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innate behaviour
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Discussion Starter · #1 ·
1. On a chainring that says something like 34t x 94bcd.....what the heck does bcd stand for?

2. How do I know if I need 94 or 104 etc??
 

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jet9n8 said:
1. On a chainring that says something like 34t x 94bcd.....what the heck does bcd stand for?
Bolt Circle Diameter

http://www.sjscycles.co.uk/article-Bolt-Circle-Diameter-Size-Chart-BCDChart.htm

jet9n8 said:
2. How do I know if I need 94 or 104 etc??
How I'd do it:
  1. Look at my current crank to see what make and model I have
  2. Look up my crank online to see if the manufacturer says which bcd it is
  3. Pick new ring that is advertised to those cranks

Alternate method:
  1. Look at my current crank to see what make and model I have
  2. Ask on MTBR, "What bcd fits my [specific make and model] crank?"
 

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411 on BCD

2. Take your chainring and measure (in millimeters) the distance between two bolt circles. Make sure to measure center to center from the two bolt circles. Mountain bike applications usually are 104 mm BCD.
 

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mtbnachos said:
2. Take your chainring and measure (in millimeters) the distance between two bolt circles. Make sure to measure center to center from the two bolt circles. Mountain bike applications usually are 104 mm BCD.
Aren't all 4-bolt mtb rings 64/104? If that's the case, then there's no need to measure anything if you have a 4-bolt crank. Just buy a 4-bolt ring and it'll fit.

5-bolt patterns have standard and compact options.

Road has its own.
 

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Most rings will be stamped w/ the bcd and tooth count (look lower right approx. the 4 o'clock position on my chainrings)

 

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Nat said:
Aren't all 4-bolt mtb rings 64/104? If that's the case, then there's no need to measure anything if you have a 4-bolt crank. Just buy a 4-bolt ring and it'll fit.

5-bolt patterns have standard and compact options.

Road has its own.
unless it's 95x series xtr
 

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4 bolt is 104/64BCD unless you have a M950/M952 XTR which is 112BCD.

5 bolt MTB is either 94/56BCD compact or 110/74BCD standard.

5 Bolt Road is 110BCD Compact or 130BCD Standard, some Campy stuff is 135BCD

Track is 144BCD.

The second number is the granny's BCD if two numbers are spec'd. And yes, road compact and MTB standard cranks take the same rings apart from the granny.

Most common MTB 5 bolt cranks are 94bcd, standard cranks became uncommon in the mid 90's.
 

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innate behaviour
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Discussion Starter · #11 ·
thecrazyfinn said:
4 bolt is 104/64BCD unless you have a M950/M952 XTR which is 112BCD.

5 bolt MTB is either 94/56BCD compact or 110/74BCD standard.

5 Bolt Road is 110BCD Compact or 130BCD Standard, some Campy stuff is 135BCD

Track is 144BCD.

The second number is the granny's BCD if two numbers are spec'd. And yes, road compact and MTB standard cranks take the same rings apart from the granny.

Most common MTB 5 bolt cranks are 94bcd, standard cranks became uncommon in the mid 90's.
Hmm...ok. Sounds like mine are 94bcd. They are 5 bolt Middleburns. What do you think??
 

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jet9n8 said:
Hmm...ok. Sounds like mine are 94bcd. They are 5 bolt Middleburns. What do you think??
I don't know Middleburns, but look at the link posted by Nat. You can't measure straight across and get the diameter on a 5 bolt design, so you measure two adjoining holes and use a conversion chart.
 

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thecrazyfinn said:
4 bolt is 104/64BCD unless you have a M950/M952 XTR which is 112BCD.

5 bolt MTB is either 94/56BCD compact or 110/74BCD standard.

5 Bolt Road is 110BCD Compact or 130BCD Standard, some Campy stuff is 135BCD

Track is 144BCD.

The second number is the granny's BCD if two numbers are spec'd. And yes, road compact and MTB standard cranks take the same rings apart from the granny.

Most common MTB 5 bolt cranks are 94bcd, standard cranks became uncommon in the mid 90's.
typo: XTR M950/952 is 102, not 112.
 

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I think Tommi meant to say M960 is 102mm. (Which is listed wrong on Sheldon's site.)
M95x came with a range of ring and spider combinations, 112mm was the DH single ring setup. But you could also get it with an integrated big ring/spider as well as compact 5 bolt and standard 5 bolt.

jw

thecrazyfinn said:
Not a typo, M950 is indeed 112mm not 102mm, it's larger diameter than XT 4-bolt (which is the genesis of 104BCD 4bolt).

Look it up yourself

http://www.sheldonbrown.com/gloss_bo-z.html
 

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