What do dimples on inner tube indicate?
 

Now that I've gotten my second puncture on the rim side of my inner tube (in the form of a rather large slit), I've also noticed little dimples at regular intervals along the inner tube. The slit puncture is on one of them. Does this indicate worn rim tape that needs replacing. I'm assuming the dimples are indentions from the spoke ends.

Also, I'd like to ask if inner tubes degrade with age. Do they become degraded from just sitting in a box for 10 years? I was thinking of buying a bunch in bulk, but don't want to do that if they degrade from age. It might take 10 or 20 years to use them all up.

The dimples are from the spoke heads. It should be easy to see if the rim strip has a problem that lines up with the slit. If you don't see anything that would cause a problem, another possibility is that you are pinching the tube between the rim and tire. When installing a new tube, slightly inflate it with the tire in place and go around the entire circumfrance on both side pushing the tire away from the rim to make sure the tube isn't pinched.

As for longevity of tubes, the tube in my wife's and my MTBs are coming up on 20 years old and are fine.

Well, the funny thing is that I can line up the tube slit with where it met the rim, but I don't see a problem with the rim tape. I even took the rim to my bike shop for their inspection/advice and they said the rim tape looked fine. So after my first tube puncture, I patched the tube and now only a week later, I have this slit in the tube in a completely different location. BTW, I hadn't even ridden the bike during that week. It just went flat suddenly while sitting in one place.

But if a spoke head is actually what caused this problem, how is it that it happened after a whole week after I installed/inflated it rather than right away?

I'd say it's time to start feeling around in there for a no-seeum

I did feel around for sharp edges poking through the tape, but didn't find anything. The only other thing I can imagine is that maybe the tape gets pushed aside by the tube as it inflates. I don't know how likely that is.

Sounding more like you are pinching the tube between the tire and rim when installing.

I've been careful to avoid that. I inflate the tube a little to give it shape, I tuck it into the tire while being careful not to let it bunch or twist, then I work it onto the rim. Once there, I make sure it is on all the way before pumping it up all the way. Is there anything else I should be doing to avoid a possible pinch flat?

Dimples are from the inner tube expanding against the spoke holes in the rim. When they expand too much into such a spot they can then burst/tear, hence the puncture hole being on one of the dimples.

About 1/6 of the flats I see in shop are from this problem, where a newly purchased bike was assembled with cheap 'rubber band' rim strips that aren't strong enough to keep the inner tube from pushing into the spoke holes.
replacing with good cloth velox rim tape keeps it from re-occuring.

So I'd say redo your rim tape, and be sure to use the good stuff made of cloth, not rubber(stretches and fails) and not nylon/plastic(turns brittle and fails)

The tube pressing against the nipples normally doesn't cause problems. If the rim tape is bad, or the spokes are protruding more than 1/32" it might cause punctures, but I've never known it to cause a split.

Do you by any chance have a narrow rim? 17mm or less? if so I suspect you're seeing the effects of hyper-stretching of the belly of the tube. Picture this in your mind. In cross section the space the tube fills is somewhat hourglass shaped, with the tire above and another space in the rim below the bead. With narrow rims the neck between these two chambers is very small.

When the tube first starts to inflate it fills the tire and lays across the neck between the beads. With more pressure this narrow section blows out in a bubble filling the rim, effectively stretching 2-5 times more than the rest of the tube. If the tube is fairly thin, it just can't go that far and gets stretched to the limit or splits.

There, no easy fix. Powdering the tube helps a bit, but the real answer lies in better quality tubes correctly sized so they can take the extra stretching.

My rim is as you described. The bottom section of the rim where the tube has to fill is 6mm in width. I measured it to see which size of Velox rim tape to buy. I just went to the bike shop and picked up a roll for $3.29, which seemed a bit steep. The bike shop guy said that I can install this new tape on top of the old one if I want to. My existing rim tape is the rubber kind, which a different bike shop guy in the past said looked perfectly fine. Well, with 2 rim side flats now, it doesn't seem fine. So now we'll be trying the velox. I'm just getting tired of spending money on this thing.

Use the Velox. Get rid of the old rim strip completely.

I'll do that.

Hey, one more thing. I got the thinnest version of the velox they had, which is 6mm, the same as what I have. Would it have been wiser to get one that's a bit wider to perhaps cover the area better, or would that have caused other problems?

Get the wider size if it fits within the bottom of the rim. It must cover all holes completely.

I`m guessing that since the OP has a rubber rim tape and has stated that he thinks the dimples might be caused by spokes - that we`re talking about a single walled rim.

If thats the case it should be a breeze to remove the rim tape and confirm that no spokes are actually protruding above the nipple head and that there are no sharp edges. If everything is good then reinstall the rim tape.

Rubber or plastic is the best choice for single walled rims. Self adhesive woven tape is intended for double walled rims. There`s also no reason to install one rim tape on top of another and several reasons not to.

Actually, I have a double wall rim. Would that create any new problems?

I had a serious case of hyper-stretching several tire tubes when I switched from a pair of thin casing 25c tires to a nearly bomb proof thick casing 37c's. The 37c's were almost destine to hang in the rafters indefinitely after several blowouts without even making a single run, that was until I wore out that set of wheels and purchased a set of wider wheels with a smaller width shoulder. Though my tubes didn't have dimples per se, but a row of small bulges all the way around the tube, and with one being the weak link in causing the tire to go flat.

Happily to say that the 37c's now have many miles on them without incident.

No, as long as you use a cloth reinforced rim strip. The only thing your hollow rim means is that the mechanic who sold you a rubber rim strip was either ignorant or or uncaring.

Anyone with any bike shop experience should know that tubes will blow down into the little hollows unless a cloth rim strip is used. At any significant pressure a rubber rim strip won't do any better than the tube alone.

Actually, I'm not sure if what is on my tube could be defined as "dimples" or not. They actually protrude outward, but when I press on them, they form inward like a dimple. They are directly over where the spoke heads are, so I guess that has something to do with it.

FBinNY: The rubber strip in my rim right now was put there by the manufacturer, which is Specialized. I assume they put it there because that's what was there originally. Of course, it's the LBS who actually assembles the bike, so maybe they swapped parts when assembling it. I'll never know for sure. But it's surprising that the top bike manufacturer would use inferior parts.

I take my tubes out of the box and hang em on hangers I made (with curved top rests)
May just be me but anything that's creased or folded for too long might be prone to failure.
Are you inflating to suggested pressure? (just a thought)
A pinched tube sounds like the problem.
Are you running wired ons of kevlar bead tires?

OK So I guess we have a bit of a communication problem - or had. If you have a double walled rim the indentations or dimples are caused by the holes over the spoke heads and not by the spoke heads themselves. The spoke heads are the next level down.

But I completely agree with FB`s previous post. A rubber rim tape has no business in a double walled rim - and if you`re running high pressure tires its just totally unacceptable. Go for a nylon or self adhesive woven rim tape and disgard the rubber altogether.

That is not a bad price for rim tape. I've seen it up to $10/roll at some shops. Even online it isn't cheap -for what is basically glorified surgical tape.