Optimizing Tire Pressure - 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 affected 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.

Vibration As A Theory

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.

60-80 psi

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.

85 psi

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.

90-120 psi

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.

9 comments

Andy,

Good questions. I’ve answered them below.

1. I am assuming that the speed and cadence of the rider were kept the same for the various tests.

Yes, you are correct. We have more tests being conducted now that will address a number of variables. We will be writing more about this as we progress.

2. It would be good to confirm what they were and if kept constant. What type of road surface was used for the test – from the image it looks like it could have been asphalt. Can the test be repeated at different bike speeds to see what the trend is?

We did test on asphalt that was smooth. We are conducting more tests now and I am excited to share the findings.

3. As a recommendation, when sharing charts like this, it is always good to keep the ‘y-axis’ the same otherwise you may be biasing the ‘visual’ results (e.g. amplitude) – even though I do understand it was the ‘x-axis’ (e.g. frequency) differences you wanted to point out.

Yes, you are correct. The y-axis is important.

Ride safe,

Jon

Jon Thornham August 04, 2020

MW,

Good question. A higher weight means you want a higher pressure. Our pressure recommendation charts include the weight of the rider and adjust the pressure based on this. I’ve added a link below. Scroll down the page until you see the pressure charts.

https://flocycling.com/products/flo-49-disc-brake

Ride safe,

Jon

Jon Thornham August 04, 2020

Pierre,

Good question. The speed was 12-15mph and the road was smooth pavement. We have a lot more coming up that discusses the variables you’ve mentioned and more.

Ride safe,

Jon

Jon Thornham August 04, 2020

Yes, weight will matter. We have more on this in an upcoming post. I weigh 158 lbs.

Jon Thornham August 04, 2020

What is the weight of the rider for these tests? I am assuming that makes a difference but wondering if you tested that?

Thanks!
Cheyenne

Cheyenne Noble August 04, 2020

Intriguing! I’ve had two spinal fusions and anything that may improve my riding experience, even with minimal gains in comfort,speed, etc., is worth paying attention to.

Bobby Sorensen August 04, 2020

Any information about the rides where you collected the data? Range of speed for instance and road quality ? Thanks.

Pierre August 04, 2020
I liked reading about this test because my newest bike came with 28c tires. The rims have a max pressure limit of 87 psi.

My question is this: How does bike/rider weight figure in? In other words, I can see an advantage here in running 65-75 psi for a 155-175 pound rider. But what about the 190-220 pound riders? If they are running 65-75 psi and run the risk of a pinch flat shouldn’t they be running 80-90 psi range? I have to add that I am using clinchers and not tubeless set up.

MW August 04, 2020

Great to see this level of analysis being done by FLO. I enjoyed the article and have a few questions. I am assuming that the speed and cadence of the rider were kept the same for the various tests. It would be good to confirm what they were and if kept constant. What type of road surface was used for the test – from the image it looks like it could have been asphalt. Can the test be repeated at different bike speeds to see what the trend is?

As a recommendation, when sharing charts like this, it is always good to keep the ‘y-axis’ the same otherwise you may be biasing the ‘visual’ results (e.g. amplitude) – even though I do understand it was the ‘x-axis’ (e.g. frequency) differences you wanted to point out.

Thank you

Andy Basacchi, P. Eng. August 04, 2020

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