Brocephus

I have a 29er hybrid with 40-42mm tires, and my road bike is currently running some wide 32mm tires. I already have a couple spare tubes for both wheelsets, but I'd like a few more, and I can get a relative deal on more Continental 700x 25-32 tubes, and it would be handy to consolidate them, so I don't have a reserve stash of different tubes.
32mm to 40mm is only about a third of an inch more, so I can't imagine that's gonna stretch the rubber to paper-thin dimensions, and start causing me problems...........or is it?

Unca_Sam

You're not hurting for spare tubes, why compromise on tubes you don't need at the moment? If I had the choice between being stranded vs using a tube too small for the wheel, I might chance it. For regular riding, you're exceeding the specifications for the tube. You got the part right about the rubber compound being stretched too thin, weakening a safety critical component.

Therefore, a deal is not a deal if you don't actually need the item!

velojym

If it were only a couple mm, I wouldn't sweat it too much, but I think when it gets that far, I'd want to have some properly sized tubes on hand.
I have heard of folks using undersized tubes to reduce weight a little, but haven't bothered with that, myself.

ksryder

No it'll be fine. Rubber stretches.

I carry smaller tubes in my kit on my gravel bike because they pack down smaller and are easier to fit in the tool roll. I'm running tubeless so the tubes are just for a puncture that won't seal, and I don't intend for them to be used long-term.

dabac

When I had a 29er MTB, I never bothered repack the saddle bag. The 26” spare tubes worked just fine. Then when I went to a 700C commuter, the 26x1.1tubes stayed in use. No problem there either.

tyrion

It's 25% more. So the tube wall will be about 80% of its intended minimum thickness. It will probably work but you're pushing the limits, I'd say.

63rickert

Rubber stretches. Good butyl stretches too. Have you ever seen a genuine Goodyear bicycle inner tube? I have one here in a bike I got for my birthday sixty years ago. Stretchy. Reliable.

Current production it's safer to match tire and tube fairly closely. To get home do what you have to. For regular riding get tubes that fit.

ramzilla

I keep a well stocked supply of tubes for all my bikes. Tube size has got to match tire size as close as possible or, you're going to have problems. It's always better to have a tube size that's slightly smaller than tire size than the opposite. Never try to stuff a larger size tube into a smaller sized tire. Try using thorn proof tubes too. They're worth the extra cost, But, you sacrifice a little on extra weight.

Kimmo 

Ye gods, no. Not unless you're prepared to prioritise puncture resistance ahead of comfort, grip, rolling resistance and acceleration.

ramzilla

I guess if you're on a professional bicycle team competing in the Tour de France you might actually notice a 2X heavier tube weight inside a tire. But for the average rider dealing with everyday concerns, heavy duty tubes can be a good problem solver. I personally want the added protection of heavy duty tubes and tires whenever I deal with extreme downhill rides in excess of 35mph.

Brocephus

That's interesting. On a related note, the front tire on my road bike has had a slow leak in it, for months. I finally got tired of pumping it up every other day, and just recently replaced it. But I still tried to find that leak, but couldn't. So I just hung it on my bike rack in the garage. The tube was pumped up pretty good, and I was surprised at the diameter of it: the thing was about as big around as a Hula Hoop !! I mean, it was HUGE !!
So assuming a 26" tube will basically do the same thing, then yeah, I could see a 26" tube functioning fine in a 29er tire. I don;t think I'd be bold enough to try and cram one into a road wheel, but in a 29er, I could see it.

Retro Grouch 

I may be a little bit sick.

I'm down to two inner tube skews at my house. That's ERTO diameter, width and valve stem length. I've given away or thrown away everything else. I'm tired of spending 10 minutes going through all of my inner tuber stock looking for the exact one that I want. I'm never going to use the rest of them anyway, so why keep them around?

tFUnK

I use 28-35 tubes for my 40-42mm tires. Works fine, no pinch spot near the valve - that's the only area you need to be careful about when tubes and tires are slightly different sizes.

Kimmo 

I'll take the grip you're sacrificing.

JohnDThompson 

The tube will expand to fill the space available. But it will also be stretched thinner than a properly sized tube, and thus arguably more prone to puncture.

JonathanGennick 

Sometimes I'll put just enough air into a tube for it to take shape, then I'll look and see how I feel about the tube size relative to whatever tire I'm about to put it into.

A 25-32 tube is probably going to be 25 mm or less when it takes shape. It probably wouldn't be my first choice for a 42 mm tire.

Brocephus

That's a fair point (not to mention, obvious, since it was already stipulated in the OP ). But we've all seen how easily a tube will stretch (cross sectionally) as big around as a soft-ball, plus, I'm not talking about using a 700x23 tube in a 2.2" mtn tire.
Mathematically, yeah, the tube has to get thinner as it expands, but if we draw a circle that's 32mm around, then draw a circle that's 40mm around, and compare the two, I'm having a tough time imagining there's a gonna be real-world difference here in the tube wall thickness, and it's puncture resistance.

biker8989

i think the 40mm are the best tires for you

Brocephus

BTW, I just found a quality,economical tube that would work with both my current road and hybrid tires......
https://www.nashbar.com/continental-...501048/p326663
I have spare 40mm's tires for the hybrid, and some spare 32's for the road bike, so I'm not likely to change tire widths anytime soon (especially not on the hybrid), so these Continental 32-47mm tubes should be nearly ideal (especially at only $6 each, with $1.99 shipping, from Nashbar. Plus, the tubes wouldn't be stretched [i.e. thinned] anywhere near their maximum, which should only enhance the aforementioned puncture resistance.)