Undersized vs. Oversized Inner Tubes

Lately, I’ve been watching a lot of BMX’ers on YouTube. Scotty Cranmer, Alfredo Mancuso, and the like. I could never do tricks on a bike and I’m too much of a Clydesdale to learn, but nonetheless I like watching them doing it.

One of the YouTubers, Alfredo Mancuso, posted this video discussing how you could use a 16″ (I believe the tube he was used was 16″, even though the video title says a 14″ tube.) Some people in the comments section thought this was the newest trick they could put up their sleeves to shave some weight on their bike and give them an extra advantage.

So, let’s talk about my thoughts and hypotheses on running undersized and oversized tubes. First, here’s the video for those interested:

Let me answer these two superficial questions first. Is it possible to run an undersized inner tube in a tire? The answer is likely yes, but it depends on how undersized the tube is. The other question – is it possible to run an oversized tube in a tire? The answer is muddier on this, but it can be done unless the tube is extremely oversized (i.e. trying to stuff a fat-bike 20×4.0″ tire into a BMX tire isn’t going to go well.)

Now let’s go deeper: down, down deep below the surface.

Oversized Inner Tubes

First, let’s tackle the idea of an oversized inner tube. You’re riding a BMX bike with 20×2.4″ tires and you get the dreaded flat. You need to get home but the only tube you have is a 26×1.75-26×2.125″ tube destined for a mountain or hybrid bike like my Sedona.

In a pinch, this can work. The quick and dirty way of working with this oversized tube is to stuff it inside of the tire and fold it on itself in one spot. However, this won’t last long – it’s a very temporary fix to just get you home or to a shop where you can pickup a properly-sized replacement tube.

The reason it won’t last long is because of the folding. Creases and folds in the tube, especially when inflated, will cause the tube to fail prematurely.

The recommended workaround to this (although still a “get me home” temporary fix) is to stuff portion of the oversized tube in itself, so that it is small enough to fit snug around the rim.

This is all assuming you can even get the tube to fit the tire. If the tube is too big or, especially, too wide, it just won’t work. For instance, there’s no way you’re fitting a 26″ (or even 20″) fat-bike tire in a BMX tire. A tube that is too big or too wide will be more prone to get caught between the tire/rim when reinstalling the beads, so be extra careful when inflating the tire and monitor both beads closely.

I have tested running an oversized tube in the Sedona just for giggles. I pulled out one of the 27×1 1/4″ tubes on the 1987 Free Spirit Pinnacle and stuffed it in the Sedona’s 26×2.0″ tire. In terms of inflated diameter, the tube is an inch too big. But in terms of width, the tube is much narrower. Nevertheless, I was able to get it to work and inflate to 40 PSI. You could feel where the tube was folded at (for some reason I wasn’t able to get it to stuff inside itself) but it wasn’t that bad. It would work well to get you home, but definitely wouldn’t last as a long-term repair or even a “I want to ride today but this is all I have” repair.

The biggest problem with running an oversized inner tube is practicality and reliability. The other problem with carrying an oversized inner tube is it adds unnecessary weight and takes up a lot more space, and may all turn out useless in the end.

Undersized Inner Tubes

This was the tip that was the craze in the video. People think that they could cut a lot of weight off their bikes or save money by running undersized inner tubes.

On cost: mostly incorrect. In most cases, a 16″ or 18″ inner tube won’t cost less than a 20″ inner tube. Just like a 20″ or 24″ inner tube won’t cost less than a 26″ inner tube. Just take a stroll through your local Walmart – all of the tubes from the same manufacturers cost the same. The Acimut/Bell/CST tubes at Walmart are typically $4.99 regardless of size.

Weight savings of running an undersized tube may be minuscule, but weight weenies look for any little way to shave off any little milligram.

An undersized tube, while it may save you weight, may cause more issues than the weight savings are worth. An undersized inner tube is more likely to be more prone to punctures and catastrophic failures (blowouts.) An undersized tube will also less likely accept a patch, and will likely need to be topped up more often as the pores in the rubber are also stretched larger.

An inner tube is typically defined as a “donut-shaped rubber balloon or bladder that holds the pressurized air inside of a tire.” The keyword is balloon here, because an inner tube is much like a balloon. Without anything to hold it in or retain it (i.e. a tire) the tube can be inflated (or stretched) much larger than the tire itself.

So an undersized tube will likely work inside of a larger tire because the tube will inflate to fill the space. For instance, the (much) narrower 27×1 1/4″ tube I stuffed in my 26×2.0″ tires inflated to fill the space.

