Originally Posted by
draganm
well I don't own a shop, but i have built about a dozen bikes in the last 30 years, so not exactly a total newb either. It's fine to have an an opinion that it's an exact science, but just in this thread we have 2 different people who used the recommended BB spindle length, or longer, and wound up with a crank that was too far in or too far out. Just saying results don't lie,
well your analogy would be great if all bicycle parts were made by Mercedes, but they're not. We have Japanese, Various other Asian, American, and quite a few European companies all trying to maintain some kind of standard , both between older and newer generations of drive-trains, all the while constantly cooking up new "standards". The bottom bracket crank interface and number chain-rings being one area where some of the greatest changes have been made on bikes. I can remember in the last few decades on more than one occasion where there were changes made to what was even considered to be the "ideal chain-line".
It just seems like calling this an exact science is a bit of stretch.
In this thread we had two different people assemble mystery bikes out of unknown combinations of parts that resulted in functional problems that may or may not have been the result of chainline issues.
So it is science, if you pay attention to everything involved in building a bike rather than assuming that problems are the result of uncontrollable random discontinuities.
For you or anyone having the problems described in this thread:
1. What make and model frame is it, and what is the correct chainline for that frame and the speed crank used?
2. What make and model crank is it, is that crank designed to work with that sort of frame, and is the BB spindle the recommended one for that use?
3. What make and model front derailleur is it, and it is designed for used with that kind of frame, crank and seat tube thickness.
You can grope around in the dark, or you can learn about bicycle mechanics and stop leaving things to random chance. I've never worked in a bike shop where it was necessary to just "try stuff" to see if it would work. Everything has a manual or a spec, regardless of whether it is Asian, US or European. Use them.
Your problem is that you have gotten away with doing things without understanding them for long enough to think that is normal. It isn't normal to not know how the parts of your bike are speccd.