A Mystery in Tremblant

There was a bit of a mystery after Ironman Mont Tremblant. When I realized I had incurred a flat, I changed the low pressure tire with a fresh 25mm Vitoria EVO CX tire. Not far down the road I realized that that tire had very low air pressure as well. I thought that I had another slow leak. In other words, I believed that I had incurred two flat tires in a single race, on separate tires. The odds of this are very low. For example, my last flat tire in a race occurred over a year ago and prior to that I raced for 4 years without ever incurring a flat in a race. This year, I have been very diligent about changing tires, and run a fresh back tire for just about every race. When I arrived at T2 in Mont Tremblant, my tire had very low pressure. When I went to get my bike out of transition around 5:30pm, the tire had no air in it whatsoever. When I got back to the condo I was curious to see where the hole was. I filled the tire with air and then filled the sink with water. I rotated the tire under the water to isolate where the hole was. Interestingly, there were no air bubbles coming out anywhere from the tire. I thought perhaps it was such a small hole I couldn’t see it, even when submerged under water. I left the tire out all night. The next morning I checked the tire and it still had a lot of air pressure. This didn’t make sense. If the tire had a slow leak, there should have at least been a bit of a change in pressure over night. There seemed to be a disconnect from what I experienced in the race, and what I was experiencing after the race. I thought perhaps the reason was because all my weight was on the tire during the race and the tire was rotating. That explanation didn’t sit well though, as there was no logical underpinning.

Fortunately, I received an email from someone today who watched the live video coverage. He informed me that the Vittoria EVO CX tire has a latex tube, and that the CO2 molecule actually will seep out of a latex tube much quicker than regular air. He told me that if I am going to use CO2 to fill a spare tire, then that spare tire should have a butyl tube (which isn’t as porous as latex, and thus doesn’t leak as fast). I had no idea that this process was at work. Being someone who often needs to see to believe, I did a little experiment today. I filled the back tire that I changed in Mont Tremblant with a full CO2 cartridge. In under 7 hours the tire lost over 80PSI. In other words, it appears that the Vittoria EVO CX tire, when filled with CO2, loses over 10PSI per hour.

I then learned something else today, and all the pieces started to fit together. The night before the race my dad asked what would happen if I emptied an entire 16oz CO2 cartridge into one of my tires. I told him the tire would explode. With hindsight, I have no idea where this false-knowledge came from. In my head, I thought there was probably 200PSI worth of CO2 in a cartridge. When I finally decided to change the tire in Mont Tremblant, this is the knowledge that was reverberating around in my head.

Today at my bike shop, as I was telling the story of what happened in the race, my bike mechanic Jamie asked me how much pressure I put into the second tire. I told him that I filled the tire until it was hard, then I stopped putting CO2 in as I didn’t want the tire to explode. He told me “You got to put the whole CO2 in.” I said, “But won’t the tire explode?” He said, “No. There’s only about 100PSI in one of those cartridges." Once again, being someone who needs to see to believe, I went home and emptied a 16oz CO2 cartridge into a 25mm Vitoria EVO CX tire. He was right. Pretty much dead on 100PSI was in the tire.

My mind was blown. 100PSI in a whole cartridge? In the race, I let some of the air out of the cartridge before putting it onto the wheel’s valve stem, mainly because it was the first time I had used that particular CO2 cartridge head. I then put about half of the CO2 cartridge into the tire because I thought the tire would explode if I put the whole thing in. Then, when unplugging the cartridge from the valve stem I let a bit of the air out.

It all makes perfect sense now. I probably got about 50PSI into the fresh tire. But, the tube was latex and I filled it with CO2, so it was likely leaking 10-12PSI per hour. I rode another 2 hours on the tire. This means as I was rolling into T2 I likely had around 20-30PSI left in the tire. And it explains why the tire was completely flat when I went to retrieve the bike from transition after the race.

Why did the tire not go flat when I aired it with my bike pump later that night at home? Because air has less than 1% CO2 in it, and thus CO2 leaving latex tubes filled with air is almost insignificant, at least during a race.

So there you go. Mystery solved. I think this is a great place to use the adage “you learn something new every day.” My words of advice: If you’re running 25mm tires, empty the whole CO2 cartridge into the tire! And if you’re running tubular, make sure your spare has butyl tubes!