Go Back  Bike Forums > Bike Forums > Bicycle Mechanics
Reload this Page >

Chainline calc / bb length

Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

Chainline calc / bb length

Old 03-18-08, 07:33 PM
  #1  
slcbob
bored of "Senior Member"
Thread Starter
 
Join Date: Oct 2007
Location: MD / metro DC
Posts: 1,302

Bikes: Cross-Check/Nexus commuter. Several others for various forms of play.

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 78 Post(s)
Chainline calc / bb length

OK, what did I screw up?

Sugino RD-48 crank on a Surly cross-check. Nexus hub.

I read that the RD-48 with Sugino's 103mm BB gives a 42mm chainline. The Nexus spec says its cog, depending on how you flip it, gives a 42.7 or 47.7. I put it on the 47.7 way, call it 48.

So I figured I needed 6mm more width on the BB, for each side, beyond what a 103mm gets me. So 103+(2x6)=115mm ought to have me spot on. I got the basic Shimano BB-UN54 in its 68x115 flavor.

Well I have it all put together, and the chainline is well off. The chainring is well outside the cog. It actually looks like if I put the chainring on the inside of the spider, rather than on the outside, it would be just about perfect. Seems to be too far to fix with a shim.

Is there fault in my basic BB math concept (the 2x deal)? Hallucination on the 103=>42 jump off point? Unclean living finally catching up to me? An uncanny difference between Sugino's square taper and Shimano's? Maybe the single chainring really is supposed to go inside the spider and Sugino just ships it assembled the other way to fake out the mere mortals?

Having fun, learning (I hope), but seem to be destined for another BB.

Last edited by slcbob; 03-18-08 at 08:06 PM.
slcbob is offline  
Old 03-18-08, 09:53 PM
  #2  
lymbzero
otherwiseordinary
 
lymbzero's Avatar
 
Join Date: Jan 2006
Location: California
Posts: 697
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
IF: a 103mm BB gives a chain line of 42mm in the front.
AND: the rear hub is 48mm.
THEN: the difference is 6mm

THERFORE: Get a BB that is 6mm larger.
UN52(54). 68x109?

If not 109 then 108 or 110 ?

You were right. But for some reason you bought one that is 12mm larger..
UN72 FTW
lymbzero is offline  
Old 03-18-08, 10:20 PM
  #3  
sivat
Geek Extraordinaire
 
sivat's Avatar
 
Join Date: Feb 2006
Location: Long Beach, CA
Posts: 1,769

Bikes: Bianchi Advantage Fixed Conversion; Specialized Stumpjumper FS Hardtail

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
slcbob assumed that all bottom brackets are symmetrical, and lymbzero assumed that any added length will only be added to the drive side. So you were both wrong.
The problem is that most bottom brackets are not symmetrical. For shimano, I know the 103 and 107 are, and the 110 is not. I'm not sure about the 115.
Why not just flip the cog and use the 103mm bb. 0.7 mm off will not be noticeable.
__________________
I'd rather have a bottle in front of me than a frontal lobotomy.

Sintesi Conversion Serotta Track
sivat is offline  
Old 03-19-08, 03:17 AM
  #4  
ultraman6970
Senior Member
 
Join Date: Dec 2005
Posts: 7,859
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
i sort of agree with sivat... other factor is that maybe the sugino cranks are ISO not Jis. I know the track cranks are ISO. If this was passed out to the other crank sets from sugino u'll have a difference also.

Thanks.
ultraman6970 is offline  
Old 03-19-08, 06:24 AM
  #5  
slcbob
bored of "Senior Member"
Thread Starter
 
Join Date: Oct 2007
Location: MD / metro DC
Posts: 1,302

Bikes: Cross-Check/Nexus commuter. Several others for various forms of play.

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 78 Post(s)
Sivat - flipping the cog would make it 5mm worse, not better. Chainrings are already further out than the cog.

Lymbzero - the "some reason" was that I assumed the L in the BB WxL measurement is the overall length, not from center line. My 68x115 sho'nuff is ~115 end to end. So 6mm as you say would seem to get me 3mm on each side. Unless this has something to do with measurement to the right of a fixed cup, rather than overall length equally distributed on a center line. Do you know for sure, or are you making as bold a guess as I did (albeit with the other measurement concept).?

Back to sivat -- you seem to be on top of that subtle nuance. And indicate it's a jungle out there. If the 110 is not symmetrical and the 107 is, is there a 3mm chainline difference between them?

Anyway, would welcome a 2nd opinion, but it does look like a 68x110 is in my future.

Ultraman -- Sugino BB says "JIS 2 degree taper" and Shimano says "Traditional JIS 2 degree taper." I assume they are the same, so I don't think that's it. I have seen some BBs (incl some Suginos) that are listed under an ISO header (e.g. Universal Cycles) but then refer in the product detail to JIS. I never knew there was a difference. Just looked it up:

C/O Sheldon at http://sheldonbrown.com/bbtaper.html
If you install an ISO crank on a J.I.S. spindle, it will sit about 4.5 mm farther out than it would on an ISO spindle of the same length.
slcbob is offline  
Old 03-19-08, 06:33 AM
  #6  
slcbob
bored of "Senior Member"
Thread Starter
 
Join Date: Oct 2007
Location: MD / metro DC
Posts: 1,302

Bikes: Cross-Check/Nexus commuter. Several others for various forms of play.

