Oft ignored — except when punctured — the lowly inner tube plays an important role in the performance of a bicycle tire. Among its important properties are puncture resistance, elasticity, air retention, and temperature stability. Those concerned about minimizing the effort required to keep up with other riders should also consider it, because it affects rolling resistance, too.
We measured the rolling resistance of a 700 X 40C Schwalbe G-One R Evo Super Race TLE gravel tire with a variety of new- and old-school inner tubes inside — from butyl to latex to TPU like Tubolito, Schwalbe Aerothan, and Pirelli SmarTUBE — as well as tubeless with only sealant inside at the Wheel Energy Oy test lab in Finland. We used this protocol and ran the tests at 2.5 bar (36 psi) and 3.5 bar (51 psi) at 35kph with a damped 40kg load on it.
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New-school tubes like Tubolito, Schwalbe Aerothan, and Pirelli SmarTUBE are extruded thermoplastic polyurethane (TPU) tubes whose ends are heat welded together. Thanks to its high strength, TPU tubes can be much thinner and thus significantly lighter than butyl or latex tubes. Based on needle-penetration lab tests, TPU tubes also tend to resist punctures better than butyl tubes.
Butyl and latex are old-school tube materials. A 700 X 35-43C Kenda butyl tube branded as a QBP Q-Tube represents our benchmark against which the other tubes are compared. In a mixture of old and new technology, the latex tube we tested was a seamless Challenge. Latex inner tubes have been around for well over a hundred years and, after being extruded like tubular pasta and cut to length, they are usually glued together at the valve with an inch or so of overlap. Challenge latex tubes are unique in that they are made with a special hand process to be a continuous, circular tube, completely seamless, without the thick, overlapped section.
Both butyl rubber and TPU are synthetic elastomers. Butyl rubber, or simply “butyl,” is approximately 98% isobutylene polymerized with about 2% of isoprene. Its initial production came during WWII when access to rubber from Asia was cut off.
TPU, on the other hand, can be considered a “family” of plastics rather than a specific formulation, and its physical properties can vary greatly depending on its recipe. TPU manufacturers adjust the formulation to optimize rolling efficiency, puncture resistance, temperature stability, and plasticity/elasticity. Tubolito makes Pirelli SmarTUBEs, which share its TPU formulation; their differences are in diameter, thickness, and color. Schwalbe’s Aerothan tubes have different TPU chemical formulation as well as dimensions.
Latex in bicycle tubes is the natural milky emulsion of long-chain polymers from rubber trees. Latex is found in 10% of flowering plants, and it coagulates when exposed to air. It can be vulcanized (cured) to cross-link the long polymer chains and increase its rigidity and durability.
Elasticity and puncture resistance
Both TPU and latex inner tubes resist punctures better than butyl ones because they have greater elasticity or “elongation” – the percentage increase in length before breaking. Elongation of a latex tube is around 800% (it can stretch to 8 times its length before breaking), while it can be as low as 300% for a butyl tube; TPU elongation depends on its formulation and typically ranges from 600-700%.
If a thorn or tack were to penetrate only 0.5mm into the interior of the tire, it would not puncture any 1mm thick inner tube, no matter how easily puncturable. However, if it were to protrude 5mm or 10mm into the tire, only a very elastic inner tube would move with it rather than being punctured.
With highly elastic latex and TPU tubes, if the tube is the correct size for the tire, the tube will be inflated but not stretched like a balloon. Its shape is constrained by the tire, so its surface tension is relaxed, and it has the capacity to stretch with the insertion of the sharp object. If the object does not stay in the tire enough revolutions to wear through the tube, it will not puncture it. This is the reason to wipe your tires off after riding through glass, tacks, or thorns; with a tubeless tire, you may be better off leaving a thorn in for the sealant to seal around.
Causes of rolling resistance
While the weight of the tube plays a role in rolling resistance, the property that matters most is conversion of rolling (kinetic) energy into heat through elastic deformation. The lower the tire pressure, the more the tire and tube deform when rolling past the tire contact point with the ground. Energy is lost both by friction between the tube and the tire casing as well as from the material of the tube heating up when deforming and rebounding. The finish of the inner tire surface and the tube’s outer surface, as well as the mechanical properties of the tube and tire affect how they move relative to each other when deformed.
