inner tube width range relative to tire width: reliability effects?

Question

[Apologies if this seems like a dup: I see there are lotsa tire+tube questions, but this one does seem to be slightly different, in that it strives for generality and concerns choice between alternatives.]

question

Suppose one is buying tubes for a given {rim, tire} tuple. Assume the following data/constraints:

1. The tire fits the rim's bead seat diameter (BSD) appropriately and has tire width w₁.
2. Tubes t_a and t_b match the rim/tire BSD and are identical in all meaningful¹ respects except width: identical cost, material, thickness, valve, etc.
3. t_a specifies a width range w₀-w₁ (with w₀ < w₁)
4. t_b specifies a width range w₁-w₂ (with w₁ < w₂)
5. One is a competent tube changer with lots practice.
6. One's primary concern is tube reliability over time: i.e., one very much wants to have fewer flats.

Given the above,

1. Are there empirically-valid reasons to choose t_a (i.e., the tube for which the tire width is at the high end of the tube's width range) over t_b (i.e., the tube for which the tire width is at the low end of the tube's width range), or t_b over t_a? Or is there no reason to prefer one over the other?
2. Are there other data/constraints which are significant for the decision? E.g., whether one typically runs at high or low tire pressure?

motivation

I drive recumbents with 20-in/406-mm front wheels, currently running unbelted 20x1.35 Primo Comets. (FWIW

• tire make/model is irrelevant for the rest of this question, so feel free to ignore.
• I don't drive a trike: I have 2 nearly-identical SWBs with swappably-identical wheels. I like to have spares :-)

) I bike for transportation, so reliability is very important to me. Unfortunately

• I recently moved to an area with much more puncturing road debris.
• The area is also less dense, causing longer mean trip distance which exposes me to more debris.
• Recent flats have now consumed my stock of 20x1.35 tubes. FWIW, I replace tubes rather than patching them (assume that preference as given :-), and I like to keep a few spare tubes (as well as ... everything else).

Fortunately there are some 20x1.5 Kevlar-belted Comets locally available at a decent price, which I intend to try. However the 406-mm tubes I'm seeing (and on which I now need to stock up) are either 20x1.25-1.5 or 20x1.5-1.75. Which raises the more general question.

possible answer

I bought the 20x1.5-1.75 for the Primo belted Comets. After 3 months: no problems installing, no flats, hold air well (i.e., takes a long time to go from 90 psi to 80 psi). Granted, this is not much of a test: n=1, not enough time for testing, change of tires is a confounding variable. But WTH.

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¹ Assuming both tubes have the same material and thickness, the wider tube will be heavier, but consider the mass difference negligible. (In this specific case, the mass difference=26 g, which is negligible for me.)

Answer 1

You say to ignore mass. Why ignore the major difference between the two tubes?

Mass = rubber. The two tubes may start with the same wall thickness but inflated to 1.5 the larger tube will have stretched less and will have a larger wall thickness at the inflated size.

The best measure would be weight but if you take 1.5 / 1.25 the inflated wall thickness of bigger is 20% greater. What is the chance an object would penetrate the smaller wall but not the larger? Very small; nevertheless, the larger tube gives a thicker wall at the inflated size.

You can buy puncture resistant tube with thicker rubber. With the larger tube you are getting thicker rubber for free. It's as if you were buying socks, and your foot was between sizes, and the different sizes were the same price. The bigger socks will have more cushion and last longer.

Answer 2

If you're getting flats regularly, my guess is that its not the tube's fault, but you to need to make sure that you have sufficient tire pressure and switch to puncture-belted tires (e.g. Schwalbe Marathon). A tube is just a balloon, and you can often put a tube marketed for a not too different size tire into one which slightly different without problems (so long as you don't have the extra width bunch up; e.g. don't put a 29x2.5 tube in a 700x18 tire; I normally do this with 700x23 marketed tubes in a 700x28 tire without problems). Just like most balloons, even the smallest prick will deflate it, so you really need to protect the debris from getting to the tube in the first place. The thickness difference between the rubber in the bigger and smaller tubes will likely be too small to make a difference. And in this case, since the tube is designed to work for tires of your size in both cases, you should expect to have perfectly acceptable behavior with both sides (that is, dumb luck should determine which works better).

Empirically, if anything, the brand of tube might have a slight effect. I've had bad luck with tube reliability from one manufacturer in particular, so I don't buy their tubes anymore.

If you're really paranoid, they make tubes which have extra thick rubber called puncture resistant tubes (useful if you have goatheads) and you can fill your tube with tire sealant (Slime). These will be more resilient to punctures compared to other tubes, but they are more expensive and Slime is messy. But I'd start with some puncture resistant tires. In short, if you don't have these speciality tubes, its likely the case that it doesn't matter which tube you have.

I'd suggest avoiding using tire liners, since they can actually increase flats unless you're really not in good shape. A puncture belted tire is a better idea for most people.

Answer 3

The things that will make your `bent more puncture resistant are

• Not riding on or through things that give punctures (not always possible)

• Higher tyre pressures

• Thicker tyres - just more rubber on them.

• Belted tyres (reinforced with something)

• Tyre liners (either real kevlar bands, or old tyres with the bead cut off)

My preference is for higher pressures and not crashing through potholes and over kerbs. I've tried puncture resistant tubes and there's not enough data yet. They are heavier and thicker than a fast tube.

ANSWER 1: Punctures are a part of cycling and cannot be completely eliminated. Carry the tools and two spare tubes. (one for each size, perhaps two for each size being a good one and a patched one for cost reasons.)

ANSWER 2: To answer the question of your question: the tube's rated tyre size is a range, and a smaller 1.2-1.5" tube will stretch more to get to 1.5 vs a 1.5-2.0 tube that will have to stretch less. A more-tightly stretched tube will be more vulnerable to puncturing over time.

However that means the foreign object has already penetrated the tyre. Expecting the tube to resist the FO means that its already on the inner line of defense against flats.

Logic says a the bigger-rated tube will do better, but the cost is the extra mass. Consider weighing both sizes of tube and compare the difference - I have this data point:

• 26" MTB tubes 190g and 212g (both with a patch) vs 560g for a new puncture resistant tube. Sadly I can't read at what width the tubes are rated.

FINAL ANSWER You'd be best off using puncture-resistant tyres and tubes if reliability is your top priority, and suffer the weight penalty. Still carry spares+tools.

Answer 4

Major edit for this, told a story before so I'll completely re-word. If the question is assuming new 26 x 1.5 Kevlar tire which tube would be used of two choices and why? A top of fit tube width of 1.25 - 1.5. Or slightly over sized at 1.5 - 1.75. Note *** All sizes in inches.

Answer: I would always go with the "top of fit" 1.25-1.5 tube for choice. My experience has been a smaller width tube that is not over sized will fit easier in the tire on installation. Why? The over sized tube means you have a greater chance to pinch the tube in between the tire bead and rim wall. Interesting over sized does work it's just not preferred. With a repair shop banging out flat repairs or personal use it's more challenging install to use the "over size" 1.5 - 1.75 tube. An over size tube also causes balance issues. The tube is slightly bent and twisted on install to achieve fit. I've ran Specialized FAT BOY slick 26 x 1.25 tires, with Kevlar and always used 26 x 1 tubes.

One caveat here, in my original answer I was just suggesting and cautioning don't let the range of size difference (tire to tube width) get too great. So as my previous example, a 26 x 2.5 tire with a 26 x 1.125 tube that combination with a 0.375 variance went BOOM and destroyed the tube and tire.