Originally Posted by
Gasman1440
I don’t think it has anything to do with being an engineer. I wanted to learn about why there is a range for how much slack and what if you dont have enough slack. The point was to learn about bikes then decide if i had to buy another chain.
Simple, you rip the derailleur off the bike. Look, this is not rocket surgery. If the pulley cage is pulled so far forward by the chain that you can't push it any further by hand, the chain is too short. If the chain is sagging in the small/small or the on the smallest cog of a 1X then it's too long. If your chain sags and is also tight your drivetrain is over the capacity spec of the rear derailleur.
For road bikes you need to have enough capacity so that you can use a range of cassette sizes. Smaller ones for flat days, bigger ones for climbing days. For modern mountain bikes there really isn't a range as there is a standard cassette that all drivetrains use, but you have to have some leeway for suspension movement.