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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 04-23-07, 04:30 PM   #1
Cassiel
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Why do I have a chronic rear flat?

My rear tire seems to repeatedly go flat. I can't figure it out. There's nothing in the tire, there is nothing poking through the rim. Am I missing something? Is there a trick to figuring this out?! I replaced it about a week ago; I replaced it again this morning. This afternoon, it is once again flat. I did two 8 mile rides (commute) today and that's all. Yes, there was glass and road crap involved. But enough to warrant another flat? I don't think so. And why is my front tire still perfectly inflated?
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Old 04-23-07, 04:43 PM   #2
Wil Davis
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Where abouts on the tube are there holes? Holes on the inside of the tube might mean that the rim strip/tape is getting nudged over to one side when the tyre is inflated (you might want to try Velox™ rim-tape; it's sticky one side and won't move as easily as the rubber rim-strips). If the holes are repeatedly in the same vicinity, then you might want to check the inside of the tyre-casing more carefully; it's doesn't take much to puncture a tube. Good luck!

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Old 04-23-07, 04:43 PM   #3
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Are you inflating to the recommend pressure? Underinflated tires will be prone to pinch flats.
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Old 04-23-07, 05:13 PM   #4
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Often you'll find you get more flats on the rear. You can avoid debris with the front wheel, but it's rather difficult to steer the rear tire to avoid junk.

Are you finding "new" holes in various areas of the tube or are they somewhat localized?
PS when you mount the tire to the rim, position the reflector strip (or size markings) adjacent to the valve hole. When you find a hole in the tube, you know exactly where to look on the tire to check for a thorn etc.
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Old 04-23-07, 05:15 PM   #5
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Yep: usually about 115 psi, and the recommended is 115-125. I have to admit I might have underinflated this morning - was in a hurry and only got to 100. Could that have done it? I swear, I don't know what else this could be. The rim tape seems fin and it's the sticky kind, not rubber. I haven't checked the tube yet...will do...
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Old 04-23-07, 05:19 PM   #6
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Quote:
Originally Posted by Cassiel
Is there a trick to figuring this out?!
The "trick" is tube forensics.
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Old 04-23-07, 05:27 PM   #7
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I've had that problem too. Turned out to be a hidden sliver of metal lodged in my tire. I couldn't see or feel it. I turned the tire inside-out at the spot where the tube punctured and squeezing the tire found the sliver almost laying flat in the meat of the tire.

Now when I get a flat I always match the hole in the tube to the spot in the tire and dig around for the problem.
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Old 04-23-07, 06:20 PM   #8
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Check and see where the hole is compared to the rim. Chances are, it's right where the "weld" is. Sometimes those welds come undone by a fraction, so when you ride, it opens and closes, pinching the tube.
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Old 04-23-07, 06:27 PM   #9
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100 psi should be fine. Is the flat always in the same spot?
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Old 04-23-07, 06:31 PM   #10
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HAd the same problem with a new set of wheels

Somebody said tube forensics and that is exactly what you need to do. When you remove the tube realign it to the wheel just like it was mounted inside the tire. See where the hole is located in relationship to the valve stem. Is the hole on the outside of the tube or on the inside of the tube? If the hole is on the outside, expect a puncture and start using mr tuffy's or flak jackets or the like. If the hole is on the inside, remove the rim tape and check each spoke hole near the hole in the tube and see if you can narrow down the culprit.

In my case there was a piece of metal shaving from when the nipples were tightened and when i looked in saw that it was poking thru the rim tape and putting a hole in the tube. Removed the shaving bit and problem was solved and my tube bill went down a bunch [i don't patch tubes].
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Old 04-23-07, 07:52 PM   #11
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You've probably got something sharp hidden in there somewhere. Best of luck finding it - fingers work well but the blood does tend to get everywhere so other methods are preferred.

