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I have a 2000s era Trek 7.3 fx 20 bl hybrid bike that recently got a flat due to a puncture in the tire near the valve. The tread was also worn down from years of gravel-y trail riding so I needed a new tread as well.

I want to learn more about basic maintenance, so I went to my local bike shop for a replacement tube and took what they gave me.

Upon inflating the tube ever-so-slightly, I noticed a lot of slack when putting it over the rim. Figuring this would even out, I put the tread on the bike and continued to inflate but noticed the tread bulging out. I can 'shift' the bulge around but the result is an uneven tire (with the tube spreading out over the rim after ~20 PSI.

tube and rim with lots of slack

After doing this a few times (and making sure the tube is seated properly) It seems like my rim is on the low end of what the tube supports, and that it's too big.

tube, rim, and tire

I wanted to double check to see:

  1. My understanding is correct
  2. How I should go about getting the right fit. My assumption is that I need a smaller tube (so not one where my rim is at the very bottom end of what is supported)

Full stats for the curious

Old

Rim: Bontrager 6000 Alloy ERRTO: 622x18; ERD: 603 Tube: 28X ⅝ X 11/14 Tread: (32-622) 700X32C

New

Rim: Tube: Kenda 700x30-43c (27x1-⅛-1-⅜) Tread: Kenda Kwick Journey 700X32C; ETRTO 32-622

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    You shouldn't judge the size of the tube outside of the tyre if it's got any pressure in it at all. When mounting a new tube I'll inflate it until it's got enough pressure to hold shape, so it can be mounted without any twists, but then it often bunches a bit at the opposite side from the tube. Deflating the tube slightly before seating the bead is all that's necessary.
    – DavidW
    Commented Jul 25 at 15:56
  • Hmm, i've done this several times (deflating, lining it in the rim, then fitting the tube+rim inside the tire, then re-inflating) and there's always a large bulge over the rim (even at low PSI). The higher the PSI the larger the bulge, it never evens out. Am I supposed to be doing something during inflation to prevent these bulges?
    – Nick Tomlin
    Commented Jul 25 at 16:13
  • What exactly do you mean by "bulge over the rim". Could you post of a photo of exactly that?
    – Paul H
    Commented Jul 25 at 16:44
  • I'll try to get a photo soon, but a text description is that the tube is popping over the rim (leading to a bulge in the tire) in one area. This leads to the imbalance in the 2nd photo (e.g. on the right side -- it's hard to see so I can try to get a better photo). This doesn't seem to be in one specific area and doesn't even out the more I inflate
    – Nick Tomlin
    Commented Jul 25 at 16:49
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    If that’s the case, then the tire is not properly seated. This tube should be perfectly fine for your rim and tire combo. But you need to get the tire seated properly.
    – Paul H
    Commented Jul 25 at 17:46

2 Answers 2

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Looking more closely at your second photo, it appears that you haven’t seated the tire’s bead in the rim at all. This is a critical step that you cannot skip. Look at your front tire and spot the difference the placement of the tire bead relative to the rim sidewall.

Here’s a video explaining that process:

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    This was it! :facepalm: For some reason I totally glossed over that in other tutorials, but this video was spot on in calling out seating the tire first. It took some elbow grease but I was able to make it work. A lot harder than I thought!
    – Nick Tomlin
    Commented Jul 26 at 2:12
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Also keep in mind that the same tube is used for both 27" and 700 tire sizes A 27" is iso 630 and a 700 is an iso 622 So the tube is going to be a little longer than needed if it's in a 700 tire.

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