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Any thoughts - my spare tube that I carry in my bag currently has a dozen or so patches on it, but still works fine. I'm tempted to keep going to see how many I can get on there until it get impossible to mend.
What is the most number of patches you reckon a tube can take ?
(And I know new inner tubes are only £5 or so, but a puncture repair costs about 10p if you buy the patches and glue in bulk, so "It's cheaper to buy a new one" is not necessarily true...)
When the patches start to overlap, or you are fixing old patches that have failed, it is time to get a new tube.
I would carry at least one unpatched, pristine tube as a spare. Put the patched ones on the bike or keep for repairs at home. The idea being that when you need it -- in the middle of nowhere, in the dark and pouring rain -- you're guaranteed that the old patches aren't peeling off or weakened and the tube should "just work". Then swap it for a patched tube when you get home if you like.
I find the bigger problem is not the number of patches, but rather the age of the patches. There is a correlation though, in that by the time you have patched a tube for the third time, I find that the first patch is starting to get a little suspect.
In my student days I would patch a tube four or five times, but I often had patches eventually fail.
Nowadays I draw the line at two. Tubes are just too cheap to have one fail on you in the middle of a ride. The time and inconvenience of having to change a tube that has failed is not worth the money, especially since it always seems to happen on wet windy nights!
I strongly contend the rules of thumb about the number of patches. I have used (and I am using) tubes in 2 different mountain bikes, with 8-10 patches, and they perform exactly as new ones. They get punctured by evident spikes, nails, etc., the same as brand new ones would suffer.
As for the age of patches, it is but a mild indicator, at best. Again, some of those patches are certainly older than 10 years (and I would dare saying some are older than 25 years), and they don't leak, as they were properly applied. I can't remember seeing a patch failing in my tubes.
The gross points to be careful about (as others mentioned) are:
Other than that, I conjecture there must be some incidence of other factors on the life of a tube (although I cannot attest for any evidence from my personal experience or any friends'):
I think any suggestion should be based on factual information.
Answer: As many as it takes till you can't patch it the last time.
I have one 26" MTB tube that has fifteen patches on it. Some of them might be two small round patches on a pinch flat, but it holds air perfectly well.
I'll replace it when I can't get a puncture to patch properly.
On the other hand I've had road tubes that won't take a patch for some reason - the butynol rubber just "rejects" the vulcanising fluid and after some minutes of test at pressures below 5 PSI it just lets go. Those get the valve cut out for bottle rockets, and are put on the "emergency tie downs" hook.
Of course I ride with a spare tube for each wheel size on the bike, and I carry sticker-patches as a last resort or to help other people. If its a long ride I'll sling a second or even third tube in. So even though a patched tube goes back into the rotation, its not the only thing I'm depending on.
My normal patches are "cure-c-cure" and weigh 9.3 grams for all 15 or 0.62g each. For comparison,
Assume the weight of cured vulcanising fluid roughly equals the weight of the backing foil:
A 26" / 559 tyre with a rolling circumference of around 1.75 metres needs 87 patches of 20mm diameter to completely surround the outer edge.
87 patches at 0.65 grams is 56.5 grams. Which is not insubstantial but represents 1/11th of a full drink bottle or one large gel.
Those patches would cost you around $20, two tubes of gloop another $20 for a total of $40. Compare that to 87 new budget tubes at $5/each is $435 total.
Upshot "patch till you can't patch no-more" unless you're competing in a race where every gram counts.
