What BB do I need???

[lackofgrip]

I recently dropped on a bargain on ebay of a Control Tech carbon comp crankset.

[Brucey]

The manufacturers should have that data.

When you have the thing in your hand you can measure the crankset and work it out easily enough, it isn't very difficult.

Just measure the depth from the chainline plane (between the chainrings) to the spot where end of the axle will sit in the crank. Suppose (for sake of argument) that is 13mm, and you want a 45mm chainline;

(13 +45) x2 = 116mm = your new BB length. [edit; assuming a symmetrical spindle]

cheers

[lackofgrip]

Eh??

[Brucey]

Eh??

http://sheldonbrown.com/chainline-multi.html

for starters.

Which bit don't you understand?

cheers

[lackofgrip]

Could I measure the chain line with the current crank? OR does it have to be the one I'm installing? If it has to be the one I'm installing I can't measure accurately without the BB.

[lackofgrip]

I measured chainline of current chainset as shown in sheldon brown

But current chainset is FSA Gossamer with mega exo external bracket

it was 113mm?

[recordacefromnew]

BTW it is not true that 113mm octalink V2's are only for triples - Shimano used them for some road doubles like the fc-3301/fc-4401, and even the new Claris I think. What is true is that for a period many Shimano mtb chainsets needed a 113mm V2 to achieve a 47.5mm chainline, and they were practically all triples.

Absent manufacturer spec and bb then as Brucey said you have to estimate what spindle length the new chainset requires. However it is not straightforward because octalinks aren't typically symmetric, iirc the longer ones are more asymmetric than the shorter ones.

First I think you should make sure it needs a V2 rather than a V1. The V2 splines are 9mm long 2.8mm wide, while V1's are 5mm and 2.2mm respectively. If you install with the wrong one it will ruin the chainset for good.

To start place the chainset with the inner face up on a table, and half cover it with a hard board, you then measure the distance between the bottom of the board and the top of the spline slots, let us call that X mm. Similarly measure the distance between the bottom of the board and mid way between the two rings, and let us call that Y mm.

If you want a chainline of 43.5mm (standard for road double), then the V2 drive side spindle needs to be (43.5 + 9 + X - Y) mm long. IF (and this is a big if as Brucey also indicated) the bb spindle is symmetric then the spindle length you need is twice that, but this is not something you should assume. If you provide the figure, we / someone should be able to tell you what the likely spindle length you need is.

[lackofgrip]

BTW it is not true that 113mm octalink V2's are only for triples - Shimano used them for some road doubles like the fc-3301/fc-4401, and even the new Claris I think. What is true is that for a period many Shimano mtb chainsets needed a 113mm V2 to achieve a 47.5mm chainline, and they were practically all triples.

Absent manufacturer spec and bb then as Brucey said you have to estimate what spindle length the new chainset requires. However it is not straightforward because octalinks aren't typically symmetric, iirc the longer ones are more asymmetric than the shorter ones.

First I think you should make sure it needs a V2 rather than a V1. The V2 splines are 9mm long 2.8mm wide, while V1's are 5mm and 2.2mm respectively. If you install with the wrong one it will ruin the chainset for good.

To start place the chainset with the inner face up on a table, and half cover it with a hard board, you then measure the distance between the bottom of the board and the top of the spline slots, let us call that X mm. Similarly measure the distance between the bottom of the board and mid way between the two rings, and let us call that Y mm.

If you want a chainline of 43.5mm (standard for road double), then the V2 drive side spindle needs to be (43.5 + 9 + X - Y) mm long. IF (and this is a big if as Brucey also indicated) the bb spindle is symmetric then the spindle length you need is twice that, but this is not something you should assume. If you provide the figure, we / someone should be able to tell you what the likely spindle length you need is.

Now my mind is truly frazzled
I have yet to collect the chainset from Royal Mail as a card was left but seller assures me it is V2