The spring on the back is an add-on, Birdies use a simple elastomer, which absorbs shock, and is less bouncy. Your chain tensioner is bent and deformed, it should be perfectly straight. When the bike is in the highest gear, the tensioner should be just above the chain. As you change to lower gears, it moves up, and farther from the chain. Your bike is also missing the retainer which prevents the chain from slipping off the front chain ring. A new rear tensioner costs about $10, as does a rear elastomer. High performance elastomers are about $40, a good shock a little or a lot more, depending on how many features you want. But overall the bike is good, though it won't ride as well as it could without the rear suspension sorted out.