PedallingATX

Hello All-

I have been Googling info about bottom brackets and cranks for the past few days and kept coming on this page, so I figured I would go ahead and join so that I could just ask y'all directly for info. It appears this question has been asked before, but I kept finding conflicting information. So, here it goes...

I bought a Shimano UN54 70mm (italian) 107mm spindle width bottom bracket for use on this Bianchi frame I got. After reading up on sheldon and other places, I decided that the 107mm bottom bracket would surely work with a variety of 46t cranks/chainrings. I took my rig to a bike shop today (although this bike shop has a BAD reputation) and they told me that I would need a longer bb to run a fixed gear crank/chainring on. is 107 mm wide enough? The type of cranks/chainrings i am looking at are all 46 tooth chainrings (Origin8, Sugino, etc)

So, will I have problems running a 46t chainring on a 107mm width bb on an old Bianchi road frame?

ALSO, I just realized that I put two Ls in PedallingATX. *bows head in shame* Not all people from TX are as stupid as me, I promise.

PedallingATX

oh, and also, what are your thoughts on spray painting a crank black. Obviously, this isn't optimal, but would it work OK?

creejoh

The crankset you buy determines the bottom bracket spindle length you need to buy.

Spraypainting it would work fine (but I would advise against it) as long as you masked everything off that would be in contact with something else, but it wouldnt last long and would probably look pretty bad.

Just get a black crankset. I think Pake's crankset is for a 107mm BB spindle and it comes in black. Make sure the square taper matches too though.

Someone correct me if I am missing anything...

Nightdiver

^This is all correct and useful info. There are several black cranks available, so there shouldn't be a reason for you to spray paint a set.

It sounds like you're not too familiar with bike components yet, so a few good places to start reading would be the beginner's sticky, and these two pages: 1 & 2, although I think you'll find all of Sheldon's site to be a wealth of knowledge.

For your situation, one thing to note is that cranksets are usually comprised of a set of crank arms (which will be the determining factor when choosing a bottom bracket), and a chainring (no influence on bottom bracket choice). So, don't let the chainring's size (46t, 48t, etc.) confuse you when checking for crank and bottom bracket compatibility.

I think it's safe to say that most people start with the crankset purchase, as this is usually the more expensive and personal choice, and let that purchase determine which bottom bracket they get. The steps I would recommend you take while shopping are:

1. Choose a crankset you like.
2. Find out what sort of bottom bracket interface that crankset requires (JIS, ISO, ISIS, Octalink, etc.)
3. Find out what length bottom bracket will give you the ideal chainline for your setup (consider here your crankset and hub/wheel setup)
4. Make sure they make that combination of interface and length bottom bracket with the correct threading for your bike's bottom bracket shell (in your case Italian)
5. If so, go for it. If not, start back at #1.

Hope this helps. The whole crankset/bottom bracket/chainline thing is a tricky bit to sort out.

Good luck.

operator

You're confused, chainring since have almost no relevance to "what bottom bracket works". This is dictated by the particular crank you have chosen. Each crank is usually spec'd with a single bb length that gives you xyz chainline.

If you happen to be running a converted double crank, or are going to then you may have a problem where the chainring or crankarm spider strikes the chainstay. In which case, you'll need a longer bb. With proper track cranks and the spec'ed bb you should almost never have this problem.

Here's a set of black track cranks:

PedallingATX

If the chainring has too many teeth, wouldn't it rub against the chain stays?? It makes sense to me that the more teeth it has, the bigger the diameter of the chainring, and the more likely it will hit the chainstays. A bb with a small spindle length (107mm) does not give as much clearance for a chain ring to clear the chain stays. Someone please tell me how this is not right and how the amount of teeth/spindle length isn't going to affect whether or not the chain ring clears the chain stays...

operator

You effectively just told us you read none of the above replies. Especially mine. Pretend you are us, are we just magically supposed to know what kind of frame/clearances/crank/chainline you're running?

PedallingATX

yeah, you're right. I posted that reply before I read your response. I must have missed it somehow. Anyways, I took an old double crank and took off the bigger chainring, so I am running a 40 tooth chainring in front and a 13 tooth cog. Friends who ride fixies/ss have told me that for austin, you want about a 3 gear ratio. I think it's kind of funny that mine is 40/13, but it works with the small bottom bracket and it gives roughly the same ratio as a 48/16. Would there be any potential problems from running such a small combination of chainring/cog?

soul05

i've been using a 107mm bb with bulletproof cranks on my beater with no problem

jtarver

Come by my house or go to East Side Pedal Pushers. It's easier to work out chainline issues in person.