However, it’s not recommended to stretch or inflate a tube beyond the size it was designed to safely fit in. Let’s go back to the balloon analogy. Take two balloons: one inflated to it’s normal size and one blown up so tight and beyond it’s normal size. The first balloon, inflated to normal size, will take more abuse than the one inflated larger.

A tube is the same way. When a tube is stretched – such as an underinflated tube – it can’t take as much abuse. The tube will be more prone to punctures because the rubber of the tube will be stretched thinner. In fact, in more extremely undersized tubes (16″ or less in a 20″ BMX tire, for instance) you’re likely to have the tube catastrophically fail (blowout) even from a pinhole puncture, as the tube may be so stretched that a small hole will stretch and rip open the tube – much like taking a pin to an overinflated balloon.

There are many variables in how reliable running an undersized tube would be. A 18″ tube in a 20″ tire will more likely behave normally and not have any issues, whereas a 16″ tube would be risky and a 14″ tube would just be madness and asking for a blowout.

But size isn’t the only variable. Tube thickness should also be considered. A standard-thickness undersized tube will likely fare better than a featherweight undersized tube. Likewise, a heavy-duty (or welter-weight) undersized tube would fare better than the standard-thickness tube. However, a welter-weight/heavy-duty tube would negate the weight savings (and purpose) of running an undersized tube.

Personally, if I was riding BMX (20″ tires) and wanted to run undersized tubes, I’d stick with 18″ standard-thickness tubes. You could run 16″ in a pinch, such as to get home – but I wouldn’t go smaller than that.

You must weigh the benefits and see if they outweigh the negatives. The benefits of running an undersized tube is the (potential minuscule) weight savings. The negatives is you’ll likely not be able to repair an undersized tube, and you’ll be more prone to punctures and blowouts. And you may have to top up pressure more often.

Weight Weenie Alternatives

There are some alternatives to running undersized inner tubes for weight weenies. You could buy featherweight tubes, but you’ll usually spend more than standard tubes. These featherweight tubes will be less prone to blowouts than the undersized tube and you’ll be able to patch the featherweight tubes, but you’ll still be more prone to punctures.

They now make specialty tubes that are made of different materials, such as the Tubolito tubes. But these are much, much more expensive than your typical butyl tube.

Tubeless: Worth it?

Now, let’s address the elephant in the room of weight-saving alternatives. The one I’ve covered numerous times on this blog. Tubeless.

Let’s look at this from the weight-savings aspect: it isn’t worth it. Most BMX wheels and tires (more on that later) are not tubeless-ready. Therefore, you’ll need to do a ghetto tubeless setup. In BMX, I’ve heard the best method is the split-tube method, which is what I did on the Sedona back in November.

The split-tube method won’t save any weight. In fact, it may add weight. You still have part of a tube in there. But you also have to add 100ml of sealant such as Stan’s No Tubes. In the end, it weighs as much or more than running the tube.

Let’s look at it from the practicality aspect: not really worth it. While BMX wheels and tires are starting to come on the market, one video I watched (from Mike Fede – below) made them not look exactly reliable. BMX is very stressful on the tires – hops, jumps and other tricks put a lot of force on the tires. Land a trick wrong on a tubeless tire on it’ll burp or get blown right off the rim. Especially if the rider isn’t running the proper tire pressure. (Once again, more on that later at the end.)

BMXer Mike Fede tries out tubeless BMX tires

To get the reliability and peace of mind, you’d want to run tubeless-ready rims and tires. And even then it’s not exactly reliable, yet. Tubeless on BMX is still far from perfect, whereas tubeless has been almost perfected on mountain and fat bikes.

The most redeeming factor for tubeless – on any bike – is the puncture protection. The sealant will seal most small punctures and repairing larger punctures is easy.

The Biggest Thing: Pressure

If there’s one thing you can do to a) make your tires last longer, b) make your tires more resistant to flats and punctures and c) make riding more comfortable/easier – it’s to run the proper tire pressure.

There is no “one size fits all” approach to tire pressure. Tire manufacturers give you a recommended range on the tire sidewall for one reason. Some people like softer tires for more suspension and smoother rides. Others like harder tires that offer easier riding. Different tires, conditions and riding styles/disciplines warrant different pressures.

While there is no right tire pressure, there are definitely wrong pressures. I’ve discussed the issues of overinflation and underinflation in this article. Essentially, both extremes can be dangerous and can do more harm than good.

One should play around with different pressures until they find what they like. Start near the maximum and keep letting air out until you find the right pressure balance.