Adjusting drive chain slack is a very important maintenance work, but it's rarely done properly.
Why does the chain need to have slack at all?
As the rear suspension compresses, the drive shaft and the rear axle move away from each other.
A chain that's too tight could cause:
Great force on the chain itself . chain can brake engine cases or worse, snap into the rider leg and cause injury.
Extensive wear of the sprockets and the drive shaft bearings.
Extensive chain slider wear.
A Chain that's too loose could cause:
The rule of thumb is to adjust anywhere between 35-50 mm of slack. But where do you measure this distance? And is the slack the same for each bike? (no)
There is one way to make it PERFECT. However, it’s somewhat time consuming:
Put the bike on central stand.
Remove rear shock.
Move the swing-arm up and down and look for the points
Adjust chain to almost zero slack (as it cannot get any tighter)
Re install rear shock
Now you have the PERFECT slack!
When we designed our “chain adjustment block” we did this process on each bike model, and when we were done, that was the point where we modeled the blocks. Using the block is equal of doing the process described above, but without the hassle.