However, don't underestimate your route - the following is rare but it does happen. I'm thinking back to the mid eighties when I commuted behind the (then) new velodrome. There was a 50m stretch where the paved path gave way to dirt. Every day for a week I picked up a three corner jack (a small, hard thorn with three spikes) - that was when I discovered thorn proof tubes ... and that exact same set of tubes is still serving in the Europa believe it or not

The other thing to ask yourself is: did you run over someone's cat recently? (a sure cause of bad luck)

Richard
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Old 04-23-07, 08:59 PM   #12
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Nope, I believe all the recent roadkills here are the work of cars - or at least, I am not responsible for them. Poor moggies.
Well, it helps to hear that it's possibly the route, because the puncture is definitely on the outside. Now, I did find one little puncture in the tire itself (possibly a millimeter in length) - I suppose that would have been the culprit. Now, is a tire cut of that size something to worry about or can I just hope that it was from the responsible thorn/glass shard/tack and put things back together with no worries? If this happens again, I'll be more careful to notice where the tub was lying in the tire and on the wheel, but unfortunately I didn't notice its alignment before extracting it this time.
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Old 04-23-07, 09:33 PM   #13
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Quote:
Originally Posted by King of Kadence
Now when I get a flat I always match the hole in the tube to the spot in the tire and dig around for the problem.
+1

A piece of chalk can be a very useful item!
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Old 04-24-07, 02:17 PM   #14
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You have something still stuck in the tire. Trust me you do. I always fold the tire completely inside out and bend it at odd angles to reveal any tiny nicks and imbedded objects. It may take you three or four passes to find the object, but it is there.
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Old 04-24-07, 04:40 PM   #15
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take the tyre and tube to the emergency ward and run it through the xray. you'll spot the culprit soon enough
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Old 04-24-07, 08:36 PM   #16
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Only you can determine the cause - too much guesswork without beingt there. Here's how:
(I've made it thorough so that others can learn as well, so please excuse the length).


1. Remove the tube from the tire. Always leave the tire in place on the rim (one bead on) being careful not to rotate it out of where it was mounted when inflated AND be careful to keep the tube oriented in the same direction as when it was mounted on the wheel as well.
2. Inflate the tube until you find the leak. Put your thumb and a finger together around the tube and go around it starting and ending at the stem. You can also put your cheek and ear close to the tube to detect the leak. Use water as a last resort - very seldom necessary.
3. Look at the hole - it it a small round puncture, a tear, two parallel slits, etc. Once you have found the leak place the tube next to the wheel in the same orientation as when mounted.
4. Now comes the payoff. I’ll do my best without graphics to explain:

Type of Hole - Location - Cause
Small puncture - Tread - Road hazard
Small bubble, hole at edge - Next to rim - Off center rim tape, long spoke
Two parallel slits (snakebite) - Next to rim - Impact, underinflation
Starburst (blowout) - Tread/sidewall - Tire cut/sidewall failure
Long slit (blowout) - Side of tire/rim - Tire bead/mounting defect
Cut Valve stem - Valve Stem - Underinflation (crooked stem)
Leaky valve - Valve stem - Defective/dirty valve
Scuffed, small leak - Side of tire/rim - Rim pinch, poor mounting

SOLUTIONS
Road hazard: Carefully check inside and outside of the tire in the area corresponding to the leak in the tube. Feel (CAREFULLY) with your fingers on the inside for the cause. Sometimes the object is still in the tire and will cause another flat, and sometimes it has caused enough damage that you should replace the tire.

Rim tape, long spoke: Shorter spoke, wider or new rim tape.

Impact, Underinflation: Check rim for damage (flat spot, bulge). Keep tires to full inflation, out of saddle over potholes, etc.

Tire cut/sidewall failure: Replace tire.

Tire bead/mounting: Replace tire if bead has separated, learn proper tire mounting.

Valve stem cut: Replace tube, keep tires properly inflated.

Defective valve: Rare – replace valve core (used valve is fine).

Rim pinch: Learn proper mounting technique (never use tools when remounting).

PREVENTION
***Keep tires inflated - prevents premature failure, less road contact and harder surface, protects rims.
-Inspect the tire tread regularly. Many hazards take a while to get through the tire.
-Check the tire's "bead line" next to the rim - should be the same distance from the rim edge all the way around on both sides
-Do not true wheels while tires are inflated unless you are positive there is no possibility of the spokes contacting the rim strip.
-Learn proper changing/mounting technique.

