Hi. I'm planning to build a hub geared alfine 8 speed commuter/light touring bike and trying to source the parts. Regarding BB length; is there a way of working out what length axle I'll need. Planning a basic stronglight square taper 68mm one. They come in various lengths which presumably will affect the chainline. Should I just get a mid-range one, and can I then spacer out the rear cog if chainline's off? Is there a system?
Many thanks!!
Dan
BB axle length
Re: BB axle length
If only!
Sadly, the makers of square-taper chainsets and bottom-brackets have always been extremely negligent in this regard. In the past they did not even adhere to standards (except the Asians, who do stick close enough to JIS - Japan Industry Standard) for width/length of taper, resulting in some serous problems of some axles not penetrating far enough into some cranks to give a secure fixing, whilst opposite combinations bottom out before they're snug. It's not quite such a minefield nowadays. Most cranks are either JIS or ISO, and the two Standards are not so very different, but different enough to create significant chainline problems. Plus there's the matter of symmetry: most axles now stick out the same both sides, but in the past they were mostly longer on the right - and some still are.
Sutherlands Handbook attempts to make sense of all this, but very few shops have an up-to-date copy and neither have I. It's very expensive.
So the usual method is you buy your crank first and poke it onto whatever Shimano UN bracket you can lay hands on. Measure things (allow for the crank pushing on between 1 and 1.5mm when torqued up) and then buy the Shimano UN bracket that's twice as much shorter or longer than the distance you want to move the chainset towards or away from the frame.
Sadly, the makers of square-taper chainsets and bottom-brackets have always been extremely negligent in this regard. In the past they did not even adhere to standards (except the Asians, who do stick close enough to JIS - Japan Industry Standard) for width/length of taper, resulting in some serous problems of some axles not penetrating far enough into some cranks to give a secure fixing, whilst opposite combinations bottom out before they're snug. It's not quite such a minefield nowadays. Most cranks are either JIS or ISO, and the two Standards are not so very different, but different enough to create significant chainline problems. Plus there's the matter of symmetry: most axles now stick out the same both sides, but in the past they were mostly longer on the right - and some still are.
Sutherlands Handbook attempts to make sense of all this, but very few shops have an up-to-date copy and neither have I. It's very expensive.
So the usual method is you buy your crank first and poke it onto whatever Shimano UN bracket you can lay hands on. Measure things (allow for the crank pushing on between 1 and 1.5mm when torqued up) and then buy the Shimano UN bracket that's twice as much shorter or longer than the distance you want to move the chainset towards or away from the frame.
Chris Juden
One lady owner, never raced or jumped.
One lady owner, never raced or jumped.
Re: BB axle length
danny1 wrote: ... and can I then spacer out the rear cog if chainline's off? ....
ha! (bitter, twisted laugh....). The alfine 8s hub has rather limited scope for adjustment. Basically you can get a few mm with the locknuts (if you are dedicated, and the frame is wide enough) and a few mm by reversing the sprocket (in some, not all sprocket sizes). But (unlike SA hubs of yore) there are no spacers on the hub spline, so no fine, easy chainline tweaking there....
As for the BB, it is like CJ says. A cunning dodge is to buy a BB unit that is close to the right size, is shouldered on the right side, is tolerant on the left (lots of threads on the LH cup) but is meant for a 73 BB shell, not a 68. Then with a few spacers you can shuffle +/- 2.5mm on the chainline. OK the Q factor of the cranks is not going to be symmetrical, but a) they often are like that anyway, and b) you mightn't notice....
cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Re: BB axle length
Any help?............http://www.lfgss.com/thread45225.html
Bike fitting D.I.Y. .....http://wheel-easy.org.uk/wp-content/uploads/bike-set-up-2017a.pdf
Tracks in the Dales etc...http://www.flickr.com/photos/52358536@N06/collections/
Tracks in the Dales etc...http://www.flickr.com/photos/52358536@N06/collections/
Re: BB axle length
Thanks for those very detailed replies! As I feared there's no simple answer! I'd hoped, since lots of bikes are sold pre fitted with these hubs, there might've been some standard way..
Re: BB axle length
Btw would using a different type of bb make things any easier?
Re: BB axle length
I'm not pretending to be an expert, and this is something I've never done, but is it really all that hard?
I just Googled "alfine 8 chainline" and I got lots of hits giving 2 different chainlines depending on which way round you fit the sprocket....the chainline could also vary if you alter the over-locknut dimension of the hub.
But surely once you have the hub in your hand, alter the width if you need to, and fit the sprocket, you can simply measure the chainline?....Then all you need to do is select the bb axle length to give that chainline with the chosen crankset.
....isn't it?.....am I missing some awful complication?
I just Googled "alfine 8 chainline" and I got lots of hits giving 2 different chainlines depending on which way round you fit the sprocket....the chainline could also vary if you alter the over-locknut dimension of the hub.
But surely once you have the hub in your hand, alter the width if you need to, and fit the sprocket, you can simply measure the chainline?....Then all you need to do is select the bb axle length to give that chainline with the chosen crankset.
....isn't it?.....am I missing some awful complication?
Bike fitting D.I.Y. .....http://wheel-easy.org.uk/wp-content/uploads/bike-set-up-2017a.pdf
Tracks in the Dales etc...http://www.flickr.com/photos/52358536@N06/collections/
Tracks in the Dales etc...http://www.flickr.com/photos/52358536@N06/collections/
Re: BB axle length
I think without a bb to "practice" on it'd be difficult to hold the crank in the right position (bearing in mind when you tighten it up it'll move in a bit)
Re: BB axle length
Crankset manufacturers specify the BB axle length for "standard" chainlines.....
