I have several pair of on-topic plastic soled cleated cycling shoes where the leather uppers (toes or heels) have separated from the hard plastic soles. This has usually happened to NOS shoes after just a few rides, so I don't think it's a matter of use or wear, but just old glue from the 70s/80s losing its grip on shoes that sat for decades in the box. My riding climate is very dry so this hasn't been the result of soaking. Also my use of these shoes is gentle, I think. I never take more than a couple steps off the bike in these shoes, and my pedal stroke is such that I pull back and up with heel up rather than pulling up on the uppers into the toe clip and strap. So I don't think I'm ripping the uppers off by sheer force.
I bought some glue from my local cobbler that he recommended, followed his directions for prep and clamping, and the Sidi upper did not re-attach to the sole. This glue reminds me of tubular cement, so is probably a type of contact cement and there may be better kinds of glue to try.
I'm considering attempting this with urethane glue, the kind that cures with moisture, because they are usually good at attaching dissimilar materials I've found. The the end results can be messy so I'm saving this for a last resort.
Gorrilla glue is often recommended for sport shoe repair stuff (telemark boots, etc.) but I haven't tried it. Is Gorrilla Glue a contact cement or urethane glue?
Also considering taking the shoes back to my cobbler for him to glue, but if his glue didn't work for me, I doubt if it would work for him. Maybe he has a better clamp to use with his last.
BTW, of all the brands of shoes that gave way, only Vittorias never have. Two pair are solid after lots of miles.
Mitch Harris
Little Rock Canyon, Utah, USA