How to Fix a Loose or Wobbly Crank On a Mountain Bike

Is the wobbling crank of your mountain bike making it difficult for you to ride it properly? Well before taking it to the mechanic, why don’t you try these methods listed below to tighten it up yourself? This can not only help you save a few bucks but also let you understand the mechanics of your bike. Furthermore, once you know how to fix a loose crank on a mountain bike, you will be able to tighten it up every time it comes loose.

How to Fix a Loose Crank On a Mountain Bike

What is a Crank?

The crank or the crankset is an essential part of any bicycle. This is because it is one of the parts which converts your pedaling to the actual movement of the bike. Cranks are a group of different parts of a bike’s drivetrain and typically consist of crank arms, gears, and brackets. And as the crank is usually subjected to frequent force from pedaling, it can sometimes loosen up. This can lead to your riding experience becoming somewhat unstable and quite annoying.

This is because wobbling cranks can disturb your balance if you are not an experienced rider. They can also make creaking noises that can be quite distracting when you are looking for a peaceful ride. Furthermore, if they are not tightened up soon, they can loosen even more and come completely off. This is dangerous because it can leave you stranded on a trail far from home. Also, as the crank comes off, it can get lodged into the back wheel, causing you to fall and badly injure yourself.

How to Tighten a Loose Crank

Now that we have established the importance of tightening up a loose crank let’s move onto the actual tightening process. The first way through which you can tighten a loose crank is by tightening the hex bolt that connects it to the bottom bracket of the bike. For this process, all you need is a simple hex key. First, locate the hex bolt; most bikes have one