Bicycle Mechanics - Bottom Bracket Question....PLEASE HELPPPP!!!

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austingoodson
08-11-10, 08:00 AM
Hi!

I'm converting a 1978 Raleigh Rampar R-1 into a fixed gear. I replaced the old 3 ring crankset with a new single ring Eighthinch crankset. I tried replacing the old spindle with a new one with the same size. Its too long for the single ring crank! The cog and crank do not align accordingly...therefore, my crank needs to be closer to the frame, ie, i need a shorter BB spindle.

I've read online to measure the back dropouts which will tell the correct BB spindle width, so the cog and crank will be aligned. The back dropouts measure 130mm, which I've read to need a 110mm BB spindle. I know the BB width is 68mm so I need a 68 x 110mm BB...IS THIS RIGHT???

This has become a pain because every bike has different lengths, threads, etc....I just wanna ride! If you can help, please inform! Thanks a lot!

Torchy McFlux
08-11-10, 08:40 AM
I know the BB width is 68mm so I need a 68 x 110mm BB...IS THIS RIGHT???
Yes.

(Eighthinch recommends a 107-110mm BB spindle right on the product's webpage (http://www.eighthinch.com/cranks.html).)
FBinNY
08-11-10, 08:42 AM
I don't know why you started a new thread, when folks have been trying to help, but I'll try again.

Let's clear some brush so we can get at an answer for you.

First of all, determine what chain-line you need. Measure from the inside of the right dropout to your single rear sprocket, and subtract that from 65mm (half the 130mm width) to get the chainline distance from the centerline.

Now, there are two ways to approach the crank centerline. You can measure what you have, (measure from the chainring tooth to a frame tube, and add 1/2 the diameter) and determine how much shorter a spindle you need. Then go to this reference (http://www.sheldonbrown.com/bbsize.html) to see what the nearest choice would be. Note that you're concerned with the right side end to bearing distance, not overall length. For example if you have a square taper 3S spindle and it's 4mm too long, you'd replace with a 3K spindle, or the nearest possible choice.

Or you can look up the crank specs, and they should tell you the suggested spindle and centerline. Note that the suggested c-line spec's for the crank may not be appropriate for your bike if your rear sprocket is on a different c-line, so you may have to make an adjustment, using the chart to find the right alternative.

Whatever calculations you do, it's important to know where you need to end up, and where you're starting from, so you know how to get there, and can adjust accordingly. Anything other than that is working blind.