TPU is not a brand name or a new bicycle tube model — TPU (Thermoplastic Polyurethane) is a plastic material that a small number of manufacturers have only relatively recently begun using to produce bicycle innertubes. Until now, the typical bicycle tube was made from butyl, or butyl rubber, an elastic rubber compound. Its construction has remained virtually unchanged for decades; it was apparently the rise of tubeless technology in cycling that prompted a rethinking of inner tube manufacturing as well. 

What are the practical differences between butyl tubes, TPU tubes, and tubeless tires?

The two tube materials, butyl and TPU, differ significantly. To reliably hold air and pressure and provide adequate puncture protection, butyl must be much thicker than TPU. This makes the butyl tube not only nearly threetimes heavier, but also means the packed size of a spare TPU tube is considerably smaller. The reduced weight plays a particularly important role in the area of wheels and tires. Not only is the overall weight of the bike reduced — every gram on the outer part of the rim has a negative effect on acceleration, because the combined mass of rim, rim tape, tube, and tire must not only be brought up to riding speed, but also set into rotation.

With a tubeless tire, these riding characteristics are similarly good to marginally better, with the key difference being that no inner tube is installed at all. Specially designed rims and tires seal against each other so effectively that, as with car tires, no tube is needed. The valve is bolted directly into the rim. In addition to the high tire flexibility, this system is considered more flat-resistant because the inside can be filled with latexsealant that automatically seals small punctures. The downside of tubeless: mounting, changing, and filling the tire with latex requires more mechanical skill and tools, and the sealant can, under unfavorable circumstances, make a mess of your bike or home with the sticky liquid. Tubeless tires also need to be topped off with air more frequently, as an undamaged tubeless tire retains air less effectively than a tubed setup due to the absence ofa barrier layer. Incidentally, both tubeless tires and TPU tubes allow for lower tire pressures without risking damage — a major advantage on low-grip surfaces like gravel, sand, or wet terrain. 

What changes in handling, and can you inflate a TPU tube with CO2?

A TPU tube is not fundamentally used or mounted on the rim or inside the tire any differently than a butyl tube. That said, there are the following small differences in handling: 

• A TPU bicycle tube cannot be patched with conventional repair kits. The vulcanizing glue included in standard kits only works on rubber. For TPU tubes, there are self-adhesive stickers (patches) or special adhesives designed for TPU patches. 
• Before repairing TPU, the bonding surface must be carefully degreased with a solvent rather than roughed up as is normally done (tip: wear gloves). 
• Both TPU and butyl tubes expand significantly when inflated outside of the tire. The typically elastic properties of rubber allow the butyl version to return to its original shape when deflated, whereas the more plasticnature of TPU means it would largely retain whatever unevenly stretched shape it had assumed. If, at the time of installation, its dimensions at one or more points are larger than those of the tire casing, folding inside thetire would occur. For this reason, it is important to only lightly inflate the TPU tube before placing it in the tire — do not bring it up to pressure (greater than 0.3 bar) beforehand! After the tire has been mounted, however, the TPU tube can handle just as much pressure as a conventional butyl tube. 
• The size of the TPU tube should match the tire as closely as possible. With butyl, the considerably thicker material allowed for some flexibility, at least in terms of width — the tube would expand to fill tire volumes larger than its stated dimensions. A TPU tube that is too small or narrow and must significantly expand within the tire would lose even more wall thickness through this excessive stretching. The excellent material propertieswould then no longer be guaranteed. 

How do you inflate a TPU tube with CO2?

When dealing with a flat and only a CO₂ cartridge and inflator are available — no floor pump or mini pump — caution is advised. Some CO₂-based pumps do not offer the ability to control the flow of the highlycompressed gas as it enters the tube. When the CO₂ cartridge is punctured, all the gas flows through the valve into the tube at once. Such inflators are not suitable for pre-inflating a TPU tube before mounting. Even thevolume from a small 16-gram cartridge would over-inflate a TPU tube outside of the tire. In that case, it's better to pre-inflate the tube by blowing into the valve — or not at all — before inserting it. 

If the CO₂ pump has a precise pressure regulator, you can carefully bring the TPU tube up to its 0.3 bar before placing it in the tire. Ideal for use with TPU tubes are combination pumps that feature both a hand pump and a CO₂ cartridge. 

TIP: Regardless of which tube system you're running — if your tire has been inflated with CO2 after a flat, it will lose pressure comparatively quickly. For physical reasons, CO2 molecules diffuse more easily through thematerial of the tube and tire than the oxygen and nitrogen in regular air. CO2 is therefore suitable for emergency use after a flat, but not as a permanent inflation medium. 

Interested in TPU bicycle tubes? Find out everything you need to know about the AERON/TPU Airtubes here.