Black art of choosing the correct length BB

[Psycolist]

I've just taken possesion of an ally frame and forks that I intend to build up to a coastal cruiser. I have a collection of parts waiting to be fitted, once the frame has been re-painted, but the frame came without a BB and I have no idea where to start in getting the right length. Any body out there know the secret or give me any advice please.

 

[RecordAceFromNew]

The correct choice of a BB is determined by both the frame (i.e. BB shell spec) and the chainset.

Unless your frame is Italian, quite old (over 20 years and/or French etc.), or falls into the latest oversized shell categories some manufacturers are adopting, most BB shells on the road are English thread of 68mm or 73mm wide. You need to establish that first.

Once that is established, you need to find out what your chainset requires. The chainset will determine the type (JIS or ISO square tapered, octalink I, octalink II, ISIS, Hollowtech, etc. etc.), and specific spindle length for a certain chainline (driven e.g. by whether the chainset is primarily designed for a track, road or mtb bike, and whether it is a single, double or triple chainset). In some instances you might want and be able to use a spindle length that is somewhat different to that recommended by the chainset manufacturer to meet your specific chainline requirement. However, shortening the spindle length could, depending on the frame and the chainset's q factor, cause a clash, and using a different square tapered spindle length spec could cause asymmetry at the pedals, since most such spindles are asymmetric, and by differing amount.

There is no secret afaik, just complicated.

 

[Psycolist]

OK thanx. at least that has put my mind at rest as to not always going with manufacturers recommendations. Over the last 5 years or so I have built 4 bikes up from scratch and it has always ended up with a trial and error selection of BB. It can go from being so long that you cant shift onto the granny, ( I always use a triple ) to so short, that the granny fouls on the frame. Those magic words " the chain line" is, I know, the most important thing for efficient drive and I just hoped there would be some magic formula.................Looks like trial and error it is then.

 

[Manonabike]

Here there's some information about BB

 

[biggs682]

my lbs always ask do you have the old spindle , when ever i take a frame in that has no bottom bracket fitted even though he knows the answer will be no

 

[RecordAceFromNew]

OK thanx. at least that has put my mind at rest as to not always going with manufacturers recommendations. Over the last 5 years or so I have built 4 bikes up from scratch and it has always ended up with a trial and error selection of BB. It can go from being so long that you cant shift onto the granny, ( I always use a triple ) to so short, that the granny fouls on the frame. Those magic words " the chain line" is, I know, the most important thing for efficient drive and I just hoped there would be some magic formula.................Looks like trial and error it is then.

Being potentially costly and time consuming, I would normally avoid trial and error by finding out what the existing measurements are, and if unavailable what the manufacturer's recommendation is, and work backwards from there if I need a different chainline. Though not all, most modern chainset specs are readily available online; this covers most older Shimano chainsets.

 

[Psycolist]

Many thanx for all the info, and for your time and trouble uploading it all.