Check Your Tyres Before Checking Your Balls

bill-nye-deflategate

“Yes, a bicycle pump *can* be used to inflate a football.”

With Superbowl Ex Licks coming up and the non-drama about deflated footballs in the AFC Championship, I thought it might be a good time to revisit tire inflation.

Here’s the summary:

  • Recommend starting point is inflating the tires so the tire drop – the amount the sidewall goes down when a load is placed on the bike – is 15%.
  • On the bike, the tire pressure will be different front-to-rear, and will be higher over the tire that has a heavier load.
  • Adjust pressures to riding conditions (e.g., slightly lower for rougher terrain) and to preference, staying within the manufacturer’s pressure recommendations printed on the sidewall

You don’t need to worry about measuring the tire drop: some very smart people have done that already. Here’s a link to the original Bicycle Quarterly article, the online tire pressure calculator, and a post about the associated Android app. (No word on a decent IOS app.)

See the formula and example after the break.

Here’s the formula (h/t Dave Adams):

TirePressure = 153.6 * TireLoad / (TireWidth^1.5785) – 7.1685

TirePressure is in PSI, TireLoad is given in pounds, and TireWidth is given in millimeters.

Load distribution front-to-rear is different for different kinds of bikes, specific to rider position, and shifts if significant weight (~25% of rider weight) is being carried on the bike. Here are front-to-rear weight distribution ratios for bike types, which will get you in the ballpark:

  • Road/Racing/CX – 40% Front / 60% Rear
  • Randonneur – 45% Front / 55% Rear
  • City/Mountain – 35% Front / 65% Rear
Measure Tire Width

50.16 millimeters of treaded chunky goodness

For the truly obsessive, you can measure the weight distribution using two bathroom scales and the help of a friend. And you can use a caliper to measure the width of the unloaded tire – see the photo.

Let’s do an example. Say a rider is 175 lbs on a road bike about 25 lbs and a typical 700C x 28 (nominal 28 mm width). Total weight is 200 lbs, so the load is 80 lbs on the front and 120 lbs on the rear. Plugging those values into the formula yields F/R tire pressures of 57 / 89 PSI.

Some notes:

  • DO NOT use these pressures absolutely. Ultimately how you feel on the bike is most important
  • DO pay attention to the minimum pressure printed on the sidewall. Too low a pressure means risking pinch flats and torn tire sidewalls
  • DO use a tire pressure gauge to measure your pressure, or at least calibrate the pressure gauge on your floor pump. Floor pump gauges can be horridly off. Presta valve adapters can be used with a Schrader tire pressure gauge you already own
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