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Tube & Tire Size Don't Match?

Old 03-22-18, 10:03 AM
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BROOKLINEBIKER
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Tube & Tire Size Don't Match?

Hi everybody,
I own Schwalbe Marathon tires size 700x32. They needed a new tube so I attempted to place a Bontrager 700x28-32 Presta Valve tube in the tire. After experiencing some frustration placing the tube in the tire, I noted that the tube's circumference even when only partly inflated appeared a bit larger than that of the tire. I resolved the issue (I hope) by deflating the tube so it was roughly 2/3 inflated. The tire seemed to have a bit of a kink in it even when partly inflated. I have some questions:
-Am I at risk for a pinch flat?
-Do tires & tubes sometimes not run true to size?
-To ensure proper fit, should I buy smaller tubes going forward, say 700x23-25?
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Old 03-22-18, 11:04 AM
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700C is 700C (622mm diameter rim) no matter what tube you buy. You did buy exactly what I would have done for my 700C X 23 tire. I buy a tube that is designated 700C X 18-25. And yes, the proper way to install the tube is to partially inflate it before putting it inside the tire. If you put too much air in the tube before installing it, slipping the last part of the tire over the edge of the rim is difficult or impossible.

The only reason you would get a pinch flat is running the tire at too low a pressure. If there is a kink in the tube, you need to uninflate it and try again to get the tube spread evenly around the inside of the tire. If you really do have a fold in the tube, it can result in eventual tube failure and who likes to go along bump, bump, bump?

Who knows what manufacturer makes Bontrager tubes? It may be Kenda or CST. Trek just buys the tubes and has their brand name put on them. For years I have used tubes from Niagara Cycle, always buying their low priced brands (Kenda, Sunlite, XLC) and frankly see no difference between them and the few times I have bought the name brand, heavily advertised tubes. I've never gotten a tube that wasn't true to the size printed on the box but that could happen.

In a pinch you could use 700C X 23-25 but you are better off using the size you bought as it was made to fit the tire you have.
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Old 03-22-18, 11:16 AM
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Originally Posted by VegasTriker View Post
And yes, the proper way to install the tube is to partially inflate it before putting it inside the tire. If you put too much air in the tube before installing it, slipping the last part of the tire over the edge of the rim is difficult or impossible.
This being the internet and all, someone is bound to disagree with you. That's me. I don't preinflate. I've never seen the point for exactly the reason you listed. Trying to put on that last little bit of tire is difficult if the tube has air in it and that's when many people get out the tools.

Originally Posted by VegasTriker View Post
The only reason you would get a pinch flat is running the tire at too low a pressure. If there is a kink in the tube, you need to uninflate it and try again to get the tube spread evenly around the inside of the tire. If you really do have a fold in the tube, it can result in eventual tube failure and who likes to go along bump, bump, bump?
While a snake bite is a special kind of pinch flat, it's not the only one. If you use a tool on the tire, you can puncture the tube between the tool and the tire by "pinching" the tube. Different mechanism but same result. Again, that's why I don't preinflate.


Originally Posted by VegasTriker View Post
In a pinch you could use 700C X 23-25 but you are better off using the size you bought as it was made to fit the tire you have.
I certainly agree. I usually carry narrow tubes (even 18-23) while touring and put them in 35 to 38mm tires. The expand to fill the space and a very narrow tube is much lighter weight than a wide tube.
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Old 03-22-18, 11:46 AM
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I routinely use tubes one size range smaller than my tires (e.g. 23-25 in 32-38mm tires) because they are lighter yes, but mostly because they're easier to install. Theoretically, tubes inflated beyond their recommended size will puncture a bit more easily and leak air a bit faster, but in reality, a thorn/nail/glass shard that would put a hole in a 23-25 tube mounted in the next size up tire would also puncture the "correct" size tube. I top off my air pressure every few days—whether a tube holds pressure for one or two days longer is meaningless to me.

I'm in the "partially inflate before mounting" camp, btw. Whenever possible, I put the partially inflated tube in the tire, then squeeze the beads together and mount the tire from the valve side half-way around (I realize this is opposite of the usual recommended way), pushing the beads (with tube protected within) into the deeper center channel of the rim. Rarely do I need to use tools when I do it like this. After getting the tire on the rim, I pull all the way around to get the beads away from the tube and close to position on the bead seats. Then I put a little air in the tube (enough to pop the tire beads into the bead seats) and pull all around again to make sure the beads are in position and the tube is completely encased. Then I bring the pressure up to where I want it, plus a few psi more. Often there will be a pop of the beads seating completely (not the bang of escaped tube exploding, if I've done it right, lol). Reduce pressure back to where I want it and ride.
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Old 03-22-18, 11:52 AM
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Originally Posted by BROOKLINEBIKER View Post
I noted that the tube's circumference even when only partly inflated appeared a bit larger than that of the tire.
1 psi, the least amount you can use to give shape to the tube, is the right amount prior to installing the tube in your tire.
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Old 03-22-18, 12:01 PM
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Originally Posted by Retro Grouch View Post
1 psi, the least amount you can use to give shape to the tube, is the right amount prior to installing the tube in your tire.
This. And given the choice, I'd rather go with too small a tube than too large, as others have said.
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Old 03-22-18, 09:40 PM
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I pre-inflate too. Makes the tube a bit easier to wrangle, plus I'm paranoid that if I just shove the un-inflated tube in there, it'll get caught on something and rip.
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Old 03-23-18, 04:37 AM
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Originally Posted by BROOKLINEBIKER View Post
Hi everybody,
I own Schwalbe Marathon tires size 700x32. They needed a new tube so I attempted to place a Bontrager 700x28-32 Presta Valve tube in the tire. After experiencing some frustration placing the tube in the tire, I noted that the tube's circumference even when only partly inflated appeared a bit larger than that of the tire. I resolved the issue (I hope) by deflating the tube so it was roughly 2/3 inflated. The tire seemed to have a bit of a kink in it even when partly inflated. I have some questions:
-Am I at risk for a pinch flat?
-Do tires & tubes sometimes not run true to size?
-To ensure proper fit, should I buy smaller tubes going forward, say 700x23-25?
Sounds to me like you already have the right sized tube, but I also find that tubes a little undersized work fine as well.

I do pretty much what you did. Inflate just enough for the tube to hold some shape when you install it. If it makes seating the tire difficult, let some air out.

And yes, tire sizes are all over the map. I guess tubes as well, but since they stretch and are not very uniform in width when inflated outside a tire, it is a hard to say what their true size is. In practice they just take on the size of the tire they are in.
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Old 03-25-18, 07:34 AM
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I run 700c 18-25c in all my tires - both my 23s and my 32s.

I put about 1 psi in before mounting. I do not need tools to fit the tire - ever. Just make sure the opposite side is sitting in the middle of the rim.

That being said, I have used larger tubes in the past for 32s. Ive swapped 23s onto those larger tubes, and while they work, it is an absolute pain to use anything with extra rubber. That is why I solely run the smallest tubes I can.
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