Asymmetric bb to symmetric bb: compensating for chainline?
 

Hi,

I'm considering using some older Dura Ace 7402 cranks. I thought I had the chainline figured out for my particular build, but then realized when i rechecked the Sheldon Brown archive (http://sheldonbrown.com/bbsize.html#shimano) that the original bottom bracket for these cranks was asymmetric. The original BB was 112mm, but had 3mm extra on the right side.

So, as I'm figuring out the chainline using a new symmetric BB, does the extra 3mm mean that I should compensate in my measurements by pretending that the original BB was 115mm? Seems like that would be the logical thing to do, but I'm not quite sure.

Thanks for any clarification.

I just use a spacer (placed under the fixed cup), and then use the same length spindle on the new bb. Your LBS should have a variety of spacers, I keep a stash of them on hand myself. I almost exclusively work on vintage bikes, and asymmetric bb were the norm.

Perhaps I should have clarified that I am converting the 7402's from double to single for a singlespeed drivetrain (and I don't have the original BB from these cranks). So, I'm not trying to duplicate the original spacing. Since I have the data from the Sheldon page, if it was symmetric, I could just do the math and reduce accordingly, without using spacers, and just pick a new BB. But with the extra 3mm, I'm not sure whether to do my subtraction from an assumption of 112mm or 115mm.

Does that make sense?

Yes

FWIW, Phil Wood Co has a tight press fit between the axle and the bearing ID.
but it can be pressed sideways, by using the crank bolt and spacers
like the 2 installation tools, by hand .. to get the chainline exactly spot-on..

if you make it short , shim washers around the bolts
between the Chainring and the crank spider.. are another adjustment.

fietsbob: I wish I could afford a Phil Wood...I'll probably be looking at something like an IRD.

Well, I'm not actually trying to match the original chainline for this crankset, which would have been 43.5. I'm trying to line up the single ring with my Sturmey Archer IGH in the rear, which is at 47mm. Luckily, the original outer ring location for the 7402 was about 46.5/47.

So if the original outer ring on the 7402 was about 47mm with a 112 BB with 3mm extra from the asymmetry, then going by my calculations and the info the other posters have given, a new, symmetrical 115mm BB would give me the 47mm chainline I need (give or take about .5mm).

Would you guys say that is correct?

Give it a try and measure. In addition to the overall width of the BB-spindle, the diameter of the taper actually varies at the tip. That means a 115mm spindle from one maker would have the crankarm sit further up the spindle than a 115mm spindle from another manufacturer. So even with exact same spindle-length, you may see chainline of 46 with one BB and 48mm with another.

Cool, thanks. Since I'm running a 3/32 chain, it'll have some tolerance room for deviation, and if I can get within a mm of 47 I'd be pretty damn happy...

Before you go too far, measure all the clearances to teh frame: crankarm to chainstay, chainring closest point to chainstay, chainring bolts to chainstay, anything else that could represent a limiting condition or a source of metal-metal interference. Recognize that you can't move that crank closer than those gaps you measure. Just know the limitations.

But I agree with Danno: put it together, see how it works, measure the clearances and see what the next one to try should be.

Thanks. I've measured as much as I can without the BB, and I think I'll start out by trying the 115mm. With my current crank, I'm at a 42mm chainline with the ring on the inside, and clear the chainstay very well. So I'm hopeful that with a 47mm chainline and the ring on the outside, metal hitting metal won't be an issue, even if I decide on a larger chainring.