THE CONVENIENT CYCLIST

Why bike tires need air so often. It’s about donuts.

Bob Forgrave

Decades ago, it was common practice to regularly check the pressure on all your car tires, and refill them as necessary. Gas stations even had a dedicated Air area. Free, of course, because . . . who sells air?

Old-time air gauge

Fast forward to today, and thinking about car tire pressure is less of a thing. Service stations, if they even have air stations, are now selling air. And still, we’re supposed to check our bike tire pressure frequently. Why?

There are basically three reasons why a bike tire needs inflating so much more than a car tire.

1. Less room for error

According to AAA, for safer car handling, better gas mileage, better braking, we should be checking our car tires a lot more than we are–preferably once a week, and at least once a month. But we don’t, because we can get away with not doing it; nobody notices a couple of MPG, and during handling/braking situations we’ve got more urgent stuff to think about than our tires.

But on a bike, where the motor is your body, everything matters more. Tire efficiency is something you can feel in your legs. You’ll also feel tire underinflation in other, more sensitive places when you go over a sharp bump. You can even create your own flat tire, called a pinch flat, when your chronically underinflated tube gets lethally pinched between rim and road.

If you’ve got a flat with one or two holes that look like incisions (maybe like an incision made by a snake), then you’ve got a pinch flat.

Pinch flat puncture holes

2. Higher tire pressures

Average car tire pressures are around 30-35 PSI. Average bike tire pressures vary widely, getting higher as the tires get narrower. Road tires often will need up to 80 to 110 PSI to stay fully inflated.

What this means is that, for every square inch of tire, a bike tire has 2-3 times the force pushing air out than a car tire. That causes a rapid decrease in pressure at first, then more gradual decrease later. Professional cyclists check their pressure daily–a painless way to improve efficiency and be faster–but other cyclists do it less often. My 110 PSI tires are usually at 80 two weeks later, so weekly is fine.

3. Greater surface-to-volume ratio of a narrower torus

A torus is a mathematical term for . . . a donut shape. And before we get into the math here, let’s make a general observation. Air can leak out only at the outside edges of an inflated surface. The more surface is available for leaking, the more it will leak.

Two thicknesses of torus
Bike tire vs. car tire: One of these has a lot more volume centrally locked up away from any permeable membranes.

And now, the math. We’ve got the formulas for a torus volume and surface area:

While it would no doubt be entertaining to see if we can plug in variables without getting tangled up in order-of-operation errors, there are simple Internet calculators even for torus volume and torus surface area. Who knew?

So let’s dive in, looking at a typical (700c/28″ diameter, 1″-wide) road bike tire vs. a passenger car tire (33 x 12.5 R15).

VOLUME

  • 700c 1″ bike tire = 67 cubic inches
  • 33×12.5 R15 car tire = 4797 cubic inches

SURFACE AREA

  • 700c 1″ bike tire = 266 square inches
  • 33×12.5 R15 car tire = 2132 square inches

SURFACE-TO- VOLUME RATIO

  • Bike tire = 266/67 = 3.97
  • Car tire = 2132/4797 = 0.4

It isn’t even close. Even at equivalent tire pressures, a road bike tire is nearly ten times as vulnerable to having air leak out. Add in the extra tire pressure, and the difference is stunning.

What all this means.

There are ways to stop punctures, like Tannus Armour for wide-tire bikes, but a bike tire will ROUTINELY lose air a lot faster than a car tire for reasons of basic physics.

Use that knowledge to your advantage. Keep your tires pumped weekly, and do it the easiest way possible, with a floor pump that you don’t have to carry with you. A big pressure readout on it is a plus, so you don’t have to deal with pressure gauges either. In our shop, we use–and recommend–the Park Tools PFP-8. We’ve had multiple pumps break on us over the years, but never this one.

Park PFP-8 pump

See you on the trails . . . with safe, efficient, fully-inflated tires.

Bob

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