I’ve been having trouble with a wheel causing slow flats.

My son rode the bike for a few hours and it was fine. The next morning the tire was flat. This is the second time this has happened. I took the tube off, inflated it and did the usual test with water. There were no leaks. But I was taking any chances, so I took mounted the tire with a new tube on the wheel. I inflated to 50 psi – within ½ hour the pressure had dropped to 40 psi.

I took that tire and tube off the wheel and mounted them both on a different wheel. After 24 hours there are no leaks. Therefore it must be the wheel.

To test the theory that it is the wheel causing the flats, I took a different tire and another brand new tube and mounted them on the troublesome wheel and inflated to 50 psi. After 12 hours the psi dropped to 40.

What could be causing the wheel to make these flats? What kinds of things can I look for in the wheel to locate the issue?

The wheel is a steel 26" one off of an old mountain bike.

Thanks,
Dave

You need to try your tube in water test again. Make sure you get some pressure in the tube for the test (2x or 3x "normal" size. Are these ordinary tubes or puncture resistant tubes? Schrader or Presta?

A common type of flat would be radial tire wire. They are small wires that are almost invisible. Finding the flat with water, then looking for it in the tire (marked valve location), can help. Also, identify whether the flat is on the tire side or rim side.

If you think it is the rim, then carefully inspect the rim tape.

Take a cotton ball and run it long the inside of the tire. If there is a foreign object, like the wire mentioned above, it will snag on the cotton ball. Then you can remove it. If the cotton ball doesn't reveal anything, make sure the rim strip is intact, and no spokes are poking through from the rim to puncture the tube.

Probably obvious, but rub that cotton ball along the rimtape and the sides of the rim too. Soap in the water can make it easier to spot the bubbles, and don't forget to check the valve as well.

Something doesn't add up.

The tube only leaks when installed on that specific wheel? But the same tube, after leaking, doesn't leak in a different wheel?

If the first wheel is puncturing the tube, then the tube is punctured. The puncture should show up in the water test and shouldn't miraculously heal itself when you install the tube on a different wheel.

Are you mounting it with the same tire on the other wheel? Or are you using a wider tire on the second wheel? If the leak happens with a thinner tire maybe the tube is simply too big for that tire.

I'd also question the valve stem. In the 'bad wheel' is the valve stem pinched or sitting at an angle or otherwise not fitting properly through the hole? While in the good wheel the valve stem fits cleaning and sticks straight up? I've seen tubes where the puncture is around or on the valve stem, and so small that it's fine and doesn't leak at all as long as there's no twisting or bending or pressure on the valve stem. But as soon as the valve stem is bent a little, the tiny hole opens up and the air leaks. A sharp edge around the valve hole in the wheel could cause that.

If neither of those....I dunno....dark magic curse? Have you angered any witches lately?

Turning the tire inside-out will put the inner surface under tension and help open up any small holes where wires or bits of glass may be hiding, making them easier to detect with the cotton ball or my current favorite, a microfiber cloth. The cloth is also larger and will cover an area quicker.

Here's the thing. It's the inner tube that holds the air pressure - not the tire and not the rim. Something in the tire or the rim may be causing a puncture in the inner tube but its the tube alone that holds the air.

You haven't found where the air is leaking from yet. Finding that will greatly help your search for what's causing the loss of air pressure.

Thanks for all the great ideas. I'll try inspecting the wheel more closely and see what I find.