Bicycle Mechanics - New chain - how long should it be?

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PDA
TCR
11-16-07, 07:58 PM
Current setup:
53 X 39 chain rings
12 X 27 cassette

Will occasionally switch to a 11 X 23 cassette.

I've been told that the rear jockey wheels should be perpendicular to the floor when in the big/small combo. However, as can be seen in the pics below it's not. So, when I install my new chain do I make it the same length as the old chain that's on there or do I make it so the jockey wheels line up perpendicular?

1st pic shows how the jockey wheels line up with a plumb-bob currently. 2nd pic shows how the current chain looks when the jockey wheels line up perendicular. Not sure how many links that would translate to but it looks like a big sag.

http://webpages.charter.net/vvbc/otherimages/chain1.jpg


http://webpages.charter.net/vvbc/otherimages/chain2.jpg

operator
11-16-07, 08:16 PM
http://www.parktool.com/repair/readhowto.asp?id=26
TCR
11-16-07, 09:11 PM
http://www.parktool.com/repair/readhowto.asp?id=26

I guess it could have been worse. You could have just posted this:
http://www.parktool.com/images_inc/repair_help/rigorous.jpg

TCR
11-16-07, 09:21 PM
Hmm, interesting, when it's in the small/small the chain is almost hitting the RD, meaning the chain is almost too long (I think).



Pic below from PT shows a chain too long. Mine isn't touching or sagging but it's really really close to touching the bottom of the RD. I'm thinking I can safely go a link or two shorter on the new chain.
http://www.parktool.com/images_inc/repair_help/chain-long3.gif
PDXJeff
11-16-07, 10:26 PM
Try using the "simple equation" for both cassettes and size it to the average.


Park Tool
SIMPLE EQUATION: L = 2 (C) + F/4 + R/4 + 1
L = Chain length in inches. Round the final result to closest whole inch figure.
C = Chain stay length in inches, measure to closest 1/8”. Use chart below to find decimal measurement.
F= Number of teeth on largest front chainring.
R= Number of teeth on largest rear cog.



Fractional conversion to decimal for 1/8” measurements:
1/8” = 0.125”
1/4" = 0.25”
3/8” = 0.375”
1/2” = 0.5”
5/8” = 0.625”
3/4" = 0.75”
7/8” = 0.875”
Retro Grouch
11-17-07, 07:51 AM
It's important that the chain be long enough to safely cover the big/big combination. If it's not and you accidentally shift into that combination it can cause a lot of expensive damage to your derailleur, wheel and frame.

I use the big/big chain sizing method as suggested in the Park Tool site. I even use it when replacing an existing chain because it only takes a minute I don't know how the old chain was sized. That way I'm sure of getting it right. If, after using the big/big method your chain goes slack in the little/little, you need to get a rear derailleur that has more slack take up capacity.