Cycling UK Forum

I've pumped some tubes up to grotesque diameters whilst searching for punctures. I know this is only low pressure & for a short period but the physical stretching doesn't appear to affect them unduly. Surely a few mm under sized won't make much of a difference as the tube is not actually stretching that much inside a tyre, the pressure is being resisted by the tyre/bead/rim. Having said that I do try & match my tube to the tyre.

one issue is that the tube rarely stretches evenly to fit inside the tyre. Even with talc applied the tube doesn't stretch evenly; the cross-section of the void it has to fill is sort of keyhole-shaped, and the tube stretches a lot more in those parts which end up pressed against the rim tape, especially in the corners.

I have experienced numerous punctures with lightweight/stretched fit tubes from quite tiny defects in the rim tape. For example

1) the edge of some PVC insulation tape (admittedly old and hard) turned out to be enough to cause a maddening series of punctures. I am pretty sure that a less stretched/thicker tube would have resisted this better. Imagine trying to cut rubber with a piece of insulation tape; it doesn't seem possible, does it? Yet it happened!

2) a Mod 58 style rim was fitted with a nominally 10mm wide, 1.0mm thickness rubber rim tape. Except that the rim well was fractionally oversize and the tape was about 9.5mm wide once fitted. The result was that there was a ~0.5 to 0.7mm wide gap at the edge of the rim tape. Amazingly the tube pushed into this gap (when stretched) and cut/split somehow.

Both the above defects caused multiple punctures before I figured out what the problem was.

Anyway they are pretty good examples of why it is that stretched/lightweight tubes are usually more vulnerable to damage than you might expect.

FWIW I think that stretched/thinner tubes will suffer more pinch flats than thicker/less stretched tubes. IIRC at one time they used to sell extra-thick tubes for use at low pressures, on the basis that they didn't pinch-flat so easily.

cheers

For 700 c Schwalbe does two inner tubes:
18 - 25
28-45

That would suggest that inner tubes are far more forgiving than some would have us believe

Cunobelin wrote:For 700 c Schwalbe does two inner tubes:
18 - 25
28-45

That would suggest that inner tubes are far more forgiving than some would have us believe

it might do except

a) pretty much every other serious manufacturer sells a 700x 35-45 tube

b) several LBSs near me (that like schwalbe tubes BTW) go to the trouble of stocking the wider tubes from other makers and specfically avoid fitting the schwalbe ones in anything wider than a 35mm tyre; they just are not reliable enough.

cheers

Brucey wrote:

Cunobelin wrote:For 700 c Schwalbe does two inner tubes:
18 - 25
28-45

That would suggest that inner tubes are far more forgiving than some would have us believe

it might do except

a) pretty much every other serious manufacturer sells a 700x 35-45 tube

b) several LBSs near me (that like schwalbe tubes BTW) go to the trouble of stocking the wider tubes from other makers and specfically avoid fitting the schwalbe ones in anything wider than a 35mm tyre; they just are not reliable enough.

cheers

DOesn't actually change anything, the tolerance is still 10 mm, which is more than the 8 mm in the OP

best to count percentages.

Where reliability is really important (eg aircraft inner tubes) the normal thing is that one tube is not expected to 'stretch' more than about 15% before there is another tube size.

Going +50% on the minimum size is just asking for trouble.

cheers

Brucey wrote:

The utility cyclist wrote:

Brucey wrote:you can do it but the tube will be more stretched than normal. This will increase the chances of a puncture (including annoying and unfixable ones such as splits on the inside, valve failures etc) and when you do get one the air will come out faster (because even a pinprick in a stretched tube will open up ).....

By how much?

Enough to make a difference.

If you want to try an experiment, try cutting rubber sheeting when it is stretched and not. It is far easier to cut when under tension. This approximates to (say) flints poking through a tyre into an inner tube, or something chafing inside the tyre.

cheers

So because you were careless/sloppy with your fitment and unable to see a problem that would have arose your anecdote becomes data.lol
How very typical of you when trying to push your point.
Again, I ask you, how much more? You can't't count sloppily fittied tubes on ratty rims that haven't been checked over properly, that can damage any tube.

simonhill wrote:If using large or even oversized tubes on wider tyres at lower pressures - are you more liable to get pinch flats?

My (probably wrong) logic has it that a smaller tube would be less liable.

Whether the tyre gets pinched at all in the first place, is purely a matter of tyre size and pressure versus the magnitude of impact. If it happens it means the rider chose a too small tyre and/or too low pressure for the load and surface roughness. But a pinch does not always cause a puncture. I think that mainly depends on how sharp and forceful is the pinch, but for a given pinch: I would expect a tube that is already stretched to the limit, more likely to puncture from also being pinched, than one that isn't under such stress.

But as I err on the wide side with tyre choice and high side with pressure, I hardly ever get pinched in the first place. And I think that also avoiding tubes for which my chosen tyre is the biggest they can stretch to is another reason I feel free to choose tyres on the basis of how easily they roll and never mind how easily they puncture! Because I rarely puncture anyway.

There, I've said it. Now I'll be for it!

