The Dark Art of Shifting
How to Avoid Chainsuck
“Chainsuck” is when your chain does not detach correctly from the bottom of your chainring, and instead wraps around the ring until something jams. Chainsuck happens when the force provided by the rear derailleur spring is less than the force causing the chain to stick to your chainring. There are many ways to reduce chainsuck, broadly speaking there are two categories:
1. Preventing the chain from sticking to the ring.
2. Maintaining chain tension while riding.
Let’s start with the chain sticking to the ring past the correct departure point. The most important thing you can do is to keep your drivetrain clean and your chain lubricated. If you do nothing else to your bike, you should at least wipe the chain down with a rag and put some lube on either before or after almost every ride. If it’s wet out then use a wet lube, like Finishline Wet Lube or Pedros Syn Lube. Sometimes it’s necessary to actually clean and lube the chain during a ride.
A common cause of sticking is a worn drive train, particularly if your chainrings are more or less worn than your chain. Drivetrains work best when they wear together as a unit. If it’s time for a new chain, then be sure to carefully check your rings. An experienced mechanic’s eye can help you identify worn rings. If your chainring teeth look like shark fins and the valleys between them look like wide U shapes, then it’s time to replace them. The back of those shark teeth make excellent chain grabbers. If you can, look at a new chainring (of the same brand and model) to compare your worn ring to a new one.
Another very common cause of chainsuck is mud. It all depends on what type of mud you have, but if the drivetrain gets too dirty, the chain is going to start sticking and it will take careful chain management to keep it from sticking.
Now we’ll discuss maintaining chain tension. Let’s assume that your chain is the minimum length needed to go around the big chainring and large cog at bottom out (standard procedure with full suspension bikes is to let the air out of the rear shock, then move the suspension to bottom out and determine minimum chain length then). The more tension you have pulling the chain off the chainring, the less likely the chain will become stuck to the teeth. The component that provides this chain tension is the rear derailleur. However, the derailleur’s pulling force is not always the same. It pulls less in smaller gears and more in larger ones. That means you should use your small chainring as little as possible. If you’re in the small ring and more than three or four gears down from the top of the cassette then it is time to shift back into the middle ring for a larger diameter gear combination. For example, instead of running a 22 front 24 rear, run the 32 front 34 rear for the same ratio but much higher derailleur spring tension (an added benefit of this ring combo is lower tension on the upper drive portion of the chain thus reduced chain wear). Another way to increase your average chain tension and still use the small ring is to remove the big chainring and shorten the chain accordingly while riding during the wet season (if you have one). This will provide significantly more chain tension. If you’re trudging though a lot of sticky mud you probably don’t need that outer ring.
There are a couple of other tricks related to tuning your rear derailleur that you might want to try. One is to tighten your rear derailleur’s B tension screw a few more turns than normal to increase your derailleur’s spring force. This nice write up from Park Tool tells how to do that: http://www.parktool.com/blog/repair-help/rear-derailler-adjustments-derailleur. The second is to increase your derailleur spring tension. This article from Park Tool describes how to do that: http://www.parktool.com/repair/readhowto.asp?id=76. Note that not all derailleurs have this feature.
Now that you’ve done what you can to increasen chain tension, you need to know about a few things that can cause a sudden decrease in tension. One is shifting from a larger to a smaller chainring. Because the ring diameter is decreasing, the derailleur has more slack to take up and it can’t do that instantly, so while the rear derailleur is moving to the new position the chain is momentarily de-tensioned. Another factor that influences the chain sticking to the chainring is to suddenly pedal harder as the higher chain force against the chain ring teeth can wedge the chain in place. This works both ways, if you ease up a bit you can actually reduce the chain/chainring stick. If you put the last two concepts together and ease up a bit right as you shift then you’ll have better results. Since you usually can’t ease up like this in the middle of a steep climb, you should anticipate and get front shifts done before you need them. Your drivetrain will thank you. Not shifting under power is one of the most important factors in mitigating chainsuck.
Suspension and Chainguides
Most of what causes chainsuck is covered above in the sticky ring and chain tension section. There are other factors that can contribute to chainsuck. The first is related to suspension. Most modern suspension designs use a little chain growth to create anti-squat and keep your bike from bobbing. “Chain growth” happens when the distance between the bottom bracket and rear axle increases as the suspension moves through its travel. When the suspension rebounds, the distance between the rear axle and bottom bracket becomes shorter and creates slack in the chain, thus reducing tension. There’s no real way around this phenomena. The good news is that provided you do everything mentioned above you should not see chainsuck due to suspension rebound.
Managing chainsuck is all about managing the combination of factors. For example, if you’re riding in the mud, stay out of your granny gear if possible. If you do have to shift down try to plan ahead so that your suspension is not rebounding at the same time as you suddenly put down full power.
One misconception we’d like to clear up is thats low chain stays do not cause chain suck. They can make it more difficult to extract the chain if it gets caught above or against the chainstay but it didn’t cause it in the first place.
Another factor that is not commonly discussed is that chainguides can promote chainsuck. The lower roller of a dual ring chainguide doesn’t provide any additional spring tension and in fact is there to keep your chain on your ring past where it normally detaches. Depending on the roller design, it can increase chainsuck because the chain is forced to go through a step on the roller at the same time it’s being forced the other direction on the rings. To do that the chain has to twist in a small space and that increases the chance of the chain sticking to the ring.
Finally, aftermarket or new parts are by no means a guarantee that chainsuck will be solved. In addition to the factors discussed above, some chainrings are better than others at releasing the chain, even when new. We have had the best luck with Shimano and SRAM chainrings.
In summary, keep that drivetrain clean, lubed and fresh, stay out of the mud and remember to anticipate your shifts. And don’t talk with your mouth full.