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Proper chain length?(5 posts)

Proper chain length?Proboscis
Sep 23, 2001 10:35 AM
Is there a formula for determining the correct chain length when building a bike? I am building a dedicated Triathlon bike. The chain stays are a full two inches shorter than the ones on my road bike and has a different geometry in general. Thanks for any advise.
as a rule of thumbJekyll
Sep 23, 2001 11:20 AM
Wrap your chain on the large chain ring and the biggest cog on your cassette. Do not run the chain through your rear derailleur when you do this. Mark the chain required to do this and add an inch (1 full link).
www.parktool.com (nm)KStone
Sep 23, 2001 11:41 AM
re: Proper chain length?Akirasho
Sep 23, 2001 11:47 AM
... actually, there is a formula...

http://www.parktool.com/repair_help/FAQchainlength.shtml

LONG Formula L = 2(C)+F/4 + R/4 + [(F – R) 2 · (0.0255)] · (1/ 2)·(1/C) + 1

L = chain length in inches. Round the final result to closest whole inch figure. Remember to round up from 0.5.
C = Chain stay length in inches, measure to closest 1/8”.
F= Number of teeth on largest front chainring.
R= Number of teeth on largest rear cog.
(Note: The truly accurate formula includes "2 x pi" as part of the denominator under (F-R) squared. An approximation of 0.0255 is substituted in the interest of simplification.)

For example, a bike has a 53 front ring and a 24 largest rear cog. The distance between the crankarm bolt and the rear axle is 15-7/8”, which using the chart below converts to 15.875”.

L = 2(15.825) + 53/4 + 24/4 + [(53-24) 2 (0.0255)] · (1/2 · 1/15.825) + 1
or
L = 31.65 + 13.25 + 6 + (21.45) (0.0316) + 1

L = 52.68

This is rounded to 53 inches. If the simplified formula were used, the length would have been 52 inches, one inch too short. Check the largest front and largest rear cog before determining which formula to use.

To measure the new chain, lay it on a flat surface with the rollers and plates aligned vertically. Pull on each end to straighten out the chain. Measure from either end. Remember, you can only shorten the chain at whole inch increments. If the chain uses a master link, install one half of the link in the chain for purposes of measuring, and measure including the link.

Fractional conversion chart for 1/8” measurements
1/8 inch; 0.125 inch;
1/4 inch: 0.25 inch;
3/8 inch; 0.375 inch;
1/2 inch; 0.5 inch;
5/8 inch; 0.625 inch;
3/4 inch 0.75 inch;
7/8 inch; 0.875 inch;

I used the above formula to calculate chainlength on two TT/Tri bikes... one with a 56t big ring, the other with a 53t. Both bikes have exceptionally short chainstays (Cervelo P2K with 700C wheels and GT Vengeance with 650C wheels) and this formula worked perfectly.

I've also used the formula on my road bikes as well. Yeah, it's a bit anal, but it works.

We abide.

Remain In Light.
universal method, no formula needed...C-40
Sep 23, 2001 12:53 PM
Two simple tests will determine if the chain is the correct length. First, it must not hang loose in the little ring, little cog combination. If there is no tension on the chain in the little ring, little cog combination, one link at a time, until there is. When the ends of the chain are brought together, some movement of the lower pulley should occur, indicating tension is being applied. On some derailleurs, one more link may need to be removed, beyond the point of absolute minimum tension, to keep the chain from rubbing on itself as it passes under the upper derailleur pulley. This will set the chain as the maximum useable length.

Second, the chain must be long enough not to overextend the rear derailleur when shifted to the big ring and biggest cog combination. If the chain passes the first test, it should always pass this test, unless you're using a largest cog which exceeds the derailleur's stated capacity.