Length Does Matter: Chain length is one of those important setup points that often gets overlooked both when building up a bike from scratch, or when replacing a worn chain. In fact, it is so common that there is a good chance that many of the bikes on your local shop's showroom floor are sporting too long of a chain, an oversight from the factory that can often lead to dropped chains or inconsistent shifting. While an over-length chain can cause some annoying issues, one that is too short can be downright catastrophic. The best case scenario is that your bike will refuse to shift to the larger sized cogs or chain rings due to too much chain tension. Worst case: you could not only rip off your bike's derailleur hanger or destroy the chain and derailleur itself but even bend over the chainring due to the massive forces involved. It's fair to say that a bit of carelessness when it comes to chain length can quickly make for an expensive repair bill.
Derailleur Cage Counts: Your bike's rear derailleur also plays an important role in managing chain length. Its hanging cage and pulley wheels take up the chain's extra slack when in small cog and chain ring gearing combos, but the spring-loaded cage can also rotate forward to compensate for added chain tension when you are in a larger sized cog or chain ring. Derailleurs are available with short, medium, and long cage lengths depending on what your bike requires. The general rule of thumb is that the larger the gearing range, the longer the derailleur cage needs to be. This is because of the massive difference in chain slack when in certain gearing combos on bikes with three rings and wide range cassettes, while bikes with only a single chain ring can often get away with a short cage derailleur. Using a short cage derailleur on a bike with a very wide gearing range (a triple crankset, for example) will require a chain that is overly long to compensate for the lack of capacity in the derailleur's short cage, while the opposite is true of a long cage derailleur on a bike that doesn't necessitate it - there will be no happy medium setup. In short, the wrong length derailleur cage will make determining proper chain length nearly impossible.
Too much chain length and you'll have shifting issues and suffer from dropped chains, but too little could rip your bike's derailleur right off. Have we scared you into checking the chain length on your bike?
Some helpful pointers
• The term 'chain growth' refers to the distance between the bottom bracket and rear axle lengthening as the bike goes through its travel. It is important to determine chain length on a full suspension bike when it is in its fully compressed position, and with the chain in the largest cog and chain ring. This will tell you the maximum chain length required. • While today's 10-speed, dual chainring setups can often be ridden in the big ring and big cog combo (otherwise known as being 'cross geared'), it isn't recommended for bikes with triple ring cranksets. Even so, it is best to set chain length when in the big and big combo simply because it isn't uncommon to accidentally shift into such a gear by accident when out on the trail. Better to be safe than sorry! • Advanced riders on bikes with double or triple ring setups will sometimes purposely run the chain too short for the bike to fully bottom when in the big cog and big chain ring combo. This setup puts the onus on the rider to not shift into such a gear in order to prevent damage, but is done to add chain tension when in more common gearing combos to limit noise and the chance of losing a chain. No, we don't recommend it. • New bikes will often ship from the factory with a stock chain length that is far too long. Just because your bike is brand new from the shop doesn't mean that its chain length is correct. It's always best to double check. • Derailleur cage length varies between Shimano and SRAM, and there can even be differences between different model years of the same derailleur. This means that a new derailleur necessitates checking chain length.
The Full Suspension Factor: Setting up your bike with the proper chain length isn't a difficult task, but the job does become more complicated if you ride a full suspension bike. How so? Many designs use a layout that, whether employed intentionally to increase pedaling performance or not, moves the bike's axle further away from the bottom bracket as the bike goes through its travel (this is known as 'chain growth'). This rearward axle path may be present throughout the entirety of the travel, or only during part of it, but it does mean that a full suspension bike will nearly always require more chain length to compensate.
Bottom the suspension - Full suspension bikes will often have the most chain growth when at full travel, so this is where you'll have to size the chain. If your bike uses an air shock, simply let all of the air out by depressing the shock's schrader valve as you compress the suspension (be sure to write down your air pressure before doing so), or remove the valve core to allow all of the air to escape. Bikes with a coil shock will require the shock to be removed, the coil taken off, and the shock to be reinstalled.
Place the bike in a repair stand and shift to the largest chain ring and cog combination before lifting up on the rear wheel in order to fully bottom the suspension. Either have a friend hold the bike in this position, or place it on the ground.
Chain too short - Your bike's rear derailleur will tell you the entire story. The photo above, taken when the the bike is fully bottomed out, shows the derailleur cage pulled fully forward and nearly parallel with the bike's chainstay, and the upper pulley wheel is also making contact with the largest cog. This setup would equal disaster if the rider was to bottom the suspension while in this gearing combo, likely resulting in damage to both the derailleur hanger and the derailleur itself. Links must be added to the chain.
Chain just right - Note the angle of the derailleur cage compared to its postion in the second step. Adding just two links to the chain allowed the derailleur cage to relax, leaving enough slack to prevent any damage from occurring.
Just loose enough - Shift the bike to the smallest chain ring and smallest cog - the combination that will result in the most chain slack. The photo on the left is close to the ideal setup, with the chain still having enough tension from the derailleur cage to keep it from hanging loosely. Conversely, the photo on the right shows the derailleur's cage in its fully relaxed position, letting the chain hang too freely with not enough tension. This setup could result in sloppy shifting or dropped chains over rough terrain.