In our last installment we were considering whether Kevlar or wire beaded tires should be more prone to failure in a downhill braking situation.
I had promised that I would write to Glenn Erickson concerning his recommendation to use Kevlar beaded tires because of a theory that the metal in wire beads gets hot and melts the tire. I couldn’t understand why the metal got any hotter than the Kevlar.
I received a very gracious reply from someone who worked for Erickson Cycle Tours. We didn’t specific discuss whether I could use her name, or quote her, so I won’t do either. I’ll just give you jist of her response, which she appears to be relaying from Glenn.
Glenn is unavailable until the fall because he is leading cycling tours in Europe. Not to puff my source, but check out the tours at their web site, fourteen separate tours this summer mostly in France, but also in Austria, Switzerland and Italy. If they thought that by being nice to this random e-mailer that they were going to get me to want to go on one of their tours, they were wrong. I wanted to go on one of their tours way before they responded to my e-mail. Now that they have been so nice, I really want to go on one of their tours.
I don’t merely mention their tours because they look like fun, but they are part of the background empirical data. In all of those tours, this is his twelfth year, Glenn has never seen a Kevlar beaded tire fail in an rim heating/braking situation. That is a pretty substantal claim, based on that experience (plus if you look at his bio, there is more). And although it would be useful to know how many wire beaded tires he has seen fail in these situations and what proportion of riders on his tours ride each, it is a fact that needs to be explained.
His current theory for why this is so isn’t really as quoted by the correspondent to RoadBikeRider Newsletter, that “the wire gets hot and melts the tire,” which if you will recall didn’t make much sense to me. I wondered how the wire got hotter than the rubber. He thinks that the wire can expand from heating which makes more sense.
I don’t however follow him to the next step, that the elongation of the wire allows the tire to roll off the rim. The wire bead on 700 mm rim is about 2.2 meters long (0.7 m x pi) and if the coefficient of expansion of that wire is between 3 and 4 micrometers per meter per degree C, the wire is going to elongate by at most a half millimeter when the temperature goes up 120 C. (There is a theoretical study here [PDF] saying that the maximum temperature on a braking rim is is 100° C.
That half millimeter means that the wire can move 0.25 mm from the rim. I wouldn’t think that would be enough to make a difference. Maybe it is.
Another possibility is that the wire and the rest of the tire (particularly the casing) expand differentially. The hot wire moving in the hot rubber while the whole is under increasing stress from the increasing pressure in the tire could cut the casing or the hot wire could move inside the casing or rubber to accelerate the melting and breakdown of the tire.
Glenn wanted me to know that while he was interested in the science of this failure, he was most interested in safety. I hope he has a great summer.
Updated – Jul 6, 2005 to fix the link to the pdf. Sorry about that.
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