Measuring Wheel Vibration – Speed Is Key

Based on all of our vibration work to date, we determined that the best way to find an impedance break point was to develop an algorithm that would calculate the amount of energy produced by a wheel and tire combination at various tire pressures. Our previous work showed that as the tire pressure increased, the energy of the system increased as well. If we could plot this successfully, our belief was that we would see an increase in the slope of a line that plotted tire pressure vs wheel energy at the impedance break point.

To test this theory, we mounted two 3D Accelerometers to a set of FLO 64 AS wheels and tested four pressures so that we could plot the total wheel energy vs. tire pressure. To be thorough, we had two riders run the same course. 

We rode the same course to collect the data. The out and back loop had small grade variations so we did not expect to see anything drastic. However, drastic is what we saw. After running the tests we plotted the data and nothing made sense. Higher pressure had lower energy than lower pressures on some tests and vice versa on others. This made no sense and showed no repeatability. 

1. The algorithm was sampling data from 20 second intervals on the course. This sampling was not in the same course location on the out and back. 
2. The slight variation in grade produced a slightly different rider speed.
3. Our theory was that the difference in rider speed could have impacted the overall energy results of the tests. Our initial test held power constant.

1. We would hold the pressure constant.
2. We would test four different speeds.
3. We would use two different riders to see if we could repeat the results. 

If our theory was true, we would notice a difference in overall wheel energy as rider speed changed and pressure remained the same.

What is really cool to see is the beauty of the graphed data. In all the years we’ve collected and plotted data to understand what is happening in the real world, I always love when data produces a clean result. In an engineering way, it’s beautiful. 

Now, we do not have enough info to draw an exact conclusion, but what you see here is potentially the following:

1. The lower speeds produce a lower increase in wheel energy. 
2. At 18mph to 21mph you see an increase in the slope, which appears to indicate the pressure tested incurred an impedance break point between 18 and 21mph. 

Here is what we now know from our data on wheel energy as speed increases. 

1. Impedance break points are not related solely to pressure; they produce a matrix of values related to speed, surface roughness, rider weight, and pressure. 
2. Velocity is much more of a factor than we originally thought.
3. Tire pressure recommendations are more like a fingerprint than a simple table.