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Axle length for 54mm chainline with TA single chainring?

Posted: 25 Aug 2014, 4:21pm
by steve browne
I am fitting a Rohloff hub to Claud Butler Black Diamond frame (Dalesman equivalent) and will be using a TA cyclotouriste single chainring. If anyone knows which length bottom bracket axle will provide the correct 54mm chain line I would be grateful for help. I am happy to use either a TA axle and ball bearings or a cartridge axle which will fit the cranks.
Thanks in advance,
Steve

Re: Axle length for 54mm chainline with TA single chainring?

Posted: 25 Aug 2014, 6:16pm
by andrew_s
The TA website says that the Pro 5 Vis uses a 111mm axle for track and 116mm for double.
The standard road bike chainline is 43.5 mm, and MTB chainline is 47.5mm (or 50mm for oversize tubes that force the front mech outwards).

The double chainline will be for a road chainline, and TA cyclotourist used as a single puts the chainring in the outer chainring position, so you'll want to subtract about 3.5mm from the 116mm (typical chainring spacing is about 7mm) to give a 112.5 mm axle.

To move the chainline out by 10mm, you'll need a BB axle that's 10mm longer on the drive side, or 20mm longer overall, giving a 132.5mm axle.
The TA Axix Light BB is available in 131mm (since I have the TA site open), which is close enough.

Chainline isn't all that critical.
A Rohloff isn't like a fixed wheel where a chain derailment is a serious problem, and almost all available chains these days are bushingless, and flexible enough sideways to easily cope with a misalignment of 10mm, as most of them have to be to cope with derailleur systems.

Re: Axle length for 54mm chainline with TA single chainring?

Posted: 25 Aug 2014, 6:43pm
by Brucey
you don't say exactly which cranks you are using, but if you know the correct length for a double chainline, you can work out the correct chainline OK.

I think if you fit a road triple chainset and simply delete the inner two chainrings, you will end up with the chainline you desire, more or less.

cheers

Re: Axle length for 54mm chainline with TA single chainring?

Posted: 25 Aug 2014, 7:56pm
by timdownieuk
steve browne wrote:I am fitting a Rohloff hub to Claud Butler Black Diamond frame (Dalesman equivalent) and will be using a TA cyclotouriste single chainring. If anyone knows which length bottom bracket axle will provide the correct 54mm chain line I would be grateful for help. I am happy to use either a TA axle and ball bearings or a cartridge axle which will fit the cranks.
Thanks in advance,
Steve


I would fit the back wheel, fit the crank to the existing bottom bracket and then eyeball it. I would have little faith in calculating the BB length.

Tim

Re: Axle length for 54mm chainline with TA single chainring?

Posted: 25 Aug 2014, 8:12pm
by Brucey
If you are going to eyeball it on the bike, it is best to stick the chainset in with any old BB, then put a straight edge back to the rear wheel from the chainring.

cheers

Re: Axle length for 54mm chainline with TA single chainring?

Posted: 25 Aug 2014, 10:18pm
by recordacefromnew
Unless the chainset is worn, 127.5mm JIS (Shimano code EL) is the closest one for delivering a 54mm chainline at the outer ring position.

Re: Axle length for 54mm chainline with TA single chainring?

Posted: 19 Sep 2014, 5:15pm
by steve browne
Thanks for all your comments. I fitted it with any old bottom bracket (which happened to be a Shimano sealed unit), eyed it up, and purchased the closest match to the desired length. Shimano UN55 at 122.5mm gives 54mm chain line. :-)

Re: Axle length for 54mm chainline with TA single chainring?

Posted: 20 Sep 2014, 6:02pm
by recordacefromnew
steve browne wrote:Thanks for all your comments. I fitted it with any old bottom bracket (which happened to be a Shimano sealed unit), eyed it up, and purchased the closest match to the desired length. Shimano UN55 at 122.5mm gives 54mm chain line. :-)


Is the chainset Cyclotouriste or just the ring, and how old is it? I am asking because according to the info from Sheldon's bb database (that 115mm UN52 delivering 43mm chainline) the 127.5mm JIS should, based on my calculations, give you very close to 54mm with the large ring while the 122.5mm would be short by a couple of mm.

Re: Axle length for 54mm chainline with TA single chainring?

Posted: 20 Sep 2014, 6:47pm
by steve browne
The TA cranks and chainring are both old. They look exactly the same dimensions as ones I bought new in 1982. It could be that the crank will bed in a bit more with use - I've not ridden it yet as the brakes aren't fitted!