7/14/15 8:18 PM

One of the 700x42 tires on my Surly frankenbike needs a tube. I have a bagful of road tubes marked 700x19-25 etc. can I just use one of those?

7/14/15 8:36 PM

Imagine how thin it might get inflated...

I can see until you get home if you flat or something...

7/15/15 4:56 AM

I agree with Sparky. In an emergency one of them will probably work but I'd not trust it for routine use. Get a tube rated for the right size range.

7/15/15 6:41 AM

I don't think they can be trusted. Run through the bag of tubes and report back frequently.

7/16/15 3:44 PM

Especially when the rim is standard road width, there is a tendency for a smallish tube in a wide tire to pin itself to the inside of the tire casing due to the air pressure, with the rubber pad at the base of the valve stem then not wanting to fall to the inside "floor" of the rim cavity.
When this happens, often the tube will rupture to either side of the reinforced area of the tube near the valve stem, as the stretching tube bulges into the rim cavity.

To some extent, one can prevent this by pulling the stem outward during the initial few strokes of the pump.

But there is still a lot of over-stretching of the tube local to the valve stem, since the reinforced area will not stretch much, possibly forcing the tube in that area to over-stretch to the point of rupture.

Normal tubes have to stretch quite a bit already to fill the inside of the tire, and an undersized one can stretch beyond limits.

An over-stretched tube loses air very fast in response to even a pin-prick, leading me to go the opposite route, fitting somewhat-oversized tubes that more-fully fill the inside of the tire with barely any stretching at all. This allows me to ride all the way home on a slow leak in many instances. These also lose less air day-to-day, and further, will enhance the level of puncture protection afforded by any kind of sealant.

I put just enough air in a tube to give it some shape, without any stretching, then fit it into the tire to see if it fits in without bulging out too far, which could interfere with installaton.

You might find that certain of the 29er tubes will fit into your 42c tires.

Tubes can easily be compared for width by measuring the width across the flat, folded tube inside of the box. The numbers printed on tubes and their boxes can vary wildly versus what the actual tube measures!

Performance sells hybrid-width tubes in two very-different size ranges, but when you take them out of their packaging, they are the same tube! Apparently 28-32c equals 35-42c at Performance, but they are not alone.

7/17/15 4:38 AM

I had that exact problem ad you described, dddd, years ago. My solution was to create a shim/protective layer using an old punctured tube and cutting out the rubber portion around the stem, then place it over the new tube, plus an extra layer of rim tape near the valve hole for good measure.

7/17/15 9:21 AM

"using an old punctured tube and cutting out the rubber portion around the stem"


That must be an old trick. I took a few apart like that when working at the LBS in 2012

7/17/15 8:12 PM

Okay, I got some marked 32-44. Love the ease of working with big, soft tires. Nice for my itty-bitty hands.