Wider wheels = faster wheels. However, the key to getting the most out of a wider internal rim width wheel is accurate tire pressure. If you haven’t lowered your tire pressure, you’re losing time. This article walks you through why lower pressures are required for larger tires or wider rims.
Casing Tension in Cycling Tires
First, we need to understand casing tension in pneumatic tires. Casing tension refers to the amount of tension or stretch that exists within the tire’s casing or carcass. The casing is made up of layers of cords or fabric that are bonded together and placed between the tire’s tread.
Casing tension is important because it affects the tire’s performance in several ways. A tire with too little casing tension is more prone to flexing and overheating, which can lead to premature wear and failure. On the other hand, a tire with too much casing tension may be too stiff and inflexible, which reduces its ability to absorb shocks and impacts.
Manufacturers carefully design the casing tension of a tire based on its intended use and the desired balance between flexibility and stiffness.
For example, a tire designed for high-speed racing may have a higher casing tension to provide better stability and handling, while a tire designed for off-road use may have a lower casing tension to provide better traction on rough terrain.
A tire with too much casing tension will push you past the impedance break point. Once we pass the impedance breakpoint, each additional psi will cost you one watt. Every watt counts, especially for competitive athletes.
Wider Rims, Wider Tires, Same Pressure, Higher Casing Tension
Here’s the thing most people don’t know about casing tension— if you keep your tire size and pressure stays the same and switch to a wider internal rim width wheel, your casing tension increases. From what we just learned above, too much casing tension leads to a loss in watts, which can be solved by lowering your tire pressure.
The same goes for larger tires. A fat bike tire with 3 psi feels just as hard as a road bike tire with 90 psi because the casing tension is the same. As a tire gets wider, the casing tension increases when tire pressure stays the same, even on the same size rim.
The impact is twofold when you consider that modern carbon fiber cycling wheels are wider and wider tires are recommended.
Why Does the Casing Tension Increase As a Tire/Rim Gets Wider?
Let’s dig a little deeper into why casing tension increases as a tire or rim gets wider and the pressure stays the same.
Hoop Stress in Bicycle Tires
Inflating a tire increases the internal pressure of the wheel-tire combo. This increase in pressure pushes the tire and rim outward creating increased casing tension, or hoop stress.
Assuming a bicycle tire is a cylinder, the stress on the cylinder wall caused by the hoop force can be calculated using the formula:
σ = F/A,
F = force applied per unit area
A = area of the material being stressed.
For a cylinder, such as a tire or a rim, the area A is equal to the product of its length L and thickness T:
A = TL
In the case of a tire or rim, force F is caused by the increased tire pressure inside the rim. This force is equal to the air pressure (P) multiplied by the cross-sectional radius (R) of the tire/rim cylinder multiplied by the length (L) of the cylinder. Since we know the radius is half the diameter we get the following equation:
F = PDL/2
σ = F/A = PDL/2TL = PD/2T
Assuming the tire thickness is the same for both narrow and wide rims, and the rim wall thickness is also constant, we can use subscripts “n” and “w” to denote the narrow and wide rims, respectively. To maintain the same stress on the tire and rim, we need to set σw = σn, which gives us PwDw = PnDn.
Let’s compare two rims with different internal widths:
- Original FLO Carbon Clinchers: 17mm internal rim width.
- FLO AS Wheels: 21mm internal rim width.
Let’s assume the tire width (cross-sectional diameter Dn) on the narrow rim is 28mm for a 700 x 28c tire. If you place the tire on the wider, 21mm internal rim width, wheel the tire will get wider. Let’s calculate how much. You can see that the cross-sectional diameter changes as shown below.
21mm – 17mm = 4mm
We know circumference is represented by C = πD so the following is true.
Cn = π * 28mm = 88.0mm
We also know that Cw = Cn + 4.0mm and:
Dw = Cw/π = (Cn + 4.0mm)/π = (88.0 + 4.0)/π = 92/π = 29.3mm
As a result of going from a 17mm to a 21mm internal rim width wheel, you change your tire width from 28.0mm to 29.3mm.
Finally, we need to calculate the new pressure. Let’s assume the 28mm tire with a 17mm internal rim width is set to 80 psi. The tire pressure for the 21mm internal rim width wheel that would create the same casing tension would be as follows:
PwDw = PnDn
Pw = PnDn/Dw = (80 * 28)/29.3 = 76.5psi
This proves that a wider rim or wider tire requires less air pressure to maintain the same casing tension.
What Tire Pressure is Best For Your Bike Wheels?
While you either loved the math above or your head is spinning, finding the right tire pressure can be overwhelming. We created tire pressure charts for all of our wheels that includes terrain, tire size, and rider weight. We highly recommend you use the charts for picking your perfect tire pressure.
If you geek out about tire pressure as much as we do, check out the FLO Air Gage. We designed the FLO Air Gage to make setting tire pressure easy and accurate wherever you go.
Co-founder at FLO Cycling. Jon manages the day to day operations and acts as the lead engineer for all FLO products.