southpawboston

I'm building up a new Soma Buena Vista mixte frame for my wife, based around a Shimano Nexus 8 IGH. I've done all the calculations on chainline, and rifled through my parts bin to find a bottom bracket and crankset that will not only achieve the desired chainline but also extend out the crank arms far enough to clear the rather wide chainstays, and on the drive side to clear accommodate a Velo Orange Porteur chaincase.

After some trial fittings, I found that an asymmetric 124mm BB (made for a triple), along with a vintage 110BCD triple crankset with all but the middle ring removed, provides adequate clearance for the VO chaincase, as well as a chainline that's just 2mm to the right from center at the crank to be perfect. My question is, is 2mm close enough? Should I leave as is, or should I try something else to get closer to a perfect chainline? Can I try a different chainring-to-crank spacer? Right now I'm using chainring bolts designed for a single (obviously) with the ring flat against the inside of the spider arms. But I suppose I could move the ring more inboard with a spacer. but if I go with longer chainring bolts (like for a double) and a typical chainring spacer, it may move the chainring too far inboard. Can you buy chainring spacers in 1 or 2mm increments?

Other suggestions?

fietsbob

You have Laid a straight edge on the chainring, to measure.. ?

note:Yes, there are shim washers to adjust chain ring placement ,
in various thicknesses .. 1 around each bolt. QBP, LBS.

I have a 127.5 UN bb on a bike with a Rohloff hub and M730 crankarms ,
It has the chainring on the inside/middle position,
disc chainguard on the outside one.

southpawboston

When I use a straight edge, it comes up more or less "perfect". Okay, maybe a 1mm off, but then again I don't have a perfectly straight straight edge, so I find that method less reliable. Rather, I used a digital caliper to determine the distances of the rear cog and front chainring from the bike's center axis:

For rear cog: distance from inner face of cog to dropout = 20mm. Dropout spacing = 132mm. Therefore cog offset = (132/2)-20 = 46.
For front chainring: distance from right side of downtube to inner face of chainring = 33.6mm. Downtube diameter = 28.8mm. Therefore chainring offset = 33.6+14.4 = 48.

That's how I'm getting the 2mm figure. Of course, I did this after properly torquing the BB into the shell and crankset to the spindle!

Good to know you can get 1mm spacers for chainrings. Can I continue to use bolts for single rings? Or will they not thread enough?

fietsbob

You should be able to see the results .. nothing wrong with Mixing
the sleeve nut from the single length bolt on the double bolt.

Charles Ramsey

I moved my crank as close as possible to the chainstays and found the 11 tooth cog skipped. I replaced it with a 12 tooth cog and it works fine. Graeme Obree a world hour record holder and Tom Ritchey both state they prefer the crank in close.

bradtx

southpawboston, Not that it'd hurt to optimize the chainline nor to contest your striving for a perfect chainline, but I've Ks of miles on the geared bikes and seldom, if ever, is the chainline perfect.

Brad

fietsbob

Internal Gear Hub, no 11 tooth cog , wrong thread, Charles.

southpawboston

Ah, yes, that makes sense.

Great, good to know!


Right! We're talking about a 21T cog here.