Bicycle chains can skip over cogs for many reasons. It’s important to differentiate and find out how exactly the chain skips. This will save you a tremendous amount of time in finding a solution. Unfortunately, bike chain skipping brings too many possible problems and solutions with it, and just trying random solutions won’t lead you anywhere. However, the good part is, more than half of these are easy and not very expensive problems to fix.
I will try to explain both situations of different chain skipping to start to analyze your own situation and see where and what to start fixing.
Bicycle Chain Skips Gears
Chain skipping gears is the more common of the two situations. It happens all the time, and there are plenty of reasons for it. What usually happens is the chain appears to move on the next cog, but it doesn’t, so it skips between two gears all the time. Most of these problems associated with gear skipping are related to a derailleur and can be detected mid-shifting.
There are, however, situations where gear skipping is not the case of a bad derailleur adjustment but an improper chain line. In this situation, it would appear that the chain simply falls off the bigger cogs, mostly when pedaling backward.
Possible Problems of Chain Skipping Gears
Bad Cable Adjustment
I’ve found that even the highest quality shifter cables stretch a bit in time. It’s not much but more than enough to mess your derailleur adjustment. Sometimes even for one full gear. Two things happen, either you are missing one gear if the cable is completely stretched or lose, or you will experience something like the chain constantly trying to shift into gear, but it can’t. This results in skipping on the cog, and the bike becomes really hard to ride.
You can easily adjust the cable by going to your lowest cog and adjusting the cable tension until it shifts to the 2nd cog without any problems. You can fine-tune this more by checking if the chain is parallel with the cogs next to it and so on until you get perfect shifting performance. Don’t forget to check limit screws as well, as they block the derailleur from pulling the chain out of cogs on each end of a cassette. Failure to adjust them can result in a chain getting stuck and your derailleur destroyed.
Sometimes, adjusting the tension on the cable makes no difference. Multiple things can have this effect, and the first to mention is cables getting stuck or having difficulty moving through the cable housing.
It’s completely normal for cables and housings to get some rust over time. I like pulling them out from time to time and pouring some chain oil in all the housings. This prevents them from ever getting stuck if done regularly. However, if they do, you can try to do this with chain oil like Muc Off Wet Chain Lube to get them moving a bit better. It won’t bring perfect results, but it will get your bike going until you get new cables and housing and change them all. Which, I recommend doing it as soon as possible.
Another of these problems on the list would be a bent derailleur or derailleur hanger. This is actually a widespread one, especially in mountain biking. People often don’t realize that derailleur hangers are made to be bent easily so that the derailleur itself or the frame wouldn’t get damaged.
And, since people rarely look at their bike to see if everything is in place, they usually get baffled by the fact that this even happened to their bike.
Bent derailleur can’t be fixed, not if you want proper shifting anyway. The only solution is to get a new one.
However, a derailleur hanger or a bike frame dropout can be straightened. With the help of a proper tool like the Park Tool Derailleur Hanger Alignment Tool, you can get it in perfect condition. The only downside is that once you do it several times on the same hanger, it gets weakened slightly, and it bends more easily each time.
Bent Teeth On A Cog
Bent teeth or a bent cog can cause the chain to skip gears. The chain goes out of alignment with a derailleur, and it makes the gear change. Even if it doesn’t change the gear completely, the chain gets unseated from the teeth and can skip over them.
Unfortunately, bent teeth and bent cogs can’t be straightened out properly. Not enough to work as they should anyway, except on cheap steel chainrings. Those can be straightened by force, and you might get them to work fine, just because the space between them is wide enough, so the chain doesn’t interfere with the cogs next to it.
With the introduction of boost components, some problems appeared for the chainline.
The Boost technology in bicycles is straightforward. It’s the widening of certain points on the bicycle that hold the rider’s most weight, like wheel hubs. Not only that, but they often get beaten on any technical stuff during a ride. The problem that appears is now two lines of bicycle components can’t really be mixed unless you really know the geometry behind all of that. This is especially important if you have a single front chainring drivetrain.
