how to calculate BB length?

[d_douglas]

OK, folks, here's a riddle for ya:

I amneed to determine what BB length to use for my wife's new Mixte. It is a Sycip with touring width chainstays, for reference.

The drivetrain consistss of:

1) HUB: Alfine 8spd at 135mm and an 18t cog
2) CRANK: http://www.bike-mailorder.de/shop/Rennrad/Antrieb-und-Schaltung/Kurbel-Kettenblatt/Sugino-XD-Kurbel-4-Kant-110mm-LK-silber::14562.html
3) CHAINRING: Surly 38t x 110mm

Is there a simple way to determine how long the BB needs to be? I ordered a 103mm Sugino BB as it said that this is what Sugino Messenger cranks used, but the guy who was selling to me said it might be too narrow, so I backed out.

I suppose that the easiest (though not least expensive) thing to do would be to track down a Phil 108mm BB that is asymmetrical (can go 5mm shorter on the driveside if need be?), but I am trying to keep the ballooning costs down for finishing this project.

Any thoughts are appreciated.

If you have a nice BB for this purpose (once we determine the length) I would be interested in hearing about it...

[oldpotatoe]

OK, folks, here's a riddle for ya:

I amneed to determine what BB length to use for my wife's new Mixte. It is a Sycip with touring width chainstays, for reference.

The drivetrain consistss of:

1) HUB: Alfine 8spd at 135mm and an 18t cog
2) CRANK: http://www.bike-mailorder.de/shop/Rennrad/Antrieb-und-Schaltung/Kurbel-Kettenblatt/Sugino-XD-Kurbel-4-Kant-110mm-LK-silber::14562.html
3) CHAINRING: Surly 38t x 110mm

Is there a simple way to determine how long the BB needs to be? I ordered a 103mm Sugino BB as it said that this is what Sugino Messenger cranks used, but the guy who was selling to me said it might be too narrow, so I backed out.

I suppose that the easiest (though not least expensive) thing to do would be to track down a Phil 108mm BB that is asymmetrical (can go 5mm shorter on the driveside if need be?), but I am trying to keep the ballooning costs down for finishing this project.

Any thoughts are appreciated.

If you have a nice BB for this purpose (once we determine the length) I would be interested in hearing about it...

Sugino messanger cranks use a 103 for a 120mm rear spaced frame and 107 for one larger than that. I'm guessing 110 because of the stays, JIS taper BTW-shimano has a UN-54 like that, Sugino does too.

Got a decent bike shop around? They can 'spec' it..put one in and see what the clearance is, etc. I do that all the time.

[salem]

Assuming the Messenger with 103mm spindle is for a 120mm track dropout, my seat-of-the-pants guess would 118mm for a 135mm dropout to keep the chainline the same. That assumes symmetrical spindles and a similar cog to axle-end distance for your hub, which probably isn't quite right. If you have ANYTHING to install and use as a reference point, that would probably be the most useful.

The other thing I've done is measure the distance from the chainring mount face to the inside face around the mounting taper. Compare this to a reference crank for which you know the spindle length and make your adjustments.

[d_douglas]

I forgot about 120mm rear spacing.

But 118mm!? Wow, I am way off. I actually think I have two crap old BBs laying around that are 119 and 107mm, so this could help me guess what increment in between. Hmmmm - a measurement btw 107 and 118 leave many more options open. Good news.

Thanks.

[EricEstlund]

Alfine runs a 42.5mm chainline, or 47.5mm with the cog flipped.

The Sugino XD triple uses a 110 mm BB for a 45mm chainline (middle ring).

It's about 5mm from the middle position to the outer. It seems like running the ring in the outer position and moving the crank in the would be pretty close- 107mm ish BB puts it within a mm.

Don't worry about the rear spacing- be more concerned with how far the cog and ring are from the bikes centerline.

[Mike748]

Assuming the Messenger with 103mm spindle is for a 120mm track dropout, my seat-of-the-pants guess would 118mm for a 135mm dropout to keep the chainline the same. That assumes symmetrical spindles and a similar cog to axle-end distance for your hub, which probably isn't quite right. If you have ANYTHING to install and use as a reference point, that would probably be the most useful.

The other thing I've done is measure the distance from the chainring mount face to the inside face around the mounting taper. Compare this to a reference crank for which you know the spindle length and make your adjustments.

You forgot to take 1/2 the axle length difference. 103 + (15/2) = 110.5

BTW, Phil BB spindles are just a press fit in the bearings and can be moved side to side... in addition to being able to move the whole bb side to side with the cups.

[khjr]

The Sugino XD triple uses a 110 mm BB for a 45mm chainline (middle ring).

I have a similar question, as I'm planning to outfit my new to me Soma Smoothie with a Sugino XD triple. What chainline would I want to shoot for to accomodate a 7 speed Shimano 130 mm rear hub? Is there a table anywhere that shows the proper chainline for various common freehubs?

[EricEstlund]

Road doubles and triples shoot for a 43-45mm chainline. Back end spacing gets wider to accommodate more gears and corresponding length needs, but the distance from center is pretty close. Remember that shifting is cross chaining by degrees- multi speed set ups are pretty forgiving in that you can be a few mm out one way or the other. If you are converting an 8/9/10 hub you will likely have to run a spacer behind the cassette anyway. Generally you are shooting for the middle ring to line up with the middle cog, but a bit in or out can work. If you need it to be perfect you can also grab some thin BB shims to space the drive side cup.