butyl inner tubes

[handlebar]

Foreseeing war shortages, German companies discovered nitrile rubber in 1934, and American companies discovered butyl. Shortly after Pearl Harbor, the Japanese seized the East Indies, source of 90% of America's rubber.

Butyl was so much better for inner tubes that America didn't go back to latex after the war. It's impermeable to air and doesn't age from exposure to oxygen. (Dunlop recommends replacing latex inner tubes every 6 months.)

BMW Motorcycles went back to latex when the R75/5 came out in 1969. Consumers often failed to check tire pressure and often rode over 100 mph. With an underinflated tire, friction could heat a butyl tube at that speed until it gave way in a blowout. Latex flexed with less friction. It wouldn't heat up as much, and if it did let go, it would release air more slowly.

These latex inner tubes were porous. I didn't have to check with a gauge because I'd know in 50 feet if either tire was 3 psi low. American bicycle tubes were going back to latex about the same time. More and more, bicyclists were buying helmets and bending way over to try to keep up with cars on bikes with narrow tires. Even with higher pressure, a smaller footprint could mean more flexing, producing heat for a smaller tube to absorb, and higher speeds would generate the heat faster. For a rider with his head down and forward, a blowout could be dangerous, so latex was preferred.

After a year of adding air to my Radrunner tires, I decided that the tubes were porous. It didn't occur to me that a 20-mph utility bike with tires as big as motorcycle tires would have latex tubes; I just thought they must be cheap. I installed butyl tubes. That reduced losses by 90%, but they still leaked and I didn't know why.

My Radmission tubes also lost air. I'd have to get out the pump every couple of days or settle for tires that were increasingly low. I hoped to seal them by adding a lot of Slime. It didn't work.

I find the chain stays on my Radmission too short. It means the back tire carries 40% more weight than the front. Today I tried 50 psi rear and 40 front. It's great. Now that I've found a good combination, I want to keep it that way. When I checked Amazon for butyl tubes, I found out why the butyl tubes I bought last time are a little porous. The description says they're made from "a butyl compound." I learned that most bicycle inner tubes these days are made from a mix of latex and butyl. "A butyl compound" must be a salesman's description of a mix.

For my Radmission, I've ordered tubes that just say "butyl." Maybe I'll finally have tires that hold air, like the old days.

[John Rose]

Well, thanks for all that.
I wonder if latex tubes influenced the adoption of nitrogen for filling tires. Larger molecules being slower to leak through, and non-oxidizing.

For my Radmission, I've ordered tubes that just say "butyl."

What about "May or may not contain nuts"? 

[handlebar]

Well, thanks for all that.
I wonder if latex tubes influenced the adoption of nitrogen for filling tires. Larger molecules being slower to leak through, and non-oxidizing.

For my Radmission, I've ordered tubes that just say "butyl."

What about "May or may not contain nuts"? 

The NHTSA tested and found the loss difference to be insignificant. So did Consumer Reports. They pumped up 31 pairs of tires, 1 with nitrogen and 1 with air. After a year, the nitrogen tires had lost 2.2 psi on average and the air tires 3.5. (Tubeless tires have a butyl coating.)

Recent research suggests that the difference may be due not to permeation through the rubber but to oxidation with the rubber. With 18-wheeler truck tires, the pressure loss with air is fastest when the tire is new, and so is the deterioration of the strength of the casing; if you buy a tire that was recapped after a year, it won't lose as much pressure and it will hold it's remaining strength better. They conclude that the oxygen is not passing through the rubber but "rotting" the inside surface. Nitrogen could pay off by preserving the casing for more recaps.

Airplanes started using nitrogen because it wouldn't support combustion; when you retract your landing gear to head across the ocean with 400 passengers, you want to be sure a tire isn't smoldering from overheated brakes. Airlines noticed that with nitrogen, the pressure difference would be smaller when you took off from a place with one temperature and landed in another temperature.

That's why race cars started using nitrogen, to reduce the pressure change between cold and hot tires. They said it was because nitrogen is dry. I doubt it. To cause a disproportionate pressure increase as a tire warms, there must be liquid water when the tire is cold. My compressor tank sits between 125 and 150 psi at 100% relative humidity; excess water condenses and goes to the bottom of the tank. If a racing tire is inflated to 10 psi, air from the compressor would be around 8% relative humidity. The moisture won't condense in the tire.

I think it comes from off-gassing. It will cause a strong odor if you deflate a new tire in the garage, and race cars use new tires.  As the tire heats, more of those gasses will come out of the rubber, and they'll be reabsorbed as the rubber cools. When you fill a tire with nitrogen, you do it several times in order to purge the gases in the tire. That could reduce subsequent off-gassing. You could purge just as well with compressor air.

Have you seen what dealers charge for putting nitrogen in tires? It causes inflationary pressure!

Oh yeah, the tubes I bought seem to be identical to the unsatisfactory ones I replaced. I guess "butyl" doesn't always mean "butyl." Latex forms tiny bubbles as inner tubes bake in a mold. That makes the tubes spongy. Some companies control the bubbling better than others, but the best way is to start with 100% butyl.

[JimInPT]

What about "May or may not contain nuts"? 

If you read Ryan's ToS carefully, you'll find that's a blanket disclaimer for the entire forum.