For many years, I've heard older people say that inner tubes don't hold air as well as they used to. It's usually the same thing - 'I never had to air up my tires when I was a kid.' Others seem to think that tubes puncture easier now than they did even 15 or 20 years ago. All of this got me wondering if there really is a difference between the quality of tubes and if it's changed through the years.
First, a (very) short history:
Originally tires and tubes were made from natural latex from rubber trees (Hevea brasiliensis) that are grown almost exclusively in Indonesia and Malaya. During World War II, Japan had invaded these countries and cut off the world’s supply of natural rubber. This led to a shortage and the need for an alternative. At the time, German engineers had already been working on a synthetic rubber, but it wasn't until 1942 that U.S. engineers were able to develop it into a useable rubber compound called Butyl. Butyl synthetic rubber is the most common material used to make inner tubes today.
Butyl retains air better and costs much less than natural rubber, making it ideal for inner tubes. It’s biggest disadvantage is that it doesn’t stretch much - as much as 5 times less than natural rubber. For this reason, Butyl is normally blended with natural rubber to give a tube the ability to stretch more.
The majority of the global supply of Butyl is produced by two companies: Exxon in the USA and Polymer Corporation in Canada. Exxon offers 5 different Butyl synthetic rubber blends. Each uses a different mix of additives to allow it to withstand heat better, increase it’s durability, or just lower the cost. These additives decrease Butyl’s ability to hold air so they must balance these additives for the best compromise. There are only a few companies that still make tubes in the U.S., but there are easily a dozen or more tube manufacturers in China alone. These companies blend the different butyl compounds with 10 -20 percent natural rubber in order to give preferred results vs cost. More natural rubber means the tube will stretch more (which also makes it more puncture resistant) but it won’t hold air as well. It also increases the cost. Again, this is a balancing act to achieve the best compromise.
Other factors that influence how well a tube holds air are the wall thickness of the tube and how closely the tube matches the size of the tire. A thicker tube wall will slow air leaking; a tube that has to stretch to fit the tire causes the wall of the tube to become thinner, allowing air to leak at a quicker rate. Thus, thorn resistant tubes with thicker rubber will hold air longer than a superlight tube that uses very thin rubber. Riders looking to save weight are willing give up a tube's ability to hold air for long periods in order to save a few grams. This is usually acceptable because these are the cyclists who will air their tires up before every ride.
The chart below shows how different brands of tubes use different thicknesses
Brand/Thckness | Ultralight | Superlight | standard | Thorn reist* |
Bontrager | .45mm | .6mm | .9mm | 4.1mm/1.2mm |
Kenda | .75mm | .9mm | 3.5mm/1mm | |
QBP/Dimension | .73mm | .9mm | ||
Michelin | .7mm | .9mm |
*thorn resistant tubes are thicker on the tire side, thinner on the rim side
Back to the original point that brought this up. Is it possible that tubes held air better 50 years ago? The answer is yes - the tubes had less natural rubber in them because of the shortage of rubber during and after WWII. As well, since the tube wouldn’t stretch as much (because it lacked natural rubber), they were made to fit the inside of the tire very close. This maintained the thickness of the tube, helping them retain air.
Finally, here's a neat YouTube video from the TV show 'How It's Made' showing how tubes are manufactured.
So what can we take away from all this?
There are slight differences in the exact formulas used for inner tubes, and that can affect it’s cost and its ability to hold air. However, with most U.S. tube suppliers, it's more likely that the reason one tube holds air longer than another is a result of the thickness of the tube after it stretches to fit a tire. Pick a tube that fits the tire better (so it stretches less) and/or a thicker, heavier tube to ensure that a tube holds air pressure longer.
note: this is a (slightly) updated post from my other blog: thebikehive.blogspot.com
No comments:
Post a Comment