Misc bottom backet and crankset questions I am planning to upgrade a few of my bikes .. some due to faulty BBs and some just to upgrade the crankset. Here are the questions I have:

1) Road vs MTB

I read from the another post about Road vs MTB Hollowtech that the BB and Cranksets gotta be the same category.


What about other types of BBs like square tapered/Octalink? 
Is it universal for both MTB and Road?


2) BB spindle width and crankset compatibility

Was browsing some online stores for cranksets, and saw a few that mention that the particular crankset is compatible with spindle width 103-113. 


Why is there this spec?  
What is the difference?


My issue is that 2 of my foldies have BB width of 125 and 127mm .. 


So what bad things could happen if I used a crankset designed for narrower BBs? 
Or for the reverse?


3) Tolerance for difference in BB width


Assuming I cannot can a BB same as the old one, what is usually a safe tolerance range + or - from the existing?
Is it better to get one that's longer or shorter?


4) Changing from square tapered to hollowtech


Hollowtech BBs don't have a spindle width specified. What do I need check if I am converting from square to hollow?
Can I convert my foldies to hollowtech? Given their BB is at the wider end of 125mm?


5) Changing from square tapered to Octalink


what is the width that I'll need to add or subtract if I want to convert from square to Octalink?


6) Changing from to Truvativ's Powerspline to Octalink or Hollowtech

I've got a road bike from 2006/7 that is using Powerspline .. which is a dying/dead product so am thinking of converting. 


what is the width that I'll need to add or subtract if I want to convert from square to Octalink?


7) Adjustments for change in Chainring/Rear Cog

Do I need to change BB with a different spindle width if:


Change the crankset between 1/2/3 chainring?
Change the rear from a 6sp to a 9sp? Thought I remember that I'll change different hub size if I want to change to a cassette of different speed.


8) Ceramic Ball bearings


I've seen some BBs using ceramic bearings. What are the advantage of ceramics?
That's a lot of questions but I think I can answer most of them with a couple of answers.

With respect to cartridge bottom brackets and cranks (Square taper, Octalink etc.):


The cartridge need to be appropriate for the frame bottom bracket shell width (traditionally 68mm for road bike and 73mm for mountain)
The axle length need to be correct for the specific crank you are using to get the correct chain line. It's critical to know that different model cranks need different length axles to get the same chainline, so you need to look up the specifications for the crank to find out the bottom bracket axle length required. 


With respect to Hollowtech II (and other external threaded bearing systems like GXP), things are a bit simpler, because the drive side crank comes with the axle attached. The axle length is just whatever is required for the bottom bracket shell width the crank is designed for. You do have to pick a crank that gives the correct chainline for your frame of course.

With respect to deviation from the correct chainline if you use a different cartridge BB than the crank is designed for, I would not recommend being more than a few millimeters out. If the chainrings are to far inboard you may get interference with the chainstay, too far outboard can result in derailleur reach problems.

If you are using Shimano products you can look up the specifications for cranks, which includes nominal chainline the bottom bracket models that work. Start here: https://productinfo.shimano.com/#/spec.