In 2016, we conducted an experiment with Union University in Jackson, Tennessee to understand how casing tension is affected by rim width, tire pressure, and tire width. (Check our original post on casing tension here for more information).
For the study, we mounted strain gauges on the inside surface of Continental GP 4000 S II tires in three sizes (23mm, 25mm, and 28mm) and measured the strain of the tire casing as tire pressure changed.
Note: The graphs in this article show measured strain. Strain is used to calculate casing tension.
What We Knew and Wanted to Show
Tire theory says that if we keep the rim and tire pressure constant, the larger tire will have a higher casing tension. We tested a Continental GP 4000 S II in 23mm, 25mm, 28mm size on a FLO 60 Carbon Clincher. The results in the graph below show that the larger the tire, the higher the casing tension at the same PSI.
What We Didn’t Know and Wanted to Prove
We determined in Part 3 of our Gravel Wheel Design Journey that if a tire size is kept constant and we increase the internal rim width of the wheel the tire is mounted on, the tire width will increase.
If we think about tire theory, stated above, we hypothesized that the same size tire with the same tire pressure on a rim with a wider internal rim width should have a higher casing tension than a tire on a rim with a smaller internal rim width. To test, we used two rims, our FLO 60 Carbon Clincher with an 18mm internal rim width and an OE wheel with a 15mm internal rim width. We tested all three of the Continental GP 4000 S II tires — 23mm, 25mm, and 28mm.
The results below show that as internal rim width increases and tire width and pressure is kept constant, the casing tension increases. Therefore, increasing rim width also increases casing tension which has the same affect as increasing tire width. In time, we will explain why you are able to lower your tire pressure with a wider rim in the same way you can lower your tire pressure with a wider tire.
The next part will discuss measuring rolling resistance by roller testing. Stay tuned.
We did get a report from Union University. Most of the info is covered here but consolidated for lighter reading.
Is there a thesis or report from the university on this procedure? Would love to read more.
Very cool! How is microstrain calculated (Delta L)/L x 1000 ?