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 78 Post(s)
P.S. thanks for prompting me to do a little more research.

Another vote for the 2x (or 6 gets you 3) concept here. About middle of the page, under chainline adjustment.

I know I'm starting to sound like a know-it-all git, which is ironic, since I'm humbly scratching my head here wondering how I'm off by so much. These darn confusing facts.

Any thoughts on flipping the chainring to the inside of the spider? There are little ledges on both sides, it seems to be viable. Might look a little less sleek, but so do I. I think I'll try that first.
slcbob is offline  
Old 03-19-08, 09:40 AM
  #7  
sivat
Geek Extraordinaire
 
sivat's Avatar
 
Join Date: Feb 2006
Location: Long Beach, CA
Posts: 1,769

Bikes: Bianchi Advantage Fixed Conversion; Specialized Stumpjumper FS Hardtail

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Originally Posted by slcbob View Post
Sivat - flipping the cog would make it 5mm worse, not better. Chainrings are already further out than the cog.
That's why I said flip the cog and run the 103mm bb. Since you're plannine on getting a new bottom bracket anyway, why not. The Sugino 75 track cranks are the only ISO crank that Sugino makes, so that isn't an issue for you. As for the spacing on asymmetrical bottom brackets, i've never found a decent list of drive-side and non-driveside lengths of the shimano bottom brackets. Yet another reason to use the 103mm bottom bracket. It's fewer unknowns to work with. I run a 103mm bottom bracket with track cranks on my crosscheck and don't have any clearance issues with a 46t chainring.
__________________
I'd rather have a bottle in front of me than a frontal lobotomy.

Sintesi Conversion Serotta Track
sivat is offline  
Old 03-19-08, 10:10 AM
  #8  
anti.team
Senior Member
 
anti.team's Avatar
 
Join Date: Dec 2007
Location: Ballard, WA
Posts: 150

Bikes: '98 Kona Kula, '8X Univega Sportour SS, '81 Trek 710

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Originally Posted by slcbob View Post
Any thoughts on flipping the chainring to the inside of the spider? There are little ledges on both sides, it seems to be viable. Might look a little less sleek, but so do I. I think I'll try that first.
I don't know about those cranks in particular, but my SS is set up with the chainring on the inside. Try it and if it works, go with it. I've never seen at as particularly less sleek than on the outside, but some people apparently think it's cheesy. Not me.
anti.team is offline  
Old 03-19-08, 11:09 AM
  #9  
slcbob
bored of "Senior Member"
Thread Starter
 
Join Date: Oct 2007
Location: MD / metro DC
Posts: 1,302

Bikes: Cross-Check/Nexus commuter. Several others for various forms of play.

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 78 Post(s)
Originally Posted by sivat View Post
That's why I said flip the cog and run the 103mm bb.
OOPS! So much for my ability to read.

Originally Posted by sivat View Post
I run a 103mm bottom bracket with track cranks on my crosscheck and don't have any clearance issues with a 46t chainring.
Good to know. I wasn't sure I'd have the space.

I think I'll try the chainring flip, and keep a new BB in reserve if that doesn't work out or I get finicky all of a sudden.

Thanks, all.
slcbob is offline  
Old 03-20-08, 01:55 PM
  #10  
Build your own
%#&*#%>?%
 
Build your own's Avatar
 
Join Date: Feb 2008
Location: Mass
Posts: 845

Bikes: Pake,Shogun,Nishiki,Motobecane

Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Despite what it says on many sites including sheldons the sugino rd gives a 45mm chainline(chainring on the outside) on the 103bb.If you use it on a shimano 107 you should be just over 47mm.
Build your own is offline  
Old 03-21-08, 09:27 AM
  #11  
lymbzero
otherwiseordinary
 
lymbzero's Avatar
 
Join Date: Jan 2006
Location: California
Posts: 697
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Let me apologize.

To tell you the truth, I've never used any chain line facts to gauge chain line on my SS bikes.
It has always been a matter of putting the pieces physically together to determine the straightness.

The one unknown that I always run into is how deep the crank will fit onto the BB taper.
Using old, new, and exotic cranks, you can never really know right off the bat.
After your tighten the bolt it can vary up to 4-10mm!

So thats why I always put stuff together before buying parts.
I know, this seems absurd if you don't have the parts,
but I've always had more than one sq.taper BB lying around.

Hopefully you now know how much difference there is for your BB.
Chainring spacers are always an option, but no more than 2mm is recommended.

PS. I found out about BB a-symmetry when I flipped my Italian threaded BB the other way to fit new cranks so they wouldn't mash my chain-stay, it was like 4mm offset.
lymbzero is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.