspension is that they are easily adjustable. By changing the air pressure and volume, you can change how the bike rides. As a rule, more volume and lower pressure makes for a more comfortable ride. This means you don’t always want to use the maximum inflation pressure. Trust that a lot of experts did some math and determined the 15% sag is the ideal figure for comfort and speed. This means that when you sit of the bike, the tires squish down 15%. These experts also put this into a chart so you don’t have to do math. Note that the loads shown are PER WHEEL. So if you’re not carrying any cargo, dividing your weight (plus your bike’s weight if you know it) by two is a good start. The angled lines are tire width; you can find this printed on the sidewall of your tires.

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For example: 150 pound rider plus 20 pound bike is a 170 pound load. For two wheels, divide the total load by two to get 85 pounds per wheel. If you are running a typical road bike with 700x23mm tires, this means your ideal pressure is about 75psi.

Even manufacturers publish this sort of chart. They would love for people to enjoy their products to the fullest and correct pressure helps in that aim.  Here is a chart from Michelin.

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A lot of road bike tires have a maximum inflation of 100-125 psi. That is a lot. You can see from these charts that most riders won’t need that much pressure.

If you think that more pressure is faster, think again. The rolling resistance decreases by an infinitesimal amount when moving from the “ideal” pressure to maximum pressure. And if you like to turn or brake, that rock-hard maximum pressure is slower and less stable. Using the ideal pressure reduces the chance that your tires will bounces or skip over uneven surfaces. This is a common fear for riders new to skinny-tire road bikes.

If you found this interesting, there is a great article here from Bicycle Quarterly.

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