question re spindle length of bottom brackets

[Jim S.]

Hi all, 

I had a question about bottom-bracket spindle length.

I recently put a bike together, and I mostly succeeded. But I failed when I got to the front derailer. I just can't get the bike to shift to the large chain ring.

After a few days of fiddling around, (I read articles, I watched the Riv videos, I watched the Calvin Jones' videos from Park Tools), I admitted defeat and took the bicycle to my local bike shop.  

He told me that I'm not setting it up wrong, it's not a matter of adjustment - I either need a different bottom bracket, or a different front derailer - he's not sure until he looks at it more carefully. 

I took some solace in the apparent cause of the problem - it isn't my mechanical ability - it was my parts selection! 

 

My question: what determines the appropriate spindle length for a given build. Is it the crankset? Is it the frame? Is it something else? I'm trying to figure out where I went wrong.

(Maybe it's not the spindle length - maybe it's the front derailer I chose, although I don't think so.)

Thanks for any information.

[Bill Lindsay]

Jim S asked: "My question: what determines the appropriate spindle length for a given build. Is it the crankset? Is it the frame? Is it something else?  "

It's the chainline.  Google "Sheldon Brown Chainline" and read that thoroughly.  From what your read you should have a pretty good guess at what your chainline should be.  You will definitely know how to measure your chainline.  If the chainline you have is a lot larger than what it should be, then you chose the wrong BB spindle, and you'll have a good guess at what you should be using.  If your chainline is good, then you likely picked the wrong front der.  

Bill Lindsay

El Cerrito, CA

 

[Jim S.]

Thanks Bill. I was hoping you'd respond. You're one of my favorite posters on this board. You write like a lawyer. That's intended as a compliment. (I mean a good lawyer who hones in on the issue and answers it directly. Not a an airbag who bills by the word.)

[Garth]

I'd like to know Jim if the FD on it's own, unconnected from the cable, adjust so that it can travel as far out as possible, can travel to where it needs to be over the large ring.

   It would of course, be helpful to know all the details, the crank model, BB model and length, frame model, FD model and shifter model !

[James Simeri]

I can’t get it to move far enough, although this wasn’t obvious to me for a while. When I disconnect the cable, and pull it towards me by hand, the outer edge goes over the chain ring, but no farther. But until I went to the shop, it wasn’t clear to me whether I couldn’t somehow correct this by doing something with the limit screws.

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