My bicycle was simply sitting at rest indoors when it exploded. A slit about ~6mm long was in the tube. I thought it might be too big to patch, but I happen to have some quit big patches to try. I cleaned the area w/denatured alcohol then roughened the surface w/the sanding tool. I covered the whole patch footprint area with rubber cement. Around 90 sec. later applied the patch and lightly clamped it down. About 5 min later: installed the tube and inflated to 4.5 bar.
Couple min later it exploded again. As the image shows, it blew exactly along the line of the original slit. AFAICT, I could not have applied this patch better. That is, the air went through the patch instead of around it.
Why did this happen? The patch is thicker and harder than the tube. So if the adhesive does its job well, then I would expect the patch to be stronger than the rest of the tube. The elasticity is lower in the patch, so I suppose that must be the problem.
So I have to wonder: would it be more or less effective to cut previously damaged spare tubes to use for patching, instead of a patch? I wonder if larger holes need patches with more elasticity.
Rubber cement vs. contact glue
Patch kits include a tiny tube of “rubber cement”. The instructions say to wait 3 min before applying the patch. That’s similar to contact glue instructions. The only difference is contact glue instructions direct us to glue BOTH surfaces then press them together a few min later, whereas the repair kits never say to put glue on the patch (why is that?).
Are the two glues chemically different? I ask because it may not make sense for me to buy another patch kit when I happen to have a very big tube of contact glue for shoe repair.
Looking at the patch it is not glued down well. The edge should be completely flatly joined with the rubber all the way round and at the edges.
If it isn’t glued correctly there then I’ll bet it wasn’t glued correctly over the original puncture. Usually with poor uneven gluing the air would leak out from one of the patch edges. The patch has failed this time though.
However I wouldn’t rule out some kind of abrasion on inflation. Whenever you get a puncture try to work out where the corresponding part of the tyre or wheel is (wheel in your case) and carefully feel the area with your fingers for roughness/sharpness. I’ve repaired a number of punctures where I couldn’t see anything initially but after feeling I realised there was a tiny sharp flint, piece of metal, etc that was just protruding enough to damage the inner tube.
Finally on the contact vs rubber cement. Use rubber cement. It is the correct adhesive for the job and as already said it is a solvent based bond so melts the rubber together a bit. Contact adhesive just creates a layer of adhesive between the two surfaces.