The thing about chainring wobble is that it messes up with a lot of stuff on your bike.
The first that comes to my mind is component damage. Chainring wobble means the chainring and the chain are moving left and right in correlation to the frame. The chain is catching sprockets at a weird angle, increasing wear on both. The riding force is unevenly transferred from the chain through the spindle to the bottom bracket bearings due to wobble. The derailleur gets damaged out due to the chain constantly scraping against its cage in some gears.
To put a conclusion to that, chainring wobble is not something that should be overlooked. Luckily for you, it’s a common sight to us, mechanics, so we know all the possible problems and solutions to apply.
How to Know if Your Chainring Is Wobbling?
A chainring wobble can be quite evident. In most situations, it will be obvious that something is wrong. As mentioned above, you can hear or feel several abnormal things happening with your drivetrain.
However, at that point, you probably won’t know for sure if it’s the chainring wobble or a dozen other things that can cause those issues.
It’s time to investigate what’s really going on.
A good start is taking a ride and determining what exactly do you feel or hear while riding.
If it was a non-constant scraping noise, that could mean two things. Either it is the chain scraping against the derailleur cage, or the chain is scraping on the sprocket teeth because of a bad angle. The noise won’t be continuous but rather in equal intervals. In both cases, you can almost be sure that it is a chainring wobble, and it’s time to find out what’s causing it.
If you can feel and hear a clicking, it can be that the chainring is wobbling. However, it can also be that there is something wrong with the chain or cassette in the back. A further checking out is necessary in this case.
How to make sure it’s the chainring wobble you are dealing with?
As I already mentioned, most of the time, it will be obvious once you turn the chainrings really fast. However, sometimes the wobble is small and not really noticeable at first. A good method is using something as a reference point. You can take a screwdriver and put it up against the frame so that the screwdriver’s head is close to the sprocket you want to check. Then you will easily notice the wobble.
There are situations where the wobble is not present during a normal rotation, but only when the bodyweight pressure is applied.
You can check that by grabbing both crankarms with your hands and trying to pull on them horizontally. You are trying to see if there is any play in the cranks or the bottom bracket. That play transmits to chainring wobble under heavy load, so it’s one of the things I will discuss in this article.
What Causes a Chainring to Wobble?
All the possible causes are equally common in my experience.
I like to start with the most logical ones. So the first thing I would like to mention is a sprocket being bent. The whole chainrings can be bent as well. It’s something that can happen on a new or used bike. On new bikes, it usually happens with the cheaper bikes on a component in a range of between Shimano Tourney and Shimano Altus. Other brands included, of course. Those components are so massively produced that mistakes and damages happen often.
On used bikes, it usually happens when crashing or accidentally damaging it with something. It’s unlikely to bend the sprockets just by riding a bike.
Bike Crank Arms are Loose
Another problem that can happen on a new bike as well as a used one.
Sometimes during a bike assembly, a component can seem like it’s properly fitted, but once you start riding it, it sort of “sits” in its place and needs to be corrected. Even reassembled in some cases. It can surely happen with crank arms, no matter which tightening system they have. Although, it happens more often on the square taper and octalink crank arms.
During usage, crank arms and bolts holding them in place can become loose during a hit on the rocks or something else on a trail. It can, in some situations, come loose on its own. Depends on the riding style and rider weight.
Bottom Bracket is Loose or Damaged
The bottom bracket being loose happens more often on new bikes than crank arms being loose. The reason for this is, there is a narrow line between tightening a bottom bracket too little or too much. When a bike is being assembled, you need to account for the time it will stay in a box and at some bike store later on before it’s ridden. It’s never recommended to overtighten anything that isn’t being used. Most of the bike components are a bit delicate, and when stuck, they can break upon trying to disassemble them.
That’s why bottom brackets are always on the verge of not being tight enough from the factory. What I recommend to every customer at my store is to keep an eye for bottom bracket play. It should always be tightened for free during the warranty period, as it is a normal thing to happen once the bike starts being ridden.
You can sense this by pressure on the pedals making clicking movements and noise at some point during rotation. That clicking point is actually the spindle moving up and down on each end, causing chainring wobble. Chainrings can wobble quite a bit, depending on how much play is there in the spindle.
On the other hand, there is always a possibility of bottom bracket damage to cause chainring wobble. Any hits to a crankarm are transferred to the spindle and bottom bracket bearings. They can take a lot of beating, even the cheaper square taper bottom brackets, but sometimes the bearings break. Usually, some weird sound coming from there gives it off. Also, the fact that you can’t get rid of the play by tightening it.
Bad Spindle Fitting Profiling
Bad profiling on the hole where the spindle is inserted in the crankarm is not so common. At least not that much to be noticed by mechanics with less experience. I’ve only seen it happen with cranksets made for square taper spindles, and it was always from Shimano. The obvious mistake is that they didn’t profile the hole at the proper angle because all of these cranksets could be mounted, but the whole crankset would stay at an angle and wobble during rotation.
If you tried to correct this by tightening the bolt, the bolt would break because the part where the bolt’s head sits on the crankarm is angled, and it bends the bolt until it breaks.
How to Fix a Chainring Wobble?
Going from simplest to the most complicated to do:
- Tighten the crankarm’s bolts.
