The Schwinn Jaguar is a Chinese made bike. I've welded on them using both TIG and MIG. I've actually been impressed with the purity of the steel used in these bikes. If the steel is bad, or dirty, believe me, you'll know it when you try to use TIG. My experience is limited to the Schwinn bikes so I can't speak for the others. As for welding aluminum either with gas or electric, it is a process that requires some practice and not for the beginner. If the weld is at a structural point, let a professional do it unless you have mastered the art of aluminum welding.
Tom

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Age and Treachery Will Always Triumph
Over Youth and Skill