Chainline refers to the distance from the frame’s centerline to the tip or the middle chainring. What is optimal chainline? It is the relationship between the cassette gears, the chainrings, and how it affects front shifting.
The crank may be too far from the bike’s centerline if the chainline is too narrow. If this happens, the chain could rub on the large ring or catch on the small rear cogs.
The crank may be too far forward if the chainline is too long. If this happens, the chain could become distorted from the middle ring if you use the larger rear cogs. Many low gears in the rear can be difficult to use when riding in the big rings due to severe chain crossover.
After shifting the chain onto both the middle ring & 5th cog, it is easy to visually check the chainline. To check if the chain is aligned with the bike’s centerline, look at the bike from the back. You can draw a line between the two rings on the bike and the fifth cog if it has two rings.
Modern mountain bike external bottom brackets come with up to three 2.5mm spacers. These are designed to fit between the threaded bottom bracket cups of the frame. These 2.5mm BB cups spacers will determine your chainline. Your frame, cranks and front derailleur will determine the exact location of these spacers. For proper application, always refer to the instructions for your crankset.
Incorrect BB Cup Spacer Use could lead to bearing damage, shifting/chainring function issues, and even bearing failure.
There are currently four standards for mountain bikes regarding the width of the bottom bracket shell. These are 68mm to 73mm, 83mm, and 100mm. Before purchasing a crank/bottom bracket system to fit a bike, measure the frame and verify the correct BB shell width. When ordering replacement parts, be sure to specify the measurement.
Race Face cranksets have one-millimeter spacers at the bottom of their brackets to simplify chainline adjustment. These spacers are placed between the crank arms and the cups of the external bottom bracket. They fit over the spindle at the bottom.
Race Face cross-country cranksets come with one 1mm spacer for the chainline, one spacer for the chainline, and one spacer to mount to the left crankarm. This gives you a nominal chainline length of 50mm. Simply remove the spacer on the left side of the chainline and place it on the right (51mm chainline). You can move the chainline forward by 1mm by removing the spacer on the right side. Install it on the left (49mm).
Our gravity cranks offer 6mm of chainline adjustment via six 1mm spacers to accommodate a wide range of frame and guide options.
MORE TECH TIPS IN JULY 2008 PRINT MASTERBASE!
This is only a small selection of tech tips found in the most recent print edition. There are 16 pages of Tips From the Troops, which include setup tips from the manufacturers of the most sought-after mountain bikes. Get your copy today
