I had a flat tire and replaced the air chamber. While doing that the rubber stripe that stands between the wheel and the air chamber ripped and it's no longer a ring. It´s hard to put it correctly and I don't have a replacement.
What is the purpose of this stripe? is it needed? leaving there (badly fitted) is worse than not having the stripe at all?
I believe what you're describing is the "Rim Tape".
The rim tape covers up the holes in the rim (wheel) that the spokes attach through. Without that tape covering the holes, the innertube (air chamber) will be exposed to holes and sharp surfaces that are likely to cause another flat tire.
If the rim tape is torn in one spot, but still covering all of those holes, it should probably be fine. Maybe feel around that area with your fingers for any edges to be sure?
Can you see the holes in the rim when the tire and tube are not on it?
If it looks like the tube could press into the rim tape holes, you can always patch it. A few layers of electrical tape should do the trick.
Yep, it's "rim tape" or a "rim strip". The "tape" version is a bit like medical adhesive tape, while the "strip" version is generally a circular strip of rubber or stretchy plastic.
The purpose of the strip is to separate the "inner tube" (your "air chamber") from the spoke ends and the holes where they reside. The spoke ends are sharp and will cut through the inner tube in short order, and if the holes are deep then the air pressure in the inner tube will force a portion of the tube into the hole, stressing it to failure.
I've tried running without a rim strip and have never gotten more than about 10 miles before the tube punctures.
Replacements are available from any cycle shop for a nominal cost, or you can use any sort of reasonably robust tape. Do be aware that there are different widths, and a wide strip in a narrow wheel will interfere with proper tire mounting, while a narrow strip in a wide wheel may not adequately cover the spoke holes.
You can use fairly cheap substitutes: My preferred one is the very same old inner tube, cut to fit. Cut a stripe a bit wider than the rim, and it's length should allow to go completely around the rim. Use patching glue to close the loop. the loop should be tight enough so it keeps in place by it's own tension. To properly glue the ends of the rubber, use sand paper to buff the ends of the stripe and use more glue than normally used for patching. Apply glue to both ends, allow to touch-dry then press ends together and let sit 10 or 20 minutes before installing. Do not forget to cut a small hole for the valve stem. The stripe should have one side completely facing out (no twisting!). These expensive "Inner Tube Protectors" have served me way more years than the tires in all my mountain bikes, which I ride at 45psi approx.
Medical adhesive tape (the cloth-like variety) is also very suitable and affordable and easy to use, but I have never used it personally (Friends of mine use and recommend it too).
Vinyl used in advertising banners can be used too, but some varieties (most, actually) are not stretchy so the ends of the stripe must be glued while already in the rim. Vinyl can be glued with rubber cement.
Many kinds of cloth can be used, they must be like painter's canvas, soft denim, the kind of nylon used in back-packs. These can be also glued with rubber cement. Stretchy cloth can also be sewn with a soft thread.
For emergencies many kinds of tape can be used, like electrical tape, masking tape, etc. As long as it is soft enough to adapt to the rim's well and not have a sharp edge or wire reinforcement, etc... However, these solutions often are not long lasting as for example Masking tape is paper-like so it practically dissolves on water, electrical tape adhesive sooner or later degrades to a messy, sticky useless substance that will make a hassle every time you have to patch or replace tube/tire.
Moneywise anecdote: On a local store inner tube cost the equivalent to $7.5 and the rim tape $40.00 (they only had fancy stuff). The old tube I was about to replace had cost $4.0 and it had enough rubber for 3 "Inner Tube Protectors". I just needed patch glue, sand paper and scissors.
