When should I tension my bike chain?
The chain should be tight enough that it only allows you to move it up and down about half an inch. If there is no slack in the chain then it is too tight. And if there is too much slack then you need to tighten that chain.
What is chain tension?
A tight chain causes additional load on the chain which increases chain wear. Excessive slack causes chain vibration increasing chain fatigue. Proper chain tension is obtained by adjusting the chain. sag on the slack side of the drive.
What does B-Tension do?
B-Tension is an adjustment using that mysterious third screw on the back of your rear derailleur. Its proper adjustment is the final step to clean and quiet shifting. It also provides a degree of chain tension assuming: Chain is properly installed and the right length.