Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  


Go Back   > >

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.

User Tag List

Reply
 
Thread Tools Search this Thread
Old 04-09-14, 07:56 AM   #1
Phil_gretz
Bike Commuter in training
Thread Starter
 
Phil_gretz's Avatar
 
Join Date: Aug 2009
Location: Alexandria, VA
Bikes: '88 Fuji Saratoga, '12 Jamis Sputnik, '13 Motobecane Fantom29 HT, '16 Motobecane Turino Pro Disc, '16 Motobecane Gran Premio Elite
Posts: 3,807
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 49 Post(s)
Chain Rub Outer Chairing From Inner Chainring

Uggh. Sorry I don't have photos available...

I'm savaging an early 80's Fuji Grand SE that had a worn out Fuji-branded Sugino crankset. The type with a five-fingered swaged outer chainring. I dug out a three-fingered Sugino Maxy (52 x 40) of the same era and mounted it to the original BB spindle, after removing and repacking the BB. The freewheel is 6-speed SunRace (blech), and the chainline appears centered (that is, the large to 3rd highest cog appears straight, as does the small-to-3rd lowest cog).

The chain is a new KMC Z-series 6/7-speed.

So here's the problem: When in the inner chainring, the chain rubs against the inside of the larger chainring, when the rear is in the highest (outermost) cog, and when in the neighboring 2nd highest AND 3rd highest cogs.

The inner chainring is removable, and there are original spacers. I'm thinking that changing the spacers so that there's no rubbing in the 2nd highest - that's the right solution. Meaning that I'll allow the chain to rub the outer chainring when fully cross chained, as a reminder to the future owner not to cross chain.

I haven't measured yet, but I'm thinking that this might be a move of about 2-3mm inboard for the inner ring. So, I'm creating a gap between chainrings (I'll need to check whether the chain can get trapped between shifts).

My other option is to swap on a different crankset with smaller difference...

Thoughts on allowing the crosschain to rub, or should I move the inner chainring further?

Thanks, PG
Phil_gretz is offline   Reply With Quote
Old 04-09-14, 09:39 AM   #2
uluchay
Squeaky Bottom Bracket
 
uluchay's Avatar
 
Join Date: Jul 2013
Location: Lake Como, Italy
Bikes: 2009 Trek 1.2 // 70's Olmo Gentleman
Posts: 164
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Seems to me that your problem (presuming you use square tapered BB) lies in the spindle length of the bottom bracket. If you pick a BB with a larger spindle length it will not rub.
uluchay is offline   Reply With Quote
Old 04-09-14, 09:54 AM   #3
FBinNY 
Senior Member
 
Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Posts: 29,660
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 217 Post(s)
It sounds like a chain line issue. it's likely that your crankset is inboard compared to the position of the cassette.

There are two common causes. Either your spindle is short for the crankset and the crank is too far inboard, or your frame is bent and the rear triangle is shifted to the right.

Start with a basic diagnostic by placing a straight edge against the face of the outer chainring on a secant, and carrying it back to the freewheel. Allowing for the distance from the outer face, to the center of the space between the rings, your straight edge should meed the freewheel about 7mm or so to the right of center.

If it doesn't line up that way, find out whether it's the spindle or frame, by checking each, measuring the chainline from the frame, and the frame via the string method (do a search if you don't know how these are done).

Two other possibilities.

If the initial crank to freewheel test passed, then your rings are too closely spaced for the chin, ie. a 7s crankset w/ a 6s chain. Or if it failed, but the chainline and frame are correct, then it's possible that the BB shell is twisted with respect to the frame's plane. This is very rare, but not at all unheard of.
__________________
FB
Chain-L site

An ounce of diagnosis is worth a pound of cure.

“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin

“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN

WARNING, I'm from New York. Thin skinned people should maintain safe distance.

Last edited by FBinNY; 04-09-14 at 10:01 AM.
FBinNY is offline   Reply With Quote
Old 04-09-14, 12:07 PM   #4
Phil_gretz
Bike Commuter in training
Thread Starter
 
Phil_gretz's Avatar
 
Join Date: Aug 2009
Location: Alexandria, VA
Bikes: '88 Fuji Saratoga, '12 Jamis Sputnik, '13 Motobecane Fantom29 HT, '16 Motobecane Turino Pro Disc, '16 Motobecane Gran Premio Elite
Posts: 3,807
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 49 Post(s)
Gentlemen,
Thank you for your feedback. So, it seems that my assumptions about correct chainline must be tested and measured. Thank you for the discussion.

By using the former Sugino spindle, I probably do have the chainrings too far inboard. Measurements will tell. Fortunately, I do have a number of steel spindles, so will likely have one that's a few mms longer, but the measurement will tell me how much more I need. Thanks again.

I'm familiar with the string method for checking chainstay/dropout symmetry.

We'll get this right yet! Phil
Phil_gretz is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

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



All times are GMT -6. The time now is 11:38 AM.