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Trying to work out if I have the right tire size on my road bike

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Trying to work out if I have the right tire size on my road bike

Old 01-31-13, 07:24 AM
  #1  
SamSam
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Trying to work out if I have the right tire size on my road bike

Hi all, I have a very basic question about tire sizes.

I got a second-hand bike, and the size printed on the tire walls is 700 x 28C. When I got a flat after several months of riding, I went and bought a couple boxes of tubes, asking for "700 by 28", and was given 'Specialized 700x20/28 -- 27x3/4" - 1 1/8"' tubes. Since the numbers "700" and "28" were there I figured these were fine.

However, both tubes blew in short order. After the first one popped a day after putting it on, I was extremely careful in checking for debris in the tires or spokes poking through, and found nothing (I generally don't have a problem fixing flats). When the second one popped after another day, I wanted to make sure I was using the right sized tubes.

I checked the wheel itself, and it had a sticker on it saying 700C x 18C/23C. This was surprising, since it seems different from the tire sizes, but the tires have been on since I got the bike and I didn't have any problems for several months.

So now I have:
Wheel: 700C x 18C/23C
Tire: 700 x 28C
Tube: 700x20/28

Are all of these matching up right, or do I have mis-matched sizes?
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Old 01-31-13, 07:36 AM
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In order for a blowout to occur the tube must escape from the tire. It it simply never caused by a puncture, but rather by a tire defect or poor mounting procedure. Any 700c tire will technically fit a 700c rim but there are recommended ranges for a given width of rim. Check the information and chart on Sheldon's site: https://sheldonbrown.com/tire-sizing.html. The rim width must be measured on the inside. that will tell you what tire width is recommended.

Your tube is fine even if you go to a narrower tire. If the tube were listed as 28-32c and you went narrower it would be more difficult to mount without creases, and if you had a much bigger tire the tube would be thinner when inflated.

The most likely problem is your mounting technique. Just Google "install bicycle tube" to see lots of suggestions - I'm not going to write it out here and have not yet made my video on tire installation - soon though, I hope.
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Old 01-31-13, 08:15 AM
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Flats that soon are often caused by improper mounting. You may have caught the tube under the tire bead or weakened it when you put it on, especially if you used levers. Always use you hands with putting a new tube/tire on and check to see the tube is completely inside the tire all the way around before inflating. Adding a few lbs of air to the tube helps.
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Old 01-31-13, 08:32 AM
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cny-bikeman is correct, punctures cause a relatively slow loss of air unless the tire and tube are badly and obviously slashed. "Blowouts" or tubes "popping" suddenly are the result of the tire escaping over the edge of the rim.

The most common cause is catching part of the tube between the rim's inside wall and the tire bead when installing the tire. After you install the tube and tire go completely around both sides of the rim to be sure none of the tube is visable outside the tire's bead.

A second cause is older rims that lack the "hook bead" of newer rims. The hook bead acts to retain the tire under high pressure and if a rim lacks them, pressures above about 80 psi can cause the tire to be pushed off the rim. Hook bead rims became very common in the 1980's so if your bike is older than that your rims may not have the hook.
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Old 01-31-13, 08:36 AM
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Also make sure that both rim bands are in good shape and properly positioned. This would be especially important if you bike has "deep V" type rims that have "recessed" spoke nipples.

While you're at it, make sure that you're running the recommended tire pressures (as specified on the tire).
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Old 01-31-13, 08:43 AM
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Originally Posted by FMB42 View Post
Also make sure that both rim bands are in good shape and properly positioned. This would be especially important if you bike has "deep V" type rims that have "recessed" spoke nipples.
Good advice but, again, bad rim strips are going to cause a relatively slow leak, not a blowout.
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Old 01-31-13, 09:08 AM
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Yeah, the OP's description of the tubes 'popping' leads me to believe it is not a problem witht he tube, but with the tire or an error in mounting.

SamSam - when you say the tubes 'popped' do you mean they went flat (and you describe all flat tires as being 'popped') or was there actually a 'pop' or 'bang' when they went flat?
Can you describe the hole in the tube - is is a small slit, a pinhole, or a jagged tear?

In general, any tube you can successfully mount between tire and rim can be inflated and will hold air.... perhaps if a tube is too small it will be stretched thinner and be easier to puncture, but this is only an issue after debris has punctured the tire, or the rimstrip has failed.
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Old 01-31-13, 09:09 AM
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I once allowed a cheap rubber rim strip cause a very rapid "blow out" (i.e. load noise and immediate tire deflation) when the tube blew through the strip where it covered a spoke access hole on a double walled rim. This could also happen on a similar "deep V" rim.

But yes, I agree in that you are, most likely, correct in your comment.
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Old 01-31-13, 09:39 AM
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One fit issue not yet mentioned is the stretch that a narrow tube goes through when installed in a wide tire. Specificly at the base of the valve stem. There is uneven tube stretch between the extra thick tube wall where the valve stem has been moulded in and the rest of the tube. As the tube gets narrower then the tire and expands to fill the tire's volume this uneveness can be problematic. I try to use the widest tube that fit's the tire's (and the rim as it is part of what controls the volume) size to minimize this stretching.

