# FLO Gravel Wheel Design #13 – On Road Rolling Resistance

We had two questions when we started designing the gravel wheel series; one, “can a gravel wheel be aerodynamic?” and two, “Can we design a rim that lowers rolling resistance?”
In Part 12 of the Gravel Wheel Design Journey, we showed we could improve the aerodynamic efficiency of a gravel wheel and save you 5-6 watts per wheel. Aerodynamics is only half of the time savings equation. This article discusses what we found after extensive on-road testing of a rim designed to lower rolling resistance. Did the results prove we did it? The quick answer, yes we did it. We have rims that lower rolling resistance. If you’re wondering what rolling resistance is, check out our article on Understanding Rolling Resistance for Cyclists.

Another key finding in our research was that this data could also be applied to our FLO All Sport Series. The data collected here shaped the design of the Gravel Series and All Sport Series.

## What We Tested

We hypothesized that since a wider tire lowers rolling resistance, then a wider rim would as well. So, we developed a wider rim, and tested internal rim width of wheels on road to determine if it affected rolling resistance. The table below shows what we tested:

Pressure (psi) Continental GP 5000 Tire Size
25mm 28mm 32mm
Internal Rim Width Internal Rim Width Internal Rim Width
17mm 21mm 17mm 21mm 17mm 21mm
65 Crr? Crr? Crr? Crr? Crr? Crr?
80 Crr? Crr? Crr? Crr? Crr? Crr?
95 Crr? Crr? Crr? Crr? Crr? Crr?
110 Crr? Crr? Crr? Crr? Crr? Crr?

## How We Tested

There were two stages of testing: first was the on road portion, as discussed in Part 10 of the Gravel Wheel Design Journey but a quick recap is below; the second part was temperature compensation for the on-road testing portion.

## Part 1 – On Road Testing Protocol

Below are the steps we took for each tested tire, width, pressure combination.  The image below shows the steps as well.

1. Install the tires and inflate to a pressure close to the working temperature.  Take a lap to warm up the tires.
2. Install the AeroLab Sensor and check that it is level.
3. Set test tire pressure.
4. Measure the casing width of front and rear tire.
5. Measure the road surface temperature.
6. Measure the tire surface temperature.
7. Weigh the bike and rider weight.
8. Record wind speed and direction.
9. Record the ambient air temperature.
10. Start the AeroLab Sensor.
11. Start the wheel speed sensor.
12. Start the power meter.
13. Ride and record data.

## Part 2 - Temperature Compensation

We know that on a hot day a tire performs differently than on a cold day and vice versa. On-road rolling resistance testing requires us to temperature compensate to make sure we have the most accurate data. To do this, we ran roller tests at two different temperatures. We followed the same protocol above with a few modifications. Below is a picture taken during temperature compensation:

## The Results

The graphs and tables below show the results. For each tire size and each tire pressure, the wider rims have a lower rolling resistance than the more narrow rims. We show a large range of results which could be related to a number of things like casing tension and ground speed. Throughout this process, we discovered something we have not heard of before and are pretty excited to share in an upcoming article on groundspeed and impedance. You can see impedance breakpoints for all of the tires in the graphs below. For more on impedance check out our article on Understanding Rolling Resistance and Impedance.

Pressure (psi) Continental GP 5000 Tire Size
25mm 28mm 32mm
Internal Rim Width Internal Rim Width Internal Rim Width
17mm 21mm Percent Decrease 17mm 21mm Percent Decrease 17mm 21mm Percent Decrease
65 0.0051 0.0050 1.96% 0.0049 0.0047 4.08% 0.0048 0.0046 4.17%
80 0.0048 0.0046 4.17% 0.0046 0.0042 8.70% 0.0044 0.0043 2.27%
95 0.0046 0.0043 6.52% 0.0051 0.0043 15.69% 0.0043 0.0042 2.33%
110 0.0055 0.0052 5.45% 0.0054 0.0045 16.67% 0.0061 0.0051 16.39%

Note:  The testing above shows road data. We have more data—including gravel specific data—coming soon.

These results tell us that we can—and did—design a rim that lowers rolling resistance. And we proved the results on road, where it matters. Not only that, but the wind tunnel results show these wheels are aerodynamic as well. We are incredibly grateful for our partnership with UNLV and AeroLab for working with us on this process. This truly would not have been possible without them. We're excited to provide more testing and results soon.