How to determine length of BB needed?
 

I'm putting together a commuter for myself that needs a new BB. Where I'm buying them there are lots of options in terms of spindle length.

The BB is for a Surly cross check, which has a 68mm wide, standard English threaded 1.37˝ x 24t . First off, what's the 1.37 x 24t mean? Is that the thread designation (another way of saying it's an English thread?)

The BB I'm looking at comes in various sizes. Obviously if you wanted to put a triple on the bike you'd need a slightly longer BB. I'm putting a 1x setup on this bike with a shimano Nexus 8 speed. The BB comes in widths of 107mm all of the way to 122.5mm, what's the correct way of choosing this? Obviously certain people want/like a certain Q factor, so they could choose for that reason, but what's the "actual" way of choosing one of these? I'd imagine I'd want to choose the right length to make a nice straight chain line to the cog on the nexus, but... uh... how do I come up with that info?

Thanks! I've never had to buy a BB myself before. My new bikes haven't needed a new ones yet and my old bikes are all cups and cones.

Identify the crankset you intend to install and buy the BB length specified for that crankset.

Id the crank, then look up the manufactures specification?

For the 1.37 x 24t, that's the thread spec, but in bike terms, it known as an English thread, You won't hear it talked about in day to day usage as a 1.37 x24t unless you frame building/designing/manufacturing BB's.

The simplest thing if looking at the crank is to get an external cup BB (i.e. HT2, MegaExo or GXP), then you don't even need to think about the spindle length, as it's fixed.

It has nothing to do with the chainline? Or is it a mix of crankset position (therefore chainring position) + spindle length gives a particular chain line?

I don't want to do the simplest thing, I want to learn how to do the thing right :) Besides, I've already had the internal vs. external debate, I'm going internal BB, most likely a simple, cheap shimano UN-55.

So if I look up the nexus, it specifies a "chainline" of 42. I'm assuming that's 42 mm, so that means I need to find a crankset + BB length that puts the chainring 42mm from the center of the BB... right? Or am I way off base here?

Well, the rightest thing to do is to make your best judgment then install said BB. Now you can check the actual chainline and other dimensions (like crank arm or ring clearance with frame or "Q" factor) and if needed remove the tester BB and install the now "corrected" size. Anything else is just assuming. Andy.

Thanks Andy. I'm finding this problem is a bit of a puzzle. Almost like building a computer, all of the parts have to fit together correctly.

I KNOW I want the nexus 8 which has a chainline of 42mm. That means I'd like to find a crankset that can also provide a chainline of 42mm. That's proving... difficult. I saw one that said it had a 45mm chainline with a 103mm BB! I didn't know they MADE 103mm BBs. And I'd have to go even smaller if I wanted a 42mm chainline. If the chainring mounts to the inside of the crank, I can use spacers, but I'd prefer not to as they just provide complication (and more of an opportunity to creak.) Puzzle indeed.

Getting a 42mm chainline is a surprisingly tricky task. The problem is that most cranksets are made for either two or three rings with an expected chainline of 43.5 (meaning the inside ring is at 41 and the outside ring is at 46) or 45 (the position of the middle ring). So you either have to accept an imperfect chainline (41mm really is close enough) or tinker -- or find a crankset that is made to have a 42mm chainline with a single ring (most common with track cranksets).

The math is really simple though. Start by finding out what bottom bracket the crankset manufacturer recommends (not always an easy task) and what chainline they expect that to give you. Then make adjustments (different spindle length or spacers somewhere).

The canonical reference: All About Bicycle Chainline

Buy several lengths and experiment..

But in today's age of interweb info why can't we just ask and get the right answer??? Really I suggested much the same. But there might be other variables too. Like does the hub's cog have dish or spaces to change the chain line? Or which kind of chain will be used? Der chain is FAR more agreeable with minor chainline offness then 1/2x1/8 single speed chain is. Andy.

If you have any old bb's throw one in, install your drive side crank and go from there? just remember to double the difference (if 2mm out buy one 4mm wider/narrower) when choosing your bb

There's a way to eliminate the uncertainty. You need to have a crankset before you choose a BB. (You can accept the manufacturer's recommended BB length for the crankset, or you can measure the crankset and be sure.)

Your chain line is indeed 42 mm, dictated by your Nexus IGH. It's the distance from the middle of your OLD to the cog.

On your crankset, measure the distance from the chain ring to the bottom of the recess where the crank bolt goes. (The advertised BB length is the distance between the ends of the tapered portions. Those points are roughly where the crank bolts will bottom out.)

1. Draw a simple diagram of your BB shell and crank as viewed from above.

2. Show the distance from the middle of the shell to the chain ring as 42mm.

3. Add the short distance from the chain ring to the bottom of the bolt hole. You now have half the length of the BB spindle. Double that number, and that's the length BB you need to buy.

If you can't find out what spindle length the manufacturer supplies, it's simply guess-and-check. You can decide where you want the chainline and get a BB to make that happen.

By the way, the guess-and-check method requires only one bottom bracket to figure everything out. I keep a trashed one around for this purpose. This is the procedure:

1. Install the "test" BB (in my case an old junk one.)
2. Install the crank. No need to tighten it all the way, just enough so that it's firmly on the spindle.
3. Measure the chainline.
4. Use math to figure out how much spindle length you need to add/subtract to get the chainline you want. For example, if you want a 42mm chainline but your test BB gives you 45mm, you need a BB that's 6mm shorter than the test BB (since the length comes off each side.)

Yes.

Does having internal vs external bottom bracket cups change the numbers any? Say I went from an internal english threaded to an external?

It changes the numbers in that they become meaningless. External bottom brackets use completely different cranksets that generally have a big hollow spindle built-in to the drive-side arm. They are not adjustable for length like square taper.

Don't they allow slight chain line adjustments, using spacers, wave washers, etc.?

Not enough to really make a chainline difference of any merit. But to attain ring/arm and stay clearance, maybe. Andy.

This chart shows specs for Hollowtech and other external BB/crank systems: Sheldon Brown's Bottom Bracket Size Database
Looks like many of the Hollowtech II models list a 43.5 mm chain line. By playing with the spacers, can they be modified to 42 mm?

Ugh. I think I may have to deal with having a chainine that isn't perfect. I'm going to be using 3/32 chain, so it doesn't HAVE to be perfect, although I'd like it to be.

I'm just going to buy a 103mm BB and a 107mm BB and try all of the decent cranks on at the bike coop and see if one gives me a ring near enough to 42 that I can deal with it. Who knew building up a single speed crankset would be so... complicated.

EDIT: Hm, apparently you can flip the cog on the Nexus to get a 47-48mm chainline. That's MUCH more workable.