Additionally I have seen MANY tubes that failed (both slits and star bursts) on either side of the valve stem where the tube tries to stretch to contact the rim's base. Often the valve stem base is not fully down in between the tire beads so the tube tries ti fill the space. You often see, on a removed old tube, two small bulges in the tube adjecent to the valve stem. This is the tube having stretched beyond it's ability to return to the origonal shape. this type of internal failure will often have a short/sharp pop or crack sound when it bursts. because the tube is still fully inside the tire/rim the sound is less then the loud sharp "fire cracker' sound of a tube herniating out a tire side wall cut or bead pop off.

As to the size lables listed. The rim is stating the best fit tire widths, the tire shows it's LABLED width (not really a measurement but we'll pretend it is) and the tube shows it's narrow enough to fit a 20mm wide tire OR a bit larger. I would try the 28/32 tube to see how well it fits this wheel. Andy.
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Old 01-31-13, 10:14 AM
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in addition to what the others said...

after you mount a tire, put about 5 or 10psi in it, then carefully go all the way around both sides of the wheel, inspecting the tire-rim bead interface... make sure the tire is evenly seated on the bead, the little 'edge' just above the tire bead should be just about level with the top of the rim all the way around. if its got high or low spots, use your thumbs to adjusticate it til its even. once it is, THEN pump it up to your target pressure.

the numbers on the rim, 700x18/23 are probably the rim width. thats 18mm inner width and 23mm outer width. a 28c tire is just fine on that size rim (you could probably use anything from a 23mm up to a 35mm on that, assuming it clears the frame).

I would inspect WHERE this tire failed in relation to the rim and tire. I always mount my tires so the color label is centered on the valve stem, this way I can match up where a tube failure happened and carefully inpsect that stretch of the tire. as others said, #1 candidate here is pinch flats, unless the tire blew off the rim.
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Old 01-31-13, 10:25 AM
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Wow, a lot of replies before I had a chance to correct a misunderstanding!

I don't think that the tire "blew out" with a sudden event, and sorry if the use of the word "popped" made it sound that way -- completely my fault. The first time the tube was flat after I left it parked for about four hours, so not sure how fast it was. The second time it flattened over the course of about four minutes of riding. In both cases when I removed the tire there was a small pin-prick puncture, in a different place each time.

So, since I had one more of the 700x20/28 tubes, I put it on this morning, being extra careful not to pinch and to make sure the tube was well seated between the beads (I usually am very careful, and never use tire irons when putting the tube on, but admit that it may be possible I messed up the previous two times). No problems on the 20 min commute this morning, we'll see how long it holds up.

What I'm gathering from the comments is that the sizes should all mostly match up, except I might possibly be better using a slightly wider tube (
28/32 instead of the 20/28 I'm using, per Andy)? Hope that's right -- the Sheldon Brown page on tire sizes really befuddles me.....

Thanks!
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Old 01-31-13, 10:57 AM
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if it happens again, before remounting the tire, carefully run your thumb all around the inside of the tire, feeling for any imperfections. first pass, be ginger, a little hair thin steel wire or sliver of glass might be poking through.

also, those pin-holed tubes are perfectly good for reuse with a patch. get a 'tip-top' patch kit, and learn how to use it



short version:
  1. use sandpaper to clean and buff the surface all around the hole for an area bigger than the patch
  2. spread thin layer of solvent goo around an area bigger than the patch
  3. wait for goo to dry
  4. peel foil side off patch and place patch centered over hole
  5. press very firmly, rolling the pressure around from center to sides of patch, for a minute
  6. pinch patch so top paper splits, carefully peel away, from center toward edges. or just leave it.
  7. reinstall tube, tire, pump up and roll away.

if the pinhole is right next to a ridge on the tube, sand as much of that ridge off as you can during step 1.
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Old 01-31-13, 11:37 AM
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Also, the next time the tires are off, take a good look at your rim strips/rim tape.

Every now and then some little sharp metal shaving or other thingie gets caught in there,
and produces mysterious punctures......
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Old 01-31-13, 12:26 PM
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When you mount your tire, put the logo (if it has one) next to the valve.
That way, when you have a leak, you can lay the tube on the tire to see where the hole is and then examine THAT area of the tire for a piece of wire etc.

I wouldn't use a tube with such a wide range as 20-28. That's a 40% "stretch factor", for lack of a better term. You could find something a bit more appropriate like 23-28 or 25-32ish.
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Old 01-31-13, 12:34 PM
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Originally Posted by Bill Kapaun View Post
When you mount your tire, put the logo (if it has one) next to the valve.
That way, when you have a leak, you can lay the tube on the tire to see where the hole is and then examine THAT area of the tire for a piece of wire etc. I wouldn't use a tube with such a wide range as 20-28. That's a 40% "stretch factor", for lack of a better term. You could find something a bit more appropriate like 23-28 or 25-32ish.
Interesting math. Fortunately the forum has the edit reply function.
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Old 01-31-13, 02:06 PM
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The other option is to mount the inflation information next to the valve (mechanics will appreciate you). In both cases it should be on the drive side. One still has to be careful not to rotate the tube 180 degrees while inspecting it, or you'll stil be looking in the wrong spot. Ideally one marks the drive side of the tube next to the valve.
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