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.  

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:

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.

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.

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 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.