Too Much Pressure?

 

by Bob Zogg

 

 

Are you riding with too much pressure? Tire pressure, that is. Many cyclists simply inflate their tires to the maximum recommended pressure printed on the sidewalls (often 115 to 120 psi). Their justification goes something like this—higher pressure means lower rolling resistance, which, in turn, means faster cycling for a given exertion level. It turns out that the conventional wisdom is not quite correct. Research by Jan Heine (http://www.bikequarterly.com/images/TireDrop.pdf shows that increasing tire pressure past a certain point provides diminishing benefits. The tire pressure is at this point if, when weighted, the tire height drops by about 15 percent of the tire width. Increasing tire pressure beyond the point that allows a 15 percent “tire drop” provides little or no advantage in riding efficiency because any theoretical benefit in lowered rolling resistance is offset by greater vibration caused by all those little bumps that the tire now bounces over instead of absorbing. More importantly, higher tire pressure means a less comfortable ride and less traction. As shown below, achieving a 15 percent tire drop usually requires different tire pressures in the front and rear tires.

Four factors determine tire drop:

• Tire width
  
• Wheel diameter
  
• Combined weight of the rider, bike, and gear
  
• The front-to-rear weight distribution of the bike.

Tire Width: Tire width is printed on the sidewalls of your tires. For example, if you see 700C x 23, that means your tires are nominally 23 millimeters (mm) wide. The wider the tire, the less air pressure needed to achieve the same tire drop.

Wheel Diameter: Wheel diameter is also printed on the sidewalls of your tires—in the example above, it’s the first number (700 in this example, corresponding to 700 mm). A smaller wheel needs a higher pressure to achieve the same tire drop. Assuming equal tire widths, a 26" wheel needs about 6% more pressure (about 1.06 times as much pressure) as a 700 mm (27.6") wheel. A 20" wheel needs about 38% more pressure (about 1.38 times as much pressure) as a 700 mm wheel.

Combined Weight: Prepare your bike and yourself as if going for a ride, including shoes, helmet, clothing, water bottle(s), food, tools, etc. Then weigh yourself while holding your bike. If that’s too much work, you can estimate your combined weight. Most modern road bikes run between 15 and 25 pounds, many of them in the 18 to 21 pound range. Clothing and gear (including bike shoes, helmet, water bottle(s), food, tools, etc.) might add another 4 to 10 pounds.

Front-to-Rear Weight Distribution: It’s easiest to approximate your bike’s weight distribution using rules of thumb. Heine suggests:

• Racing-style road bikes: 40%/60% (front to rear)
• Randonneur bikes: 45%/55%
• City bikes: 35%/65%.

If you want to get it precisely right, round up a friend and use your bathroom scale. Prepare your bike and yourself as if going for a ride, including shoes, helmet, clothing, water bottle(s), food, tools, etc. Place one wheel on the scale and the other on a block (so that your bike is level). With your friend helping you balance, or while leaning lightly against a wall, mount your bike and assume your normal riding position. Record the weight. Turn your bike around and repeat for the other wheel.

Once you know your tire width, wheel diameter, combined weight, and weight distribution, determine your optimum tire pressures through one of four ways:

• Use the chart in Heine’s article (originally from Frank Berto) at: http://www.bikequarterly.com/images/TireDrop.pdf
• Use Dave Adams’ spreadsheet calculator at: http://www.biketinker.com/tire-pressure-calculator/
• Use Dave Adams’ equation, available at: http://www.biketinker.com/2010/bike-resources/optimal-tire-pressure-for-bicycles/)
• Use the table below (generated using Dave Adams’ equation), which assumes 700C wheels and 40%/60% front-to-rear weight distribution.

If you’re heavier than the average rider, you may find that the optimum pressure exceeds the tire manufacturer’s recommended maximum pressure. Frank Berto (http://www.bccclub.org/documents/Tireinflation.pdf) points out that manufacturers generally test their tires to twice the recommended maximum pressure. So, you’ll probably be OK going 10 to 20 percent over the recommended pressure. You may, however, want to consider switching to a wider tire. This will let you run a lower pressure, for a more comfortable and sure-footed ride.

All this fuss about tire pressure is only meaningful if you have a reasonably accurate way to measure tire pressure. Berto found that most gauges are pretty accurate when new, but tend to read high as they wear out. Gauges built into floor pumps get extra wear, so they may read high sooner than hand-held gauges. A hand-held gauge has the additional benefit of releasing less air when removed from the valve stem, so that you end up with a tire pressure somewhat closer to the pressure you actually measured.

Some additional tips:

• If you just pulled your bike out of a cold garage or basement, or out of a hot car, let your bike sit outside a while before inflating the tires so that the tires are at about the same temperature as the outdoor air.
• If your bike is in a car that might sit parked in the hot sun, let some air out of the tires. Otherwise, your tires could blow out as they heat up.
• Check your tire pressure regularly. For most road bikes with skinny tires, you’ll want to check tire pressure each day before riding.
• Pay attention to how your bike feels on the road. If your bike bounces or scoots to the side when hitting bumps, your tires may be over inflated. If the ride feels soft or sluggish, your tires may be under inflated. If your bike rocks fore and aft on bumps, you probably have one tire that is either over or under inflated.
• When riding, carry a pump (or cartridge), tire levers, a spare tube, and a patch kit. If it’s raining, consider carrying two spare tubes. You are more likely to get a flat on wet roads, and your patch kit will not work.

It may seem a bit complicated, but you only need to figure out once the optimum tire pressures for your bike (unless you change tire sizes). You’ll have a safer and more comfortable ride, and you won’t have to work any harder to keep up with your friends.