In Thursday’s RoadBikeRider Newsletter, reader John B. writes as part of an e-mail on another matter.
I’m not one for crawling downhill with the brakes on, but I have experienced tyres melting on descents in the Alps and Dolomites. The section that contains the bead melted and stuck to the rim. This is obvious on a descent when you get a puncture and are faced with a lovely goopy mess.
… framebuilder and tour leader Glenn Erickson recommends Kevlar-beaded tyres vs. wire beaded ones. His theory is that the metal in wire beads gets hot and melts the tyre.
The physics of this situation elude me. Friction between the brake pads and rims is causing heat. The heat goes into the air and into the rim. The rim heats the rubber and casing of the tire, and the tire heats the wire (or not). I don’t see how the wire gets hotter than the rubber.
What I could imagine it that the wire would conduct the heat better than the Kevlar, but this conduction would be along the circumference to the tire. Since the heat source, the rim, is more or less equidistant from all parts of the wire, that doesn’t seem to make any difference.
My first thought is that the heat was localized by the wire in some way. The geometry of if the situation seems symmetrical and if anything the wire would work like a heat sink to draw the heat from the hottest part of the system. I don’t see why having more heat on the inner surface of the tire would cause failure.
Maybe there is structural reason? Perhaps the construction of Kevlar-beaded timers is more tightly coupled than the construction of the wire beads. I could imagine that Kevlar is woven into the threads of the tire so that even when the tire heats it doesn’t fail, whereas the wire bead is simply surrounded by a wrap of fabric and covered with rubber.
I need to cut open a couple of tires – I just junked a Kevlar-beaded tire last week and investigate the construction, or do some experiments. It would be great to get my hands on some tires that have failed this way. Any body know of a source.
Google doesn’t immediately reveal any construction diagrams, and nothing on heat related to bicycle tire constrution.
Googling Glenn Erickson shows a phone number and e-mail address for his shop. I will drop him a line for his thoughts. If he responses, I’ll let you know.
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