In our test, more energy loss occurred in the butyl tube than in the latex and TPU tubes, probably due primarily to the lower elasticity of the former material. Higher elasticity allows the tube to respond more quickly to deformation and vibration. Hysteresis (the lag in rebound after deformation) losses as heat when flexing and stretching a tube are a function of its thickness, the frequency and magnitude of its deformation, and its inherent material properties.
A thinner, lighter inner tube is certain to have lower rolling resistance than a thicker tube only if the two tubes are of identical material; the lighter one will be faster since less material equals less energy dissipation. However, when comparing tubes of different materials, the properties of the materials are more important than the thickness. Only in climbing and when changing speed and direction will a lighter tube gain additional advantage.
Petri Hankiola, founder of Wheel Energy Oy, says that his data from extensive measurements also indicates that for lowest rolling resistance the elastic properties of the inner tube and its response to vibration must be “almost the same as that of the tire” (or more movement occurs between tire and tube). He says, “a latex inner tube is generally compatible with almost all (tire) models and is a safe choice,” and that some TPU tubes “can work very well with a few tires, but with some tires it can even increase rolling resistance compared to a butyl inner tube. Measuring rolling resistance with different inner tubes is the only way to ensure 100% which is the best inner tube for that outer tire.”
The Schwalbe Aerothan tube was the fastest tube at 3.5 bar (51 psi) pressure by a tenth of a watt over the latex tube. However, when the tire was more deeply deformed at the lower, 2.5 bar (36 psi) pressure, the Challenge latex tube absorbed 0.7 watt less power to roll at 35kph than the Aerothan tube, and over two watts less than the Tubolito and Pirelli TPU tubes and the butyl tube.
While the Aerothan formulation appears to be more efficient inside the 700 X 40C Schwalbe G-One R Evo Super Race TLE gravel tire than the Tubolito and Pirelli TPU formulation, it is double the weight of those tubes. Since Tubolito and Pirelli tubes are made from the same TPU material, the slight difference in performance between them can perhaps be attributed to a difference in thickness indicated by the higher weight of the SmarTUBE or to sizing.
All the tubes except Pirelli were sized the best possible, and the Pirelli TPU and Challenge latex tubes were undersized relative to the tire. The Pirelli tube was sized 700 X 23-32C, so it was stretched beyond its intended use inside of the 40C tire. This would affect its ability to resist punctures, being both stretched to the point of being very thin and less relaxed and able to move with the object puncturing the tire. How that would affect its rolling resistance, I don’t know. It’s also available in a 700 X 33-45C size. The Challenge latex tube was sized 700 X 29-38C, which is the biggest size it comes in.
TPU tubes are, by far, the lightest solution, as well as the most expensive. They have the advantages inherent to light weight of speed when climbing and quickness when accelerating, and they are very compact as a spare. They are faster than butyl tubes in every condition. Even if they are slightly slower in steady-state conditions than latex tubes, TPU air retention is much better than latex, resulting in stable performance throughout the ride.
A downside of some TPU tubes is that, to reduce weight, the valve core may be glued into a plastic valve stem, rather than screwed into a metal one. This eliminates the possibility of adding a screw-in valve extender for a deeper rim as well as of removing the core to inject sealant inside. Even though the valve may have wrench flats, don’t put a valve core wrench on it. If you try to unscrew the core, you will ruin a very expensive tube. Ensure that the valve is of sufficient length for your rim.
Latex tubes were the fastest solution for well over a century and continue to be competitive, especially at low pressure. Like TPU tubes, they resist punctures better than butyl tubes. They are heavier than TPU and somewhat less expensive. They require at least daily re-inflation and bleed enough air over a multi-hour ride to significantly affect performance and resistance to puncture and rim damage with sharp impacts. In Paris-Roubaix or any long event, tire pressure at the start should take into account pressure loss over the course.
The only advantage of butyl tubes over TPU tubes is price and perhaps ability to screw in a valve extender, and relative to latex tubes its price and air retention.
Finally, the Schwalbe G-One TLE tire rolled easier tubeless than with any inner tube, ranging from 0.9 watt less power than with the Aerothan tube at 3.5 bar to 5.1 watts less than with the butyl tube at 2.5 bar.