How Velocity Affects Impedance

We knew when we started developing our FLO All Sport and Gravel wheels that rolling resistance would be a large focus of the process. We believed that a wider rim would lower rolling resistance. To test this hypothesis, we spent close to two years developing an on-road testing protocol to measure rolling resistance which included advanced hardware that allowed us to measure accurate rolling resistance values on road.  (If you are wondering what rolling resistance is, this article is for you.)

Previously, work from Silca and Tom Anhalt showed a phenomenon called “impedance.” Impedance happens when rolling resistance increases drastically once you hit a certain pressure. This finding was contrary to lab results which show that as tire pressure increases, rolling resistance decreases. The spot where rolling resistance starts to increase due to tire pressure has been coined the “impedance break point.” (Wondering what impedance is? This article explains it.)

We also found impedance break points in our data, which we expected. However, we also determined something new. The data shows that rider velocity has an effect on your impedance break point.

To paint the scenario, we were measuring rolling resistance on road for a given tire pressure, tire width, and rim width, expecting to find a single coefficient of rolling resistance value. In most cases, we did. However, when we were near the impedance break point we started to notice that our data converged on several different values for a given test. Naturally, our first question was, “why?”.

In order to test rolling resistance on road, our protocol included riding at three different power outputs over the same course. As we looked more closely in the data, we noticed that rider velocity was the only variable that changed. In the cases where the Crr value was converging to multiple different values, the lowest power output always had the lower Crr value, and the highest power output always had the highest Crr value. This means that as a rider increases their velocity, the impedance break point happens at a lower pressure. When you plot the data, there is a shift in where the break point occurs. The graph below shows one example where we saw the break point shift.

I had two theories. I called three of the smartest people I know who are known in the industry to see if I could get their backing. They all agreed with theory one and two said theory two may hold some weight. I believe in both. But first, here is a quick refresher on hysteresis.

The National Academy of Sciences explains hysteresis in a wheel as:

“A characteristic of a deformable material such that the energy of deformation is greater than the energy of recovery. The rubber compound in a tire exhibits hysteresis. As the tire rotates under the weight of the [rider+bike], it experiences repeated cycles of deformation and recovery, and it dissipates the hysteresis energy loss as heat. Hysteresis is the main cause of energy loss associated with rolling resistance and is attributed to the viscoelastic characteristics of the rubber.”

When you are moving forward faster (increasing velocity), the wheel of your bicycle is moving faster as well. As a result, the tire is impacting the ground more rapidly and with a higher force. This means that the hysteresis effect increases causing a break point earlier.

The second theory has to do with the cyclical loading and unloading of rider mass at higher power outputs. Most of the tests performed were are at close-to-max efforts. Anyone that has put out a strong effort can attest that at max effort, your body starts to bounce up and down as you pedal. But, when you are soft pedaling, your stroke is much smoother.

The theory is that since there is more of a cyclical loading effect of rider mass, the downward pressure of the bounce adds to the hysteresis loses.

These findings show that your average speed will impact your rolling resistance and what your tire pressure should be. Today, it’s tough to say specifically how this effects the break point but we know that it does.

All FLO wheels have a tire pressure recommendation chart that considers your tires size and your weight. That chart will give you a starting pressure. We encourage you to experiment on your own, but you will want to lower the pressures from the suggested starting points. Remember, it’s always better to be slightly low than slightly high. Crr spikes quickly once the break point is reached but before the break point, Crr decreases quite gradually.

Currently, we are working on a way to make this type of testing easier for cyclists but we have our work cut out for us. More on this in a future blog post.