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Discussion Starter · #1 ·
Can someone tell me how to determine chainline position. For example in RaceFace instruction after determining BB requirements ie. 68MM and installing the appropriate spacers you have to determine Chainline Position Requirements. They state the number of spacers required for 48,49 and 50MM chainline.

Question is where and how is it determined if you 48,49 or 50MM? Thanks you in advance
 

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it is measured from center of seat tube to the middle ring on a triple, or the inner ring on a double.

If you have indexed front shifting I think shimano prefers 47 or 50mm chainlines.

another rule of thumb is that the chain should line up front to back when on the middle ring and middle cog (effectviely means the crank is centered with the cassette)
 

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There will always be a tiny bit of variability, but in general a 9 or 8 speed cassette installed on a hub mounted to 135mm dropouts will have a chainline at around 47.5mm. This doesn't vary from bike to bike, but may vary a little with different hub and cassette combos.

Front derailleurs generally have a working range of 47.5 to 50.0mm. You can get away with a touch wider, but you run into trouble going narrower -- especially if you're mounted to a fat 34.9mm seat tube.

Generally 47.5 is considered an ideal chainline, but you may go wider for a few reasons:
- Fat aluminum chainstays or suspension links cause crankarm clearance problems
- Short chainstays cause the chain to rub on the big ring, when shifted to the middle ring and smallest cog
- Fat seat tubes cause difficulty shifting into the granny ring
- 73mm bottom bracket shells cause the crank arm to rub the BB cartridge

What you can garner from this is that a 50mm chainline is acceptable for many reasons, and therefore is acceptable if you meet none of those criteria.

Be mindful of the actual chainline of gear combinations you commonly use. If you regularly find your self in the mid ring - big cog combo, every millimeter you bump the crank out results in that much more wear and tear on your chainring, cassette and chain.

Based on this, I'd attempt to set it up as narrow as you can go while maintaining necessary clearances and shifting effectiveness.
 

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Discussion Starter · #6 ·
Wow! excellent explanation speeddub. Between you and 05 stumpy I think I have it nailed. It does make sense. What I did was I put my digital depth gage though the crank arms where the pedals mount then position the crank to measure the distance on each side to the down tube to determine if the cranks are centered to frame center line. I then measured the second chain ring center to the center of the seat tube and walla! The measutrements were less than .5 mil difference equalling just over 448.7MM. Thanks again
 
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