Chain length is often overlooked when assembling a bicycle or replacing a chain. In fact, it’s common to see the chains on display bikes in shops being too long. This small oversight from manufacturers can easily lead to chain dropping or poor shifting during rides. On the other hand, a chain that’s too short can have catastrophic consequences, as it prevents the chain from engaging the largest sprocket on the cassette and keeps the chain under excessive tension. The forces exerted on the chain during riding are tremendous—they can even damage the derailleur or snap the chain. Not to mention the costly repairs resulting from such negligence.
About Rear Derailleur Selection
Your bike’s rear derailleur plays a crucial role in determining chain length. It allows for extra slack in the chain when using the smallest sprockets on the cassette and chainring, and provides tension compensation via the derailleur spring when using the largest sprockets. Rear derailleurs come in long, medium, and short cage variants. The general rule is: the higher the gear ratio you need, the longer the cage you should choose, as this accommodates more gear combinations. If you opt for a short-cage derailleur with a high gear ratio crankset (like a triple crankset), you’ll need a longer chain to compensate for the reduced compatibility. Choosing the wrong cage length can easily result in an incorrect chain length. (Editor’s note: In other words, if you’re using a triple crankset with a high gear ratio cassette like those on mountain bikes, go for a long-cage derailleur. For others, a medium cage is usually fine. Based on our experience, short cages are not recommended for casual riders—they’re typically used for single-chainring folding bikes paired with road cassettes, as well as for aggressive off-road bikes at the FR to DH level.)
(A chain that’s too long makes shifting difficult and increases the risk of dropping the chain, but one that’s too short will quickly spell the end for your derailleur—so checking your chain length is essential!)
Some Suggestions
The maximum straight-line length of the upper part of your chain is equal to the distance between the largest chainring and the largest sprocket. Doubling this figure gives you an approximate chain length. (For full-suspension frames, ensure the suspension is fully uncompressed.)
With the popularity of 10-speed and double-chainring setups, many have abandoned triple chainrings because they can’t accommodate the largest chainring paired with the largest sprocket for optimal performance. However, it’s still safest to measure chain length based on the largest chainring and sprocket.
Some experienced riders use short-cage derailleurs to improve shifting speed and maximize chain tension on climbs. But this requires avoiding the largest sprocket on the cassette—otherwise, as mentioned earlier, your derailleur could be toast. We don’t recommend this choice for average riders.
New bikes often come with stock chains that may not be the ideal length, so always double-check upon purchase.
Shimano and SRAM derailleurs have slightly different cage lengths, and even models from the same brand and year may vary—so always verify before installation.
Full-Suspension Bikes
The correct chain length is actually quite straightforward. For full-suspension frame designers, the rear axle is often positioned farther from the bottom bracket to enhance pedaling performance (Editor’s note: because the rear suspension compresses, altering the position of the bottom bracket and rear axle). As a result, full-suspension bikes usually require longer chains to compensate.
This is what a full-suspension bike looks like when the suspension is fully compressed. If your bike uses air suspension, release the air (be sure to note the original pressure), or if it has a coil spring, remove it.
Place the bike on a repair stand—don’t leave it on the ground or ask someone to hold it (Editor’s note: this ensures maximum suspension compression. Releasing air from air suspension prevents it from slightly compressing due to internal pressure, which could protect the rear triangle from damage). Shift to the largest chainring and largest sprocket.
This is what a chain that’s too short looks like. Your rear derailleur will indicate whether the chain length is correct. As shown, the derailleur is pulled all the way forward, with its cage parallel to the down tube. If you ride in this gear and pedal hard, prepare to say goodbye to your derailleur and chain. In this case, you’ll need to add a few links to the chain.
This is the angle of the derailleur after adding two chain links—it’s just right, leaving enough slack to handle sudden stress.
Shift to the smallest chainring and smallest sprocket. The left image shows the ideal setup, while the right image shows a chain that’s too long and overly loose. The derailleur spring pulls the chain backward on the guide pulley, reducing tension and causing chain dropping or poor shifting.
Xingzhe Wuyu Outdoor Network Cycling Knowledge (http://bbs.xzwyu.com) From: Bicycle Network