I thought building a bike would be much easier but I'm getting more and more lost. The drivetrain is what's confusing me so much. So I contacted titus (Ihave a titus motolite) about bottom bracket dimensions and they said the BB I buy should be 73mm and that spindle length doesn't matter. But then I was reading that you need a certain length spindle to have a proper chainline. Can anyone clarify...can I just use whatever spindle length I want?

If you buy one of the new external bearing 2 piece cranksets for mountain bikes ,
get the whole set and have it put it on, by a bike shop.
they make your choices simpler by eliminating them.

so what's the difference between two piece cranksets and traditional? And suppose I do buy a 73mm BB will spindle length effect the chainline? I want to try and avoid having a shop do all this just because I enjoy doing it all myself. The only thing I'm gonna have the shop do is install the fork but if the drivetrain is a major pain I may have to have them do it. But I haven't thrown in the towel just yet. I'm trying to get a grasp on all the specifics of the drivetrain, it seems pretty complicated though.

what crank do you want? you can fit pretty much anything in a 73mm bb shell

Yeah my only concern is if it will throw off the chainline. Basically I'm just wondering if spindle length is important to the operation of the drivetrain or if it's purely just personal preference. I know the BB length is important depending on the BB shell on the frame but I'm not sure if a specific length spindle is required in order for the drivetrain to work efficiently. I'd imagine you'd want a chain that's perfectly straight going from the cassette to the crankset rather than being slightly offset which I would thing would be inefficient. Again I'm clueless about bicycle mechanics so I'm just assuming a certain spindle length may be required for it to work properly.

To sum it up:

-BB length has to match the BB shell length
-BB threading must match the threading in the shell

Things I'm unsure of:

-BB spindle length is personal preference?
-Crankset has to fit the BB but size doesn't matter?
-Any cassette will do?

3 piece sets , what crank you buy dictates the axle length it uses
low profile means the arm is forged in a shape that comes in to meet a shorter spindle, the older rightangle cranks use a longer spindle to do thhe same job because of the arm profile.

they both can result in the same chainline ,
.. distance from the Center line thru the frame.
but they must be considered in the context of each other..


2 piece cranks , by definition, spindle and right crank arm are one piece,
the left crank arm is the second piece,

the external bearing cups thread into the frame.

the spindle, attached to the right crank and the chainrings,
passes thru the bearings, and is secured by the left crankarm.

as you say going to one of the many Bike shops in the Chicago metro area,
buying the appropriate crank from them,
and having them put it in for you will be a more satisfactory result.

i think you have a lot more reading to do.

It can be confusing but, all due respect to advice above about going to bike shop, you can figure it out and the best way to do that is to do it yourself, which, in my experience, sometimes means making mistakes. Not sure I understand the point of this forum if novices are supposed to go to bike shops and not ask any questions. You just need to remember, as Fietsbob notes, its the crank that dictates the spindle, not the bottom bracket. If you are going with three piece bottom bracket, not only will the crankset tell you what spindle length to use (crankset manual will have that info) but also the type of spindle interface (i.e. part of the spindle that physicaly interfaces with the crank) of which there a few variations depending make of crank and bb (e.g. square taper, octalink, ISIS splined). Going with a crank with integrated spindle and external bearing BB makes things less complicated since the spindle is built into the crankset so you cant get one that isn't a match.

Yeah I respect his advice, but I'm the type that likes to do things myself because then I learn how everything works. BB spindle length was my initial question but now it seems I need to read up on two piece, three piece, etc. cranksets.

I do have a book for all this but I don't think it covers enough. It's called: Bicycling Magazine's Complete Guide to Bicycle Maintenance and Repair for Road and Mountain Bikes

Try doing some searches on www.parktool.com (lots of pictures to go along with step by step instructions for just about every maintenance issue) , Parktools' Big Blue Book of Bicycle Repair, and Zinn's "ARt of Bicycle maintenance" books (he has one for Road bikes, one for mountain bikes).

edited to add: Also there is this https://www.sheldonbrown.com/bbsize.html. Lots of info on 3 piece bbs; although some info may be dated since I am not sure anyone has been keeping info updated since Sheldon Brown died.