Using Vibration To Measure Impedance

Vibration has been on my mind. When designing the All Sport and Gravel wheel lines, we aimed to prove that a wider internal rim width lowered your rolling resistance. During that process, we ran hundreds of tests at different tire pressures, tire sizes, and internal rim widths. After these tests proved our theory correct, our All Sport and Gravel wheels were designed with wide internal rim widths to make you faster.

While testing on-road rolling resistance, I acquired a greater understanding of how tire pressure affects the vibration of the bike and how that vibration affects me, the rider.

When you’re riding a bike on the road, you eventually hit an impedance break point. When this point is reached, your rolling resistance spikes, slowing you down. (Read here for more information on impedance break points.) What was interesting was that it also felt like vibration increased at the break point.

To test this theory, we ran a test to determine if tire pressure altered the wheel vibration in the same way that impedance did. If the break point really does create more up and down motion then, in theory, the vibration would increase and my perception would be accurate.  We set out to answer the question, “Can you use vibration as an indicator for impedance break point?”

For the test, I reached out to a good friend and fellow UNB Engineering grad, who just so happens to be a vibration expert.

First, we mounted a uni-directional accelerometer to the front skewer. This allowed us to measure acceleration in the y-axis (up and down).

Then, we connected the accelerometer to a data acquisition unit that sampled from 0-1000 Hz. This meant we would pick up any vibration in this range.

Next, we tested a range of pressures from 60-120psi in 5psi increments.

To prove the concept, we tested 28mm Continental GP 5000 tires on the FLO 64 AS Disc wheels. We ran a Fast Fourier Transform (FFT) calculation on the acquired data to visualize the data acquired.

Here is what we found. I am going to discuss three key locations in the data.

Between 60 and 80psi, there was very little excitation in the FFT plot. This means that the accelerations in the y-axis (up and down) were minimal.

At 85psi, we started to see a spike in excitation in the 30-50Hz range. This means we were starting to see vibration in the y-axis. What is really awesome about this is that this is the same pressure at which we saw the breakpoint during our on-road testing. My theory was starting to hold some weight.

As pressure increased from this point on, the excitation got larger and larger, which means the system experienced more and more vibration. This is also in line with our on-road rolling-resistance testing. As pressure increases past the break point, rolling resistance gets worse.

In the plots above, the increase in vibration happens around the 30-50Hz range. This is because the wheel has a natural frequency in this range. We’ll visit this topic another day.

To summarize, based on our first tests, it appears that an increase in vibration could be an indication of the impedance break point. Stay tuned for more. And yes, the picture below shows milk and cookies. My wife is awesome.