For example, these XD2 doubles want 107mm JIS taper BB axle .......http://www.spacycles.co.uk/products.php?plid=m2b0s109p2003
...That puts the big ring at a standard (road) chainline, I think that's 45mm......(but check me)
To get a 2mm different chainline you need to alter the bb length 4mm, ie. 2mm each side
For example, these XD2 doubles want 107mm JIS taper BB axle .......http://www.spacycles.co.uk/products.php?plid=m2b0s109p2003
...That puts the big ring at a standard (road) chainline, I think that's 45mm......(but check me)
To get a 2mm different chainline you need to alter the bb length 4mm, ie. 2mm each side
Bike fitting D.I.Y. .....http://wheel-easy.org.uk/wp-content/uploads/bike-set-up-2017a.pdf
Tracks in the Dales etc...http://www.flickr.com/photos/52358536@N06/collections/
Tracks in the Dales etc...http://www.flickr.com/photos/52358536@N06/collections/
Re: BB axle length
I think that you can often do OK like that, if all the bits are well made and you get your ducks all lined up in a row. But it is ever so easy to get caught out by
-a set of cranks that doesn't do what it should
- a BB axle that isn't symmetric like it should be
- a frame that is a gnat's out of track
- a set of chainring bolts that are hell-bent on gouging into the chainstay (e.g. when you are trying to run the outer ring position of a double chainset at a 46mm chainline...)
and life can quickly turn miserable. Remember also that a chainline error of 3-5mm might be lost in the noise on a derailleur geared bike, but with an IGH is sat there laughing at you every time you ride the bike. It is the lack of fine adjustment when getting it from 'nearly right' to perfect that is so often a pain.
cheers
-a set of cranks that doesn't do what it should
- a BB axle that isn't symmetric like it should be
- a frame that is a gnat's out of track
- a set of chainring bolts that are hell-bent on gouging into the chainstay (e.g. when you are trying to run the outer ring position of a double chainset at a 46mm chainline...)
and life can quickly turn miserable. Remember also that a chainline error of 3-5mm might be lost in the noise on a derailleur geared bike, but with an IGH is sat there laughing at you every time you ride the bike. It is the lack of fine adjustment when getting it from 'nearly right' to perfect that is so often a pain.
cheers
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Re: BB axle length
danny1 wrote:Btw would using a different type of bb make things any easier?
Yeah sure, it's called "cottered". Your grandad should know all about it!
Chris Juden
One lady owner, never raced or jumped.
One lady owner, never raced or jumped.
Re: BB axle length
I started a whole thread on this subject and never did learn how other axle lengths differ apart from 107 and 110mm lengths, which didn't measure up as you would expect. If anyone has got any other UN** axle lengths they could measure, I'd be grateful.Brucey wrote:
- a BB axle that isn't symmetric like it should be
BTW, quoting an axle length for a chainset must be dependant on what rear hub width is being used, particularly with a "hybrid" style CS such as the XD2, which gets used on road and tour bikes. It was originally a MTB chainset.
Re: BB axle length
Just in case it helps some bikes used to use BB's with asymmetric axles where the right hand (drive side) is longer than the left. I swapped out one such BB last year.
The important thing is to decide what chain-line you are trying to achieve which is probably easier for an alfine 8 speed geared hub than it is for a dérailleur set-up where there is an additional range of possible chain-lines depending on whether you are using a double or triple at the front and also whether you are following a"road" or "MTB" specification.
The main issue I have found is working out the correct combination of BB axle lengths when using BBs from one manufacturer and chainsets from another. It is confusing enough that Shimano recommend 4 different axle lengths for the various variants of their Alivio chainset (see their SI-1LZ0A-001-00 document). This emphasises that it is essential that you check for a particular model number rather than just a generic series name.
Following on from the comments about the XD2 chainset in CREPELLO's post above I used the Spa Cycles equivalent triple on an old touring frame with a 68mm wide shell last year with a Shimano UN55 107 mm BB and achieved a chain-line of 45 mm (as intended). I cannot remember why I specifically chose a 107 mm Shimano UN55 rather than a 110mm one (the Spa website suggests a "110mm JIS square taper bottom bracket. eg Stronglight JP400" but does not say what chain-line they are trying to achieve) other than the fact that at the time Spa were out of stock of Aluminium JP400s. I do remember deliberating over it for a while and then being pleasantly surprised when the chain-line worked out as intended.
The important thing is to decide what chain-line you are trying to achieve which is probably easier for an alfine 8 speed geared hub than it is for a dérailleur set-up where there is an additional range of possible chain-lines depending on whether you are using a double or triple at the front and also whether you are following a"road" or "MTB" specification.
The main issue I have found is working out the correct combination of BB axle lengths when using BBs from one manufacturer and chainsets from another. It is confusing enough that Shimano recommend 4 different axle lengths for the various variants of their Alivio chainset (see their SI-1LZ0A-001-00 document). This emphasises that it is essential that you check for a particular model number rather than just a generic series name.
Following on from the comments about the XD2 chainset in CREPELLO's post above I used the Spa Cycles equivalent triple on an old touring frame with a 68mm wide shell last year with a Shimano UN55 107 mm BB and achieved a chain-line of 45 mm (as intended). I cannot remember why I specifically chose a 107 mm Shimano UN55 rather than a 110mm one (the Spa website suggests a "110mm JIS square taper bottom bracket. eg Stronglight JP400" but does not say what chain-line they are trying to achieve) other than the fact that at the time Spa were out of stock of Aluminium JP400s. I do remember deliberating over it for a while and then being pleasantly surprised when the chain-line worked out as intended.