Hi,

coming from a cup/cone style BB with a square taper, i see that there are a variety of spindle length available with the UN300 BB from Shimano. Given that I will be using a 2x7 setup, how impactful is the chainline really? As long as i don't hit the chainstays with the crank and the middle of the cassette is somewhat close to the middle of the front chainrings will there be any real, tangible issues? I do not race, like to climb hills and use DT friction shifting.

Thanks !
P!

Get it off too far the and the FDER won't be in the correct location.

The point of a good chain line is that every thing functions and you run the straightest chain possible for the gears you use most.
That may not necessarily be the "perfect" chainline.
I tend to use the middle ring and outer cogs the most.
Therefore, I use a "slightly" longer spindle if convenient, because sometimes you use what you have handy.

Since you currently have a serviceable BB, why not tell us what the "code" printed on the spindle?
It should be something like 3-P or other letter.
Then go to Sheldon to see the symmetrical equivalent.
https://www.sheldonbrown.com/bbsize.html

Thanks - i would tell you the spindle length, but currently waiting on a pin spanner to remove the NDS cup.
It should be a JIS taper.
FDER? Front Derailleur?

With the wider chains and cog to cog spacing that 7 speed systems have the chainline being spot on is a lesser need, than with the current 10+ systems. Still, aiming for the center is a good goal and the minor unavoidable tolerance will be OK.

Not mentioned is the chain drop that has motivated so much 1 by chainring tooth design and cage pivot clutches. Some will leave the ft der on the bike just to act as a guide and help prevent chain drop. Back in the day ring teeth had a higher peak, engaged the chain link deeper, than is current with rings designed to shift (as in a 2 by). So if you plan to just use one of the 2X rings and a rear der without a pivot clutch be aware that you could easily loose a chain at the wrong moment. Andy

thanks. I will be using both my front chainrings, so my FD is not just to keep the chain on but to enable shifting.
Since I've heard terrible things about the Shimano UN300 square taper BB, what are alternatives that last longer than just 500km, if I were to change the BB?

Short answer, it's moderately important, but not critical.

Given that Derailleur bikes obviously can manage with poor chain line, then it's more about degree and the combinations you prefer. For example, if you tend to ride the outer half of the cassette most of the time, outboard chainline might actually be an improvement, while inboard could be problematic.

So, this is about considering the specifics and using some judgement, rather than painting by numbers.

The easy answer, though not cheap, is Phil Wood. You can get exactly the chainline (and left crank location) you want, they last near forever and can be re-built or changed for other bikes and applications. There are Phil Wood BBs from the '70s that are still going and Phil Wood still backs and services them.