By widening the hubs, you push the cassette out for around 5mm. So for the chainline to be straight, you need to move the front chainring as well. However, since the number of cogs on a cassette increased over time, the effective chainline changed, and the difference is 3mm between boost and non-boost components.
Long story short, if components are not mixed properly, you might end up with a bad chainline, and that can cause gear skipping and even chain skipping over teeth because they can’t hold a chain that is at a big angle.
This isn’t a common problem, but people usually make this mistake when buying new components. I advise everyone to always check the manufacturer’s webpage for component details. Everything related to the drivetrain will have a chainline specification, so you know exactly which component fits your drivetrain.
Non-boost chainline – 48-49mm
Boost chainline – 51-53mm
Downhill and some other wider chainline types – 54-56mm
Bicycle Chain Skips Over Teeth
The bigger problem is when a chain starts to skip over teeth. In other words, the chain won’t sit properly on the cog. What happens is the chain starts to skip over teeth when pressure is applied on the pedals. This usually happens on climbs because the chain is tightened the most. The pedals will start to drop when stepped on, and it will be impossible to ride uphill. You might even get hurt or fall from the bike.
Possible Problems of Bicycle Chain Skipping Over Teeth
Worn Out Chain
It might just be that your chain is worn out. In my experience, customers at my shop often don’t even realize that chains have limited mileage. Usually, I have to do the whole explanation of how that happens to them. I’m sure my average reader knows this stuff, but I will try to give a short and simple answer just in case someone is new to this.
The basic principle is that chains have those circle links. Since those rotate to place the chain on the chainring, the metal slowly grinds itself away. The tension of the chain is always directed along the chain. So basically, each link gets stretched a bit, and when you add up 100+ links that a chain usually consists of, it easily gets an extra inch in length. This makes the chain sit poorly on the chainring. When tension is applied, it won’t lock onto teeth but rather skip over them.
To check if your chain is worn out and when it’s time to replace it, I always have a Park Tool Chain Checker in my toolbox. It’s important to measure because if you wait until it starts skipping, you’ve already damaged your chainrings too much, and you might need to change them as well.
You can mitigate this process of chain wearing out by regularly cleaning and lubricating it. Making sure that your bike is kept in a dry environment helps as well, as it will prevent rusting. Check my article on chain lubes for more info.
Worn Out Cogs
Cogs wear out much slower than the chain does. The material is grinded away similarly to chain links, except this time the links are grinding against the teeth edges. The better the chain works, the slower this process is. What I mean by that is, a new chain won’t wear out chainrings nearly as much as a worn-out one. When the chain starts to stretch, the force won’t be equally distributed on the teeth, and they will get damaged quickly.
You can prevent chainrings from wearing out quickly by changing the chain before it gets worn out completely and by cleaning and lubricating it regularly.
Stuck Chain Link
It’s usually effortless to notice when a chain link is stuck. Simply because it happens when your chain is completely dry and rusty. However, sometimes it can happen when the chain seems completely fine. In that case, it might not be related to rust and poor maintenance but other kinds of chain damage.
I once had factory damage to one of the chain plates. It had a rough edge which they clearly skipped or had a malfunction while manufacturing. That rough edge was causing that chain link to get stuck each time it went over a derailleur pulley. In the end, I had to use my warranty claim on it and get a new one.
A chain can get damaged during a ride as well. Especially mountain biking chains because you might be forced to jump onto a bigger chainring under a full load of your body. Chain plates can get bent during this, or even worse; a chain-link can snap. That bent chain part can cause chain links to move a lot harder than they should and, in turn, cause chain skipping. But that’s the cost of having lots of fun while mountain biking.
As you see, several things can cause chain links to get stuck. What I do when I suspect this is trying to move each link by itself with my fingers to see if I can detect any resistance on it. Also, checking if the chain is bent somewhere is a good idea while you are at it since it usually goes hand in hand with this problem.
