|Tire Bead Test
by Damon Rinard
On many occasions Jobst Brandt has written about a tire bead test. Recently:
"[T]he tires we ride are in fact clinchers that stay on the rim primarily by the clinch of the hooked sidewall that retains the tire bead. To test this, I cut the bead wire in five places (on a tire that was worn out) and mounted it on an Mavic MA-2 rim where it stayed in place while inflating it to 100 psi."
From: firstname.lastname@example.org (Jobst Brandt)
Subject: Re: Tight fitting tires needed
Date: 30 Aug 2000 00:00:00 GMT
For fun, a friend and I replicated this test, with photos below. An old worn out Specialized Armadillo from my tandem was a good candidate tire.
We cut both beads in five places around the circumference.
You can see the steel bead is in fact cut through. We then mounted the tire with an inner tube onto a good used Velocity Aerohead rim.
The mounted tire was a shockingly loose fit on the rim. The handle of the diagonal cutters easily slipped fully between the rim and both beads of the tire with its larger dimension vertical.
We carefully inflated the tire. Even though we were wearing leather gloves, long pants, ear and eye protection, I chickened out after we reached about 110 psi. Result: The tire did not blow off the rim despite the cut bead making a tight fit impossible.
Conclusion: Clincher tires stay on the rim primarily by the clinch of the hooked sidewall that retains the tire bead, not the circumferential tension in the bead. What this means is that a tire does not need a tight fit for it to stay on the rim; it needs a rim with a hooked sidewall, and air in the tire.
Note from John Allen: This is a very interesting experiment, however the behavior of a deflated tire depends greatly on its fit on the rim, and is a safety concern. Prof. David Gordon Wilson, author of the book Bicycling Science, and his students, have conducted experiments showing that only a tight fit on the rim allows the bicycle to remain controllable in case of a sudden deflation or blowout of the front tire, see the article in Human Power, issue #51. I expect that a tire with broken beads would be even more of a problem than a loose tire. I also expect that a tire without a bead wire, even a broken one, would need more pronounced hooks in the rim flanges to stay in place.
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