This tire was just about worn through but no spots where the rubber was thin enough to see casing threads. Then the rubber just separated from the casing as shown. Most other tires I've had would still have the tread well adhered to the casing even when the rubber is really thin, but this has been the death mode of several recent ones - all Vittoria Evo.

What's going on here? Is the bond failing or is my idea of the bonding/vulcanizing incorrect?

After several similar failures I would be considering switching brands.

I think these weren't bonded/vulcanized adequately. Most of my experience with tire wear is the same as yours, normally the tread rubber stays attached to the casing until it is worn completely through and the casing shows.

However, I recently had two Kenda 700-23 "Konstrictor" tires fail the way yours did, the tread rubber separated from the casing before it was worn completly through. In fact, one was a front tire and showed no tread wear (front tires usually don't) when it failed. I e-mailed Kenda about this but never got a reply.

I don't consider this a failure, just a different way to wear out. I love the way these tires ride and I'm happy tossing them with a little more rubber than I otherwise would with different tires.

I inspected this tire before my century Saturday; it had some thin spots and tread cuts but I figured I had at least another 100 miles on it but I knew I'd be replacing this one very soon regardless. Finished the century just fine and noticed this when I got home.

If it is a vulcanizing issue at the manufacturer, or if the vulcanizing somehow fails after a few thousand miles, it doesn't show up until the tire's almost worn out anyway.

These tires, like most high quality tires are probably of "colle a main" construction. In this method the tread is molded separately from the tire and glued to the built tire. The advantage is that the fabric tire plies aren't heated, the drawback is that the glue can fail, leading to separation, usually along the edges, where water and UV can weaken the bond.

Most long time riders of top end tubulars are used to this, and will carefully reglue good tires if their overall condition warrants the effort. But, given the age of your tire, I'd thank it for the long service and give it a decent burial.

BTW- looking at the damage, I have a question. This isn't by any chance on a fixed wheel bike you "skip" to stop, is it? The intense shear loads of this type of hard braking can delaminate colle-a-main tires prematurely.

Thanks, that phrase gave me lots of interesting google results... several of them yours.

Colle a main is French for hand glued and referred to the construction technique for top end tubulars, especially silk tires which couldn't be heated the way cotton ones could. Until fairly recently only tubulars were built this way, but with the demand for lighter, more supple, higher pressure clinchers this construction is now used in the clincher world too.