Chains get worn, if it's really old get a new one, or clean and lube it if it's filthy. If the chain is slack, you can increase the tension by moving the back wheel in the frame ends.
@Skülly started
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Chains get worn, if it's really old get a new one, or clean and lube it if it's filthy. If the chain is slack, you can increase the tension by moving the back wheel in the frame ends.