Can a bike tube lose pressure without having any leaks?

bobthurstonMember

I've never seen this before but I pumped my bike tires up to my customary pressure (85 psi with those tires) last night, calibrated this morning and got numbers in the expected range, but by late afternoon I had apparently lost lots of air pressure.

First indicator was calibration, which went from 11080/km at 9:30 (temp 77 F), to 11121 at 4:45 pm (temp 87 F). Second indicator was the feel of the tire, it felt pretty soft. I stopped at a bike shop and asked to use a pump with a gauge: the pressure had dropped to around 35-40 psi.

Just a slow leak, I assumed. But I took off the inner tube and tested it in water, and saw not a bubble.

What is going on here? Has this happened to anyone?

I should add that there are some extremely bumpy parts of the course I rode today, and I rode some of them faster than I should have (for control and safety I mean; while measuring I went slower).

8/31/139:11 PM

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pauladamsMember

One possibility is a small amount of debris in the valve which was blown out when you checked the tire pressure the second time.

Good thing we calibrate before and after.

9/1/1311:09 AM

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ronfitzpatrickMember

If you use some type of slime for puncture protection, it could be that the pressure was lost before the slime fully sealed the leak.

My Trek Ride+ (electric assist bicycle) came with green slime installed in both the front and rear tubes. It even seeped out the valve stem. Very messy. After discovering the slime, I replaced both tubes with thornproof tubes and added kevlar strips in both tires. Thornproof tubes have very thick rubber and stuffing them into the tire is a bit of a chore, but once it's done, it's almost like having a solid rubber tire. The only flats I've ever had came from failed valve stems.

9/3/134:08 AM

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bobthurstonMember

How is the ride on thornproof tubes, and how do they work out for measuring?

9/4/134:38 AM

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ronfitzpatrickMember

I don't notice any difference in the ride. I inflate to 100 psi, so the ride is pretty firm anyway. As for measuring, they work fine. I've used them for so long that it's hard to make a comparison.

Most of the cal courses that I use are 1000 ft long, and when I have about a 10 degree temperature change from pre-cal and post-cal, the difference in the average counts is usually 1 count or less.

I will say that you need to take care when installing the tube, espcially near the valve stem. The rubber at that location is very thick and makes it difficult to seat the tire bead. But, with a bit of effort it can be done.

I've read that cyclists in the southwest are familiar with thornproof tubes. Apparently at certain times of the year, their roads are littered with burrs from some roadside weed that can easily puncture lightweight high-pressure tires.

9/4/135:12 PM

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bobthurstonMember

That is intriguing, I'll look into those tubes. I wondered if they shared some of the characteristics of solid tires, which I used to use for measuring. I loved the fact that the calibration constant didn't change very much, but eventually I realized that they were not measuring well on rough surfaces. Instead of an error in the safe direction, mine were off in the other direction. On rough surfaces (I eventually concluded) I was registering more counts and thus laying out short courses.

9/4/138:15 PM

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