To unstick the link, I like to use Muc-Off MO-94 Multi-Purpose Spray since it’s perfect for getting anything unstuck. And if you can’t get it unstuck at all, you might want to consider replacing that chain link with a quick link. That is, if the chain is newer and other parts of it are perfectly fine. To remove the damaged chain link, you can use Park Tool CT-3.2 Chain Tool. There are cheaper chain tools, but they get bent really fast, tend to damage chai links, and barely any of them can fit all the chain sizes. They are usually 5-10 speed or 11-12 speed only, which is ridiculous, while Parktool’s chain tools work on all chains while its precision and durability are just fantastic.
Chain Is Too Long
Derailleur tension plays a huge role in holding the chain properly on the bigger cassette chainrings. However, derailleurs are, in this case, limited by the length of the chain. If the chain is too long, the tension will be reduced, and you might get some chain skipping on bigger cogs.
Luckily each brand of derailleur has its instructions on how much tension it should have in the biggest cog, so it’s easy to cut it just the right amount. That’s another reason to have a Park Tool CT-3.2 Chain Tool in your toolbox.
Wrong Chain Link Width
The problem appears when people start mixing different speed chains than how many gears their cassette has.
Now, you may ask why anyone would even do something like that. However, there are some occasions where you can get away with this and benefit.
For example, by changing the whole drivetrain to a different number of cassette gears, you might save some money by reusing the old chain. In some other cases, there is a durability issue. There are technologies on some chains that allow them to shift better or be stretch proof for very long mileage. Best brands usually don’t offer these on their cheaper chains, so many people try to improvise their drivetrains with the best they can get.
The thing is, there are different widths of chain rollers (those metal circles around pins that connect the links). Since they have a bit of movement to turn freely, the exact measurement is made between two inner plates.
Single-speed chain – 1/8″ (3.175 mm)
5-8 speed chain – 3/32″ (2.38 mm)
9-12 speed chain – 11/12″ (2.18 mm)
From this, you might assume that 5-8 and 9-12 speed chains are interchangeable. However, they have very different widths of outer plates. Luckily though, they can still sometimes be used for a different number of cassette sprockets with some restrictions.
For example, you can use a narrower chain where it isn’t supposed to be used. Like, you can use a 12-speed chain on a 9-10-11 speed cassette because they both are made for a 3/32″ inner plate width. In some cases, you might lose some shifting performance unless it’s some higher-end derailleur that’s really precise.
Unfortunately, you can’t do the opposite, a 9-speed chain on a 10-11-12 speed cassette. You can’t because the outer plates are wider, and the chain would get stuck between cassette sprockets and break. Not in every combination but lots of time, that’s the case.
The beauty of this is that since the inner width is the same, you can effectively use any 9-12 gear setup with single front chainrings made for 12-speed drivetrains. That allows you to combine the best one by front chainrings, which are the key for these drivetrains to work well, with cheaper components for the rest of the drivetrain. Allowing for some really affordable one-by-drivetrain conversions.
In some rare cases, freehub might start skipping. The tricky part is, it feels exactly like chain skipping over teeth, and unless you can see what’s happening, you wouldn’t know the difference. Unless it’s making noises while pedaling, then it kind of gets obvious because the noises are usually nothing like a chain can sound.
Freehubs have these pawls that allow it to turn in one, but they block the movement in the opposite direction. They are constantly retracting during rotation. In older freehub bodies, the grease gets dry, and sometimes some rust appears on pawls. They start to have difficulty retracting and can’t properly block the rotation of the freehub. This results in chain pulling freehub rotation, but the wheel doesn’t move. However, in most cases, this happens from time to time, and the result is the same as chain skipping.
It would be best to change the freehub body or the whole wheel if you can’t get the exact one. However, I’ve found that Shimano Hydraulic Mineral Oil can get it to work well once again, although it doesn’t last forever. If you know how to take the freehub off the wheel, you can easily do this by yourself every so often to keep it working.
In some cases, freehub pawls are on the outside, so when you take them off, you can actually service them completely. Clean them from any rust and grease/oil them properly.