i'm putting together a 42-tooth front ring with a 9speed 12-26 cassette on my poprad frame. as it is i'm not much of a wrench so i'm sorry if this is kindof a stupid quesiton. is there any specific rule of thumb for chainlength? -i'm running a medium-cage derailleur
the reason i ask is i'm wondering if standard rules for chain-length do or do not apply....like, should my chain be shorter on this rig to increase chain tension and decrease chain-drop concerns? (i'm also running a third-eye on the seat-tube and a bashguard outside)
is this as simple as eye-balling my derailleur when it's in the smallest and largest cogs or what?
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.
Example: A bike has a 42-32-22 front chainring set up. Use only the 42 for the equation. The rear cog set has 32 tooth largest cog. The bike measures 16-3/8” from the center of the rear axle to the center of the crank bolt. The decimal equivalent for 16-3/8” is 16.375 inches.
L = 2 (16.375) + 42/4 + 32/4 + 1In the example above, this becomes 32.75 + 10.5 + 8 + 1 = 52.25 inches. Chains cannot be joined at 52.25 inches, so this length is rounded to 52 inches.
For this system, round up from 0.5. For example, a chain length figured to be 55.5 would be rounded to 56 inches.
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 it on one of the chain for purposes of measuring, and measure including the master link.
I'm a little more "hand-on". Shift the bike into the largest cog on the back and route the chain through the derailers. I use an old hanger or spoke that is cut into a piece 6 inches long and bent to a L shape on both ends to hold the chain together. Like this but inverted L_______J. Then try different lengths until you find the one you want and put the chain back together.