Why did my road bike tire patch fail?

Question

Long story short: I put a patch on a punctured road bike inner tube, it was fine for two days and 20 km, then the tire became completely flat, due to air leaking from the edge of the patch; why?

On Saturday, I patched a punctured inner tube on my road bike (rear wheel, 25 cm tire). I have patched countless inner tubes, including on road bikes. I followed all the steps: found the leak, roughed up the surface of the inner tube around the leak, applied rubber cement, let it dry completely (about five minutes), put on an appropriately sized patch (trimmed down with scissors so that it is not too big), found and removed the cause of the puncture (a bit of glass in the tire).

On the following Monday, I rode my bike to and from work (total 20 km), no problem. Then on Tuesday morning as I was getting ready to ride to work, I discovered that the rear tire was completely flat. I dipped the inner tube in water to find the location of the leak: air was quickly bubbling from the edge of the patch that I had just put on the inner tube two days earlier. I easily removed the patch (it just peeled right off). I thought maybe the repair had failed because I had not made the surface of the inner tube under the patch sufficiently rough before applying the patch, so this time I made sure to make it quite rough. Then I reapplied rubber cement, let it dry for five minutes, and put on a new small patch.

I rode to work that morning (Tuesday), no problem, but then in the evening I discovered my rear wheel was completely flat yet again. Fortunately, there was a bike shop within walking distance. Air was again leaking from the edge of the patch again, but this time the air was bubbling slower than the previous time.

The guy at the bike shop said, as a rule, they do not patch punctured road bike inner tubes (except in emergencies), because the patches sometimes fail due to the narrow width of the inner tube. I had never heard this before (and as I mentioned, I have repaired countless road bike inner tubes), and a subsequent internet search turned up nothing about this, but I guess their experience is worth considering. (They replaced the inner tube.)

My question is, why did my patched inner tube work fine for a few days, and then start to quickly leak air?

Answer 1

Bike shops don't patch tyres because at $60-100 per hour its not worth their time compared to a new tube.

Road tubes can absolutely be patched, however its harder with thicker patches - try and get the thinner ones. Also, avoid pre-glued "sticker" patches because they tend to not hold for long at road pressures (anything under 50 PSI is generally OK, or last-resort.)

Old stale vulcanising fluid can cause issues, as can a contaminated tube surface, or insufficient surface prep. You need to lightly scuff/sand/abrade the tube around the hole to expose fresh rubber.

However in your case, the only thing I can point at is trimming a patch with scissors - generally the patches will taper off smoothly. By cutting you've made a sharp ending which may have been a weak point. There should be at least 10mm of patch overlap around the edges of the hole in all directions.
Also, while cutting you may have contaminated the rubber side of the patch. Never ever touch the pre-treated side of the patch, not even a little bit. You'd normally use the clear top-layer to apply the patch to the tube.

The other common failure point I see generally is lack of time for the fluid to set-up on the tube. You have to leave it for 3-8 minutes before sticking on the patch, and the area should be larger than the patch in all directions.

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From a preparation standpoint, I always ride with two spare tubes, sometimes more on long rides. I never bother patching on the roadside, I just swap out a bad tube for a known and tested good one.

So when I do patch, it happens in the warmth and comfort of the garage at home. This is not rushed and when done I can inflate tube and leave it for hours or overnight. That way tube is tested before I wrap it for storage on the bike/bag.

Summary You're doing fine - flats happen. The trick is how you prepare for and deal with them.

Answer 2

My road tubes are definitely harder to patch than my MTB tubes. I also blame the width, but specifically the curvature. Some of my patches are simply too stiff to stay nicely on the tube, while being just too wide to clamp flat. When they do work, they appear to only bond at the flexible edges, but I have to press them down by hand even to get that. So I use those on bigger tubes and got some better ones for the road tubes - only a little narrower but much more flexible.

This means that if anything else is marginal, the patch will fail. That could be cleaning/abrading the tube, condition of the adhesive etc.

I tend to patch at home, in good clean dry conditions, and do a batch at a time, so every step can be unhurried, and I can be sure of using glue in decent condition. I always test my tubes at least overnight before reinstalling them or packing then away

Answer 3

I followed all the steps: found the leak,

No you didn't. This list does not contain all the steps.

roughed up the surface of the inner tube around the leak

You don't rough up the surface. The purpose of using a sandpaper isn't to "rough it up", the purpose of using a sandpaper is to remove the mold release. It doesn't have to become rough. You just need to remove enough material from the surface that the mold release is gone.

Tubes contain mold release on their surface because without it, they would stick to the mold in which they are made. Unfortunately, with the mold release, patches do not stick. So you need to remove it with a sandpaper to make a durable patch.

If your patch kit contains a "cheese grater", toss it away and replace it with a proper sandpaper piece.

applied rubber cement, let it dry completely (about five minutes), put on an appropriately sized patch (trimmed down with scissors so that it is not too big), found and removed the cause of the puncture (a bit of glass in the tire).

Generally I would advise not to trim down a patch. Not sure if this is the cause of the leak, but that's something you probably don't want to do. It could increase the likelihood of a slow leak. Instead, I propose using patches of the proper size (e.g. Rema Tip Top TT04 sport kit has quite small patches).

But your list is missing the most important action item. It is to "put a spare tube in and leave the patched tube rest for a period of at least a day".

If you insert a recently patched tube into the tire and inflate it, it can create a leak, either a slow leak or maybe in some cases even a faster leak.

However, since your problem repeated many times, I suspect it could be some quality issue in the patch kit rather than using the patched tube too quickly.

For safety, buy a new patch kit of good quality (e.g. from Rema Tip Top), and try again. This time leave the patched tube rest for a period of at least a day, and use a spare tube in that period.

Answer 4

One last thing that others have not mentioned: in the past, I didn't spread the vulcanizing fluid over a wide enough area, so I had some patch jobs fail. You want the whole patch to be covered in the glue, i.e. the edges should adhere as well.

The original post says "rubber cement". This may incorrectly conflate rubber cement and vulcanizing fluid. The latter is what you actually get in a patch kit. Also, you can buy separate mini tubes of vulcanizing fluid at bike stores - they will be able to order them from a supplier like QBP. You should consider doing this if you have a partly used tube of vulcanizing fluid that's been in storage for a while. Rubber cement does not patch successfully, as I found out when I bought a roll of Rema patches and a can of rubber cement.

Answer 5

I've had terrible problems in the past when the cement gets too old and loses its potency, whether sealed or not. But I would guess you might have run into that if you've done a lot of patching.

Another thought - you say you trimmed the patches with a scissors. Did you leave enough of the thinned margin on the patch? A thick edge might get loosened by the action of the tire and tube rolling about.

Answer 6

As far as I know, it is still a vulcanising agent being used for patching tubes. The problem comes up, when the tube being used is made of NBR/SBR Rubber and not classic vulcanised rubber. I never heard of contact cement being used in those kits. The patches are a black top layer of vulcanised rubber and an unvulcanised bottom layer on a piece of foil right? If the tube is not vulcanised rubber, then it will never hold as good as it may have with classic tubes...

It may hold the air in but it will never bond completely to the tube in this case.