- Check and tighten the bottom bracket.
- Straighten the bent sprockets.
Now, into the details…
Tighten the Crankarm’s Bolts
Shimano’s Hollowtech II is probably the easiest to troubleshoot and fix. The crankarms can actually loosen up and have some play in them, but it rarely happens. Even when it does, it’s easy to tighten back.
Start by unscrewing two screws on the side. Not completely, but enough to make sure the crankarm can move. The arm cap which goes into the spindle is actually the one that tightens the whole crankset. The specifications say it should be at a maximum of 2 nM of torque. However, it’s hard to measure since the plastic arm cap is tightened by a SHIMANO Hollowtech-II Crank-arm/Cap Tool. I prefer to do it by tightening it a bit and checking for play by pulling on crank arms. Once it feels fine, you can tighten back the two side screws.
Sram’s DUB system is a bit more complicated, and it does have lots of problems when you need to service it. Though, the thing that is great with it is that it rarely gets loose. It actually tightens even more while riding.
However, if you need to tighten it, you need to tighten the arm cap to a 54 nM torque with an 8 mm Allen key. Then you need to adjust the preload adjuster to get rid of any play in the crankset. Turn it in the + direction until it stops. You can stop once it made contact with the bottom bracket as well. You need to use a 2 mm Allen wrench to tighten the pinch bolt until the preload adjuster’s gap completely seals. Do not tighten it anymore from that point because you can damage it.
Square taper and Octalink
Cranksets for square taper and octalink spindles are the simplest of all, but they have major design flaws.
One of the flaws is that putting pressure on the pedals constantly loosens them. Not only that, but the way the crank arms are attached to the square taper spindle causes material wear. That’s what made Shimano invent Octalink, but it had even worse problems with getting loose all the time.
The Octalink system couldn’t be tightened more than the bolt could go in. You would be stuck constantly tightening it back in a never-ending cycle.
A Square taper spindle could be tightened that it doesn’t get loose. At least not for a very long time. The problem is no one ever told the customers the proper way to do it. However, the trick is actually straightforward; checking the other articles, I’m seeing that everyone still does it completely wrong. It all comes down to a simple question;
Should you grease chainring bolts?
Yes, but not the thread of the bolt. Park Tool HPG-1 High-Performance Bicycle Grease should be applied to the inner side of the bolt’s head. That way, the bolt won’t “bite” into the crank arm’s material before the crank arm sits completely on the spindle. Once the crank arm is fitted on the spindle to its limit, you will feel that the bolt can’t be tightened anymore. This is the only way that works permanently.
Other crankset systems are all quite straightforward with tightening the one arm cap; therefore, I don’t need to mention them separately.
Check and Tighten the Bottom Bracket
Adjustable cup and cone bottom bracket
It’s an old mountain bike bottom bracket system. It has an adjustable cup that presses onto the bearings and a lockring that locks it in place. The way to tighten is simple, but it sometimes requires some outdated tools. The lockring requires Park Tool HCW-5 Lock Spanner to turn, while the cup can require several different tools depending on the type. The most common cups can be tightened with a simple adjustable wrench.
The procedure is to untighten the lockring on the left side first. Then untighten the left cup by a bit. It’s important to do so because now you can check if the right cup is tight enough; once the right cup is tightened, tight back the left cup until there is no play in the spindle. The right cup is reverse threaded. Now you can use Park Tool HCW-5 Lock Spanner to tighten the lockring. Be careful while doing so because the lockring could turn the cup with it as well, so you either hold the cup in place with a wrench or adjust it back and forth a bit until you get it right.
Sealed Cartridge Bottom Bracket
A similar procedure always takes place, untighten the left side, tighten the right side if it isn’t already, and then tighten the left side until there is no play in the spindle. In this case, you can tighten the left side even more; it won’t do any harm, except you can damage the left cup if it’s a plastic one. The right side cup is always reverse threaded.
Threaded Bottom Bracket Without Spindle
Bottom brackets without a spindle are used in almost every decent crankset. If the bike frame is made with a threaded shell, then the bottom bracket’s cartridge cups are also threaded.
The tool needed is Park Tool BBT-9 16-Notch Bottom Bracket & Rotor Lockring Tool.
The tightening process is the same as sealed cartridge bottom bracket, except here you don’t tighten as hard. The play is usually coming from the cranks and spindle not being tightened enough.
Press Fit Bottom Bracket Without Spindle
Same as with threaded ones, the play almost always comes from the crank arms and the spindle. The difference here is, there is nothing to tighten. Once installed, the bottom bracket can’t loosen up. If a play isn’t from the crankset, that means the bottom bracket bearings are damaged and need to be changed.
Straighten the Bent Sprockets
If you have finally concluded, there is no play at all, in the crank arms or the bottom bracket, that leaves only one repairable situation. The sprockets of the crankset are probably bent. To make sure it’s best to take the chainrings off the bike and put against a flat surface. You will easily notice if they aren’t straight.
To straighten them, you need to apply some force to them. A Park Tool HMR-8 Shop Hammer and Channellock 440 Tongue and Groove Pliers can come in handy, but you should give it some time and care because hitting it too hard in the wrong spot can make it even worse. Also, make sure not to bend the teeth at all; you won’t have much use of the chainrings if you do.