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.
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