I was fixing my son's mountain bike up... I'm pretty good at bicycle repairs and such having build several over the years from frame up.
But I am stuck.
Some modern shift systems have a different derailleur ratio. His bike had SRAM grip shift on it, so I ordered some replacements for them, same ones I figured.
Turns out maybe not so.
How can I determine the shift ratio of the derailleur on the bike to determine if I need to replace it as well, or if the cluster is the issue. It skips gears up, or down depending on how I tension it.