Heat them with a hair dryer to soften the glue and they should peel off pretty readily. Clean up any remaining glue with Isopropyl Alcohol (IPA) then have an Indian Pale Ale (also abbreviated to IPA) beer; it is very important not to mix up these two materials in the second stage!
If they're new-ish they'll just peel right off, don't even need to heat them. Usually there's no glue residue left over either, and if there is, you can roll it off with your finger, don't even need alcohol or anything.
Most newer decals that have not had much time spent in the sun drying out will peel right off. You may have to find a certain angle so the glue stays with the stickers, but they come right off.
Generally, heating them up too much can cause the glue to stay on the wheel or whatever surface they are stuck to because the plastic/vinyl heats up faster than the metal surface they have been applied to. This causes the glue to separate from the material. There is a fine line between warming them and heating them up hot. Hot decal = messy. Cold-mildly warm decals pull off the best.
Not all decals are created equal, but majority of Bike wheel decals are of similar quality and pull off with no special tools. Just pull slow. You rip them off and glue will stay put...not a fun clean up.
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