Proper road bike tire inflation - In-depth discussion of current science-based views and recommendations
How much air pressure should we be inflating our bike tires to? That debate has been raging ever since the first pneumatic bicycle tire was invented in 1887.
We all know that underinflated tires are at risk for getting “snakebite” punctures, so the most oft-heard advice is pump the tire to the max pressure printed on the tire's sidewall. Unfortunately max pressure is actually too much air in most cases which can cause safety problems like an exceedingly harsh ride. To understand proper tire inflation, one needs to understand the science behind a nice-rolling tire, and it starts with understanding what slows down a rolling tire, aka “rolling resistance.”
How a particular brand and model of bicycle tire is designed greatly affects its rolling resistance, such as the formulation of the tread compound and how thick and flexy is the tread and casing, but that's beyond the scope of this article which will be focusing on tire pressure. (For rolling resistance tests of various makes and models of bike tires to help you decide what tire to buy, http://www.bicyclerollingresistance.com/road-bike-reviews is a great resource). And aside from the normal frictional forces that we are all familiar with from high school physics class, there are three other major contributors to rolling resistance in a road bike tire: flexing in the tire, bumps in the road surface, and the weight of the tire. (Aerodynamic drag also plays a role, but the width of the tires commonly found on road bikes ranging from 700x23c to 700x32c have such small differences in drag as to be almost insignificant.) We need to look at and understand those three major contributors, starting with tire flexing.
Flexing in the tire as it rolls along dissipates your pedaling power as waste heat. Try this: Take a ball of play-doh, and knead it vigorously a few times. You will feel the mass of play-doh get hot from the kneading. The scientific term for this phenomenon is internal friction, or “hysteresis”. This is exactly how rolling tire flex absorbs your pedaling power and wastes it as heat, contributing to rolling resistance. We all feel this when riding underinflated tires: As the underinflated tire rolls under the rider's weight, the sidewall flexes to bulge out where the rolling tire meets the road, giving a mushy, sluggish feel. The thing to understand about tire flex is that not all tire flex is bad; we do need just enough tire flex to soak up all the bumps in the road for a smooth, controllable ride. Imagine if you are rolling on solid metal wheels that don't flex (i.e. on bare rims with no tire), it would be a supremely dangerous, teeth-chippingly jarring and uncontrollable ride. In other words, too much tire flex is bad, too little tire flex is bad, just the right amount of tire flex is awesome.
Bumps in the road are the next major contributor to rolling resistance. Just to define what we mean by bumps in the road: Even freshly-steamrolled pavement is not totally smooth and has a texture of centimeter-scale roughness features (the individual small rocks held in the asphalt's matrix of tar), and a bicycle tire needs to have just enough flex to roll over that texture smoothly. Obviously a smoother surface will have less rolling resistance than a rough one, and cyclists best see this effect coasting down a relatively smooth road versus a rough one: On a smoother road one can coast further. To accommodate a normal range of road surface roughness we encounter on a typical bike ride, we cyclists have to compromise a little and have tires that flex just enough to smooth out the ride. In effect we are conceding a little bit of rolling resistance from having that small amount of tire flex so that we can negate as much of the rolling resistance from the rough surface as possible. We cyclists have to perform that balancing act to find that optimum amount of tire flex for the road surface.
Lastly, tire weight is also a major contributor to rolling resistance BUT only during acceleration, when you are pedaling hard trying to push the tires' rotational weight up to a faster RPM. The heavier the tire, the more rotational weight you have to push when trying to accelerate, the more sluggish the tires will feel. Tire manufacturers like Continental have shown through research that It is indeed true that wider tires have both lower tire flex rolling resistance and rough surface rolling resistance, because a wider contact patch means the tire has to flex less than a narrower tire to roll smoothly over the same rough surface. So why not put 4-inch-wide fat bike tires on a road bike you ask? The simple reason is weight-- An increasingly wide tire gets so heavy that it negates the wider tire's lower rolling resistance advantages in tire flex and ride smoothing. This is why a bike with 2-inch-wide heavy tires take more effort to pedal the same distance on the same paved road as a road bike with 1-inch-wide lighter tires. The best compromise for road bikes between the lower rolling resistance of wider tires and the accompanying weight penalty seems to be 700x25c, which is what the racing pros are overwhelmingly preferring nowadays over the older narrower 700x23c standard. Tires wider than 700x28c is where the weight penalty starts to outweigh the lower rolling resistance advantage of the wider tires.
So minimizing your road bike's rolling resistance requires you find a balance: A tire just wide enough to be able to smooth out the ride over a surface textured with centimeter-scale bumps with minimal tire flex, but at the same time not so wide (hence heavy) to make acceleration sluggish.
So where does tire pressure fit in all this? Quite simply, you need enough air in the tire to give just enough tire flex to smooth out rolling over bumps on the road surface. Insufficient tire pressure will increase the tire flex rolling resistance far beyond what is necessary to counteract the rolling resistance from road surface roughness, resulting in an exceedingly mushy ride. At the other extreme, too much tire pressure will have very little rolling resistance from tire flex, but will increase rolling resistance from road surface roughness so much and give an exceedingly bumpy ride, that it vastly overwhelms any benefit from eliminating the rolling resistance from tire flex.
So at last the 64-million dollar question: How much tire pressure is enough tire pressure to achieve that balance in minimizing both kinds of rolling resistance from tire flex and road surface roughness?
The 64-million-dollar answer is: How much tire pressure depends on two variables: 1) the total weight the tires are supporting (total weight of bike + rider + items carried) and 2) how wide the tires are.
Breaking down the answer:
1) Total weight supported by tire - Tire pressure and rider weight is a mathematically proportional relationship: The less you weigh, the less tire pressure you need. The more you weigh, the more tire pressure you need. So a light rider with excessively high tire pressure will be riding on too-stiffly-inflated tires, which has no tire flex and too much road surface roughness rolling resistance. This is why inflating to the max PSI printed on the tire sidewall is not always the best thing to do. We all need to find that optimum tire pressure for our individual weight.
2) Width of tire - Tire pressure and tire width is an inverse relationship: The wider the tire, the less air pressure it needs. A 1-inch wide tire needs a higher pressure than a 2-inch wider tire to support the same total weight (bike + rider + items carried) without bulging out at the bottom and causing excessive rolling resistance from unnecessary tire flex.
Bicycle tire manufacturer Michelin has published a very handy chart (I included it at the top of this page) on what tire pressure they recommend given the rider's weight and width of the tire. I would highly recommend using that chart as a starting point, then add or subtract a little pressure experimentally and see what feels best for you to determine your customized optimum tire pressure.
I would also like to add that many of us ride bikes that position the rider pretty upright (hybrids and relaxed-geometry road bikes), and for these bikes we need to use 10PSI less on the front tire because on an upright-riding bike more of our weight is supported by the back tire. Having less air pressure on the front tire also allows it to absorb shocks to give the rider better control, so you won't lose control and crash from the sudden shock of bouncing off a big bump. If it feels like you have to hold onto your handlebars with a death-grip to barely stay in control over the shocks and vibrations on your ride, you need to lower your front-tire pressure!
So there it is, my science-based recommendations on tire pressure in a nutshell: Use the Michelin guide chart at the top of this page as a starting point and try the pressure they recommend for your weight and tire width, then experimentally add or subtract a bit of pressure to find what feels good to you, and that's your custom tire pressure. 10PSI less on the front tire if you ride upright, decrease the front tire pressure until you have good control.
Hope this contributes to your safe riding and enjoyment! See you on a 5BBC ride.