We break down many of the myths surrounding tire width, pressure, and aerodynamics to better understand why the tri world is riding wide.
If you were to ask 10 different pro triathletes the size of their tires and what pressure they pump them to, you would likely get 10 different answers. Finding the best combination of tires, wheels, and air pressure is not simple, and there is no “one size fits all” answer. The perfect combination for one stretch of road is different for another, but with a better understanding of this complicated subject you can make better decisions leading to faster bike splits and more comfortable confidence on the bike.
The Contact Patch
Before jumping into all the details, it’s important to be familiar with one critical element of bicycle dynamics where the rubber literally meets the road: the contact patch. This very small section of tire that physically touches the ground gets a lot of attention from engineers because the shape and size of the contact patch affects the rolling resistance and traction available at any given moment. The contact patch is influenced by many variables, including tire size and inner rim width, the material properties of the tire and tube, the tire pressure, external forces (turning, braking, accelerating), and wheel lean angle among others.
Wheel manufacturer Flo Cycling performed a study a few years ago comparing the size and aspect ratio of the contact patch created by wheels and tires of different widths. Below is a quick graphical representation of the results. (The third measurement is the width x length of the contact patch.) As you can see, a wider tire and rim creates a wider, but shorter, contact patch, which requires less force to roll. This phenomenon is responsible for many of the benefits of using wider wheels and tires.
If there ever was a topic that people seem to be generally misinformed about, this is it. Rolling resistance is the resistive force created by the tire rolling on the ground, it is primarily affected by tire properties and air pressure. In the dark ages, and unfortunately still in many circles today, it was common practice to pump the tires to the maximum amount permitted by the print on the sidewall. A highly pressurized tire certainly feels fast, but feelings aren’t always the best judge of reality. Today, in the enlightened era, we know that there are two aspect of rolling resistance that need to be considered: tire deformation and impact forces. On a perfectly smooth road, a tire pumped to a high pressure will experience less deformation as it rolls and will therefore have less rolling resistance. Unfortunately, there aren’t too many perfectly smooth roads and impact must be considered as well. When you impact a bump in the road—even very tiny ones—it causes the tire to deflect and cause forces applied to the tire. These forces oppose forward motion and slow you down. A tire pumped to a lower pressure will experience lower forces when impacted, therefore reducing rolling resistance from bumps. Since higher air pressure reduces deformation losses and increases impact losses, striking a balance of tire pressure for maximum overall efficiency is essential. A wider tire, pumped to a lower air pressure than a narrow tire will exhibit the same amount of rolling resistance from deformation thanks to the shorter contact patch but since the tire is more compliant, it will roll with less resistance to impact and deflection. The important takeaway is to remember that bumpier roads are better ridden with wider tires pumped to a lower air pressure to a point, just be sure to stay within the recommended range on the tire sidewall.
Wider tires have a larger air volume than narrow ones, and because the higher volume creates greater vertical distance between the road and the rim, they can be left at lower pressures without running the risk of a pinch flat.. The higher volume and lower pressure also creates a superior shock absorber than that of a narrow tire, meaning that rough roads physically feel better with a wider tire.Tire size and air pressure can have a significant influence on the ‘harshness’ of your bike’s ride quality. Since aero bikes often have a stiff ride quality that can abuse riders during long days in the saddle—and on race day, you’ll still have to run—it’s important to factor in tire size and air pressure when thinking about your comfort. If your bike is too rough, first consider riding a wider tire to help soak up the bumps before running out to buy a new frame.
The wheel and tire interface is one that wheel designers play close attention to. Since wider tires have been generally accepted to be better than narrower ones for the reasons we’ve mentioned, many wheel manufacturers now are making wheels that are also aerodynamically optimized for wider tires. This typically means that rims are getting wider as well. Josh Poertner, formerly of Zipp and now with Silca, coined the term ‘The Rule of 105%’ which essentially says that the rim should be at least 105 percent of the width of the tire at its widest to improve aerodynamics. In the case of a wheel designed for a 25mm tire, they are likely to have less aerodynamic drag when using a 23mm tire, according to Nathan Barry, a design engineer with Cannondale. Bear in mind, the aerodynamic gains may not offset the increase in rolling resistance or comfort from the smaller tire.
What does it all mean?
Since it is generally accepted that a wider tire is better than a narrower one, why aren’t we all rolling around on motorcycle tires? The answer is two-fold:, bike compatibility and weight. Many of the road and triathlon frames manufactured today still have restrictions with 25mm tires, so there is little point in making wheels and tires that are even wider. So before you jump on the superwide bandwagon check your frame and fork’s compatibility.
The beauty of all this research and tech behind tires means that triathletes can now arm themselves with a substantial upgrade for far less than a new frame. If you’re willing to let go of the old misconceptions around the skinny wheel/super high pressure mindset, today’s rubber and wheels can open a whole new world of comfort, speed, and confidence on the bike.