The Chainlink

So last night riding home (on relatively good road conditions Augusta Blvd.) I begin to feel some "uneveness" to the rear wheel, but keep riding. A minute or so later I hear the dreaded 'pop' and the tire goes flat. Get home (it was 2 blocks from my house) and start to inspect things : there is a long gash (maybe 2") in the tube; the puzzling thing the tire was blown out (about the same width as the tube's gash) just under the wire bead : this is a two week old tire so it cannot be worn out. But it does not look like the tire damage was from a foreign object. i.e. no nail, glass, etc. in the tire and no small holes in the tube. Question : Without seeing the evidence, any ideas as to how this happened ? Was it a result of me mouting the tire/tube incorrectly ? thanks in advance

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Got enemies with knives?

Some months ago I read that it would be better for my chronic low back pain (no spasms in a year now) to keep the tires slightly less than fully inflated, so that they have a bounce and they minimize the impact of each bump in the road.  I was getting lots of flats.  In the end, inflating the tire fully put an end to that.  It was that simple.  I too was looking over the rims carefully for something sharp, but no, it was just me bouncing over a less than optimally inflated tire.

older (up to 1970s?) rims didn't have the hook edge that modern ones do. Even most racing bikes could manage only about 65 lbs pressure. If you have say an old Schwinn and put a new tire on it that says 90 lbs, if you fill it up higher than 65 the edge slips off, the ride gets wobbly, and it goes bang with a big long gash in the tube. Could that be it? Probably not with the tire injured too, but hmm.

Straight side rims are still useful and can be used safely with modern tires. The first well-distributed approximately modern tire was the Schwinn LeTour, introduced in 1973 and labelled for 85psi. There were darn few hook-edge rims around to mount it on and the Schwinn engineers were extremely conservative.

Bike shops are super cautious about this one because they know the customers do crazy shite to bikes and the shops can become liable. Here are some cautions--  We are talking old bike parts. Inspect thoroughly before use. Use a wide tire. Wide tires need less pressure. Old tires were all wide by current standards. The rims are wide too. Big heavy people or riders who always carry expedition loads are better off with new parts. Above 200# rider weight the old rims won't last long. Above 220# forget it.

Allen Wrench said:

older (up to 1970s?) rims didn't have the hook edge that modern ones do. Even most racing bikes could manage only about 65 lbs pressure. If you have say an old Schwinn and put a new tire on it that says 90 lbs, if you fill it up higher than 65 the edge slips off, the ride gets wobbly, and it goes bang with a big long gash in the tube. Could that be it? Probably not with the tire injured too, but hmm.

One other note about brake pad adjustment as it relates to tire sidewall preservation and damage: if your brake calipers are flexible, as is often the case with sidepull brakes on road bikes from 30 or more years ago, and with department store bikes of any era, the attitude of the pad to the rim will change upon hard braking. As the pad grabs the rim from the friction created on contact, the caliper will flex in the direction of wheel rotation, lifting the leading edge of the pad toward the tire. If the pad was already set close to the tire, hard braking could cause the pad to cut a groove into the sidewall. Similarly, if the pivot(s) of your brakes are too loose and have play, the pad could behave the same way as above. This issue won't be noticeable if you simply squeeze the brake while the bike is stationary, as it requires the friction of braking action to manifest. Unlikely situation, but theoretically possible in your case.

I think it's more likely the tube was pinched under the bead, given how new the tire was.

You have a cut tube and a cut tire just above the bead. Normally cut tire carcass at the bead is caused by tire removal using a tool scraping along the tire bead cutting the cords of the carcass. A new tire should not have be removed yet. A tire with nylon cording can be cured to over 200 degrees Fahrenheit melting the fibers. Once in a while the rubber will not fill the tire mold fully and cause thin bead on edges of a tire making it very hard to seat correctly, this will not normally cut the tire at the bead. 

If I were you mechanic I would inspect the tire to see if the carcass was cut or if it tore. If it was cut by a scraping motion the rubber over the nylon cord would be removed, if it tore the rubber would be split but not scraped off. A tear is a manufactures issue, a cut would be either a mechanics installation or a consumer removal/ tool usage issue. 

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