Cunobelin wrote:For 700 c Schwalbe does two inner tubes:
18 - 25
28-45

That would suggest that inner tubes are far more forgiving than some would have us believe

They also do 28-32 tubes (SV16).Here they are from SJS, & I expect, other places (that was the first link that Google gave me).

if you look on p89 of the 2019 Schwalbe catalogue they have a chart indicating their inner tube fitment advice. You will see that in narrow 622 tubes you have a standard weight option #13 and a lightweight option #14. Similarly in the wider 622 tubes there is a #17 standard weight and a #18 lightweight option. In each case the wider fitments are only applicable to the standard weight tubes, i.e. they certainly don't have a blanket recommendation that their tubes will go +50% on minimum size.

The #16 tube is built between a #17 and a #18, weight-wise. I would imagine that it is a very good fit within its (relatively narrow) recommended range of fitments.

BTW no-one has asked what limits the smallest width tyre that a tube will 'fit'; the answer is that if the tube doesn't stretch at all, you are quite likely to get wrinkles, and wrinkles tend to rub and cause punctures (again, the wrong size tube is OK as a get-you-home but not much else). Before the tube gets so baggy you get wrinkles, the chances of nipping a tube are considerably increased.

Nearly all tube manufacturers have a 'blind spot' in their range where the packet says it will fit certain sizes of tyres but actually the fit is very poor, and you would be better off with something else. If the tube says it fits only one rim size (eg 622) then it is simple; the tube will normally work well in the minimum width quoted, very well in the +5% to +10% range, and decreasingly well in widths above that.

However if the tube claims to fit two quite different rim sizes, this should ring alarm bells. This normally only works at all if the smaller diameter is also the narrower fitment. However in (say) 20" tubes there are some tubes that are sold as fitting a wide tyre in a small diameter as well as a narrow fitment in a larger diameter. In practice such a tube can be nearly useless; it doesn't fit either tyre at all well.

cheers

Can't say I have ever noticed a real difference. I use roughly the correct size if possible but wouldn't worry if I didn't have one and would probably forget to swap it if the wrong one went in. For spares on my rough stuff bike I carry two narrow ones. Can't fit two big ones in the bag and its a safer bet I reckon.

Brucey wrote:BTW no-one has asked what limits the smallest width tyre that a tube will 'fit'; the answer is that if the tube doesn't stretch at all, you are quite likely to get wrinkles, and wrinkles tend to rub and cause punctures (again, the wrong size tube is OK as a get-you-home but not much else). Before the tube gets so baggy you get wrinkles, the chances of nipping a tube are considerably increased.

Having tried some tubes by inflating far enough to give a measurable circular cross section, the minimum size to avoid wrinkles is significantly smaller than the minimum size printed on the tyre/box.

Hutchinson 32/35/?/? (a patch covered the other sizes) --> "rounds out" at 22 mm dia.
Vavaert 35-45 --> 29 mm
Vittoria 38-45 --> 28 mm
Schwalbe "17" 28-47 --> 23 mm

I generally fit tubes where the minimum quoted width matches the size of my tyre, as near as possible. I don't have problems with chafed tubes, and a significant fraction (~20%) of my punctures are slow enough that I can swap tubes at home.

that's interesting; I'd have guessed that many tubes are a bit bigger than that. Of course if the tyre is 'size' on the outside then the cavity inside is smaller than that; eg a 28mm tyre is probably between 25 and 26mm ID.

Some tubes assume a very flat shape indeed when there is no air in them; these seem more likely to become nipped if they are not rounded out when installed. When flattened, the tube is ~1.57 times wider than the 'rounded out' size, so for example a tube that is ~25mm diameter when rounded out is likely to be about 39mm wide when flattened.

I think that fitting a tube which is not stretched at all inside a typical bicycle tyre is very difficult; even if the tube is 'size' when rounded out this doesn't exclude the possibility of wrinkles, since the tube has to be able to slide lengthwise easily in the cover in order for it to settle into the right shape.

Anyway since most tubes are stretched a bit even in the smallest size of tyre, this means they are more likely to be stretched to an unacceptable degree when installed in much larger covers.

cheers

Brucey wrote:Some tubes assume a very flat shape indeed when there is no air in them; these seem more likely to become nipped if they are not rounded out when installed. When flattened, the tube is ~1.57 times wider than the 'rounded out' size, so for example a tube that is ~25mm diameter when rounded out is likely to be about 39mm wide when flattened.

The Hutchinson is one such tube.
It measures about 29.5 mm when flat, rather than the 34.5 mm that pi/2 would give for a 22 mm diameter.
I expect it's a combination of having been slightly inflated at 22 mm, and the edges of the flattened tube actually being a little rounded (the tube is about 2.6 mm thick at the edge, but 1.6 further in - measurements subject to squishy rubber error).

I was expecting the tubes to be under size compared with the quoted minimum tyre size, as there's the thickness of the tyre sidewall to allow for, and the fact that many tyres are narrower than they say they are. However, they are more under-size than I was expecting.

I measured a new tube I had kicking around and it 'rounded out' at 20.7mm and measured 27mm when flattened. Again it seems a little stretching must occur during rounding out. The manufacturer (Schwalbe) reckons this size tube is OK for a 40mm tyre... . I think there are much better tubes for that purpose! These tubes are going inside 25mm tyres and I think they will be 'about right'.

So could a sensible recommendation (for a tube that is easy to fit, doesn't nip too easily, and is not stretched too much) be that the flattened width of the tube ought to be about 10% wider than the nominal width of the tyre?

cheers