Last edited by cny-bikeman; 04-24-07 at 08:48 PM.
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Old 04-24-07, 09:01 PM   #17
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if everything said above fails, always be open to the possibility that you might just have sh*t luck with regard to tires and tubes.
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Old 04-24-07, 09:13 PM   #18
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Onother thing you have to watch out for in Portland is roads made with recycled car tires. Brett the service manager at the Bike gallery there in portland told a story at a seminar about how some of the roads were made with recycle car tires and the metal belts had not been removed which left small wires poking up from the street causing bicycles to get flats on a big ride.
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Old 04-24-07, 10:52 PM   #19
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Quote:
Originally Posted by Cassiel
Yep: usually about 115 psi, and the recommended is 115-125. I have to admit I might have underinflated this morning - was in a hurry and only got to 100. Could that have done it? I swear, I don't know what else this could be. The rim tape seems fin and it's the sticky kind, not rubber. I haven't checked the tube yet...will do...
What is your weight?


Let's not pretend that tire pressure has nothing to do with body weight. Political correctness won't work. Depending on whether you are 50 Kg or 120 Kg, 100 PSI could be wholly adequate or wholly inadequate.

+1 on inspecting the tire near the puncture spot. Carefully.

Last edited by wroomwroomoops; 04-24-07 at 11:01 PM.
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Old 04-24-07, 11:09 PM   #20
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I weigh 140, so is 100 psi adequate?
Ok, well, having turned the tire inside out to try and find the embedded sharp, I had no luck. Whatever it was seems to be gone and I assume I simply have horrific luck. I went and spoke to someone at the LBS who I trust, and he recommended tougher tires (the ones I currently had were original to the bike, and I bought the bike in August of '06, so they were probably done anyway). I got a couple of these "gator" tires and we'll see - they certainly feel tougher.
That is really interesting about the recycled car tires - I had no idea!
Thanks again for all the thorough information. Immensely helpful. :-)
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Old 04-25-07, 03:56 AM   #21
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Quote:
Originally Posted by Cassiel
I weigh 140, so is 100 psi adequate?
Ok, well, having turned the tire inside out to try and find the embedded sharp, I had no luck. Whatever it was seems to be gone and I assume I simply have horrific luck. I went and spoke to someone at the LBS who I trust, and he recommended tougher tires (the ones I currently had were original to the bike, and I bought the bike in August of '06, so they were probably done anyway). I got a couple of these "gator" tires and we'll see - they certainly feel tougher.
That is really interesting about the recycled car tires - I had no idea!
Thanks again for all the thorough information. Immensely helpful. :-)
140 Kg? Well, your rear wheel is underinflated, for such weigth. The rear wheel always carries more weigth than the front, and should be inflated to a higher pressure to avoid pinch flats.

I don't know many cyclists of such high weigth, so I don't have experience with the set of problems they might have. I am not even sure many tires are designed for such heavy cyclists.

The conti gatorskin are certainly good tires, though.
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Old 04-25-07, 06:56 AM   #22
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Originally Posted by wroomwroomoops
140 Kg? Well, your rear wheel is underinflated, for such weigth. The rear wheel always carries more weigth than the front, and should be inflated to a higher pressure to avoid pinch flats.

I don't know many cyclists of such high weigth, so I don't have experience with the set of problems they might have. I am not even sure many tires are designed for such heavy cyclists.

The conti gatorskin are certainly good tires, though.
OP is from the US, so I'm assuming he meant 140 pounds. 140kg, you'd have a pretty large contact patch even while pushing the pressure limits of the tire (125 psi), be like me running at 60 psi and 150 lbs. Not optimal, and would be somewhat prone to pinch flats and the like, but averaging one every 20 miles is still kind of extreme. But if the unit of measure is in fact kg, I'd be running something larger than 23mm tires, for sure.

As far as the recurring problem - I'm not sure what kind of tires he's using, but if it's not a racer they're probably not that expensive - I'd get a new tire and throw that on while I worked on diagnosing the old one. If the current tube half life is about 15 miles, I'd drop an extra $20 for a tire while I figured it out.
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Old 04-25-07, 08:16 AM   #23
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I like light tires with Mt Tuffy strips to avoid flats. They ride and handle better than Armadillos. I dont have any experience with other armoured tires.
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Old 04-25-07, 08:25 AM   #24
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Oh, sorry - yes, the OP weighs 140 LBS, not kg! So hey, one more question - I went with the gator tires but I only changed the rear one so far since it was the one with the problem (though I did buy two new tires). Is there any reason I can't run two different tires until I get time to change the other one?
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Old 04-25-07, 09:23 AM   #25
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Quote:
Originally Posted by Cassiel
Is there any reason I can't run two different tires until I get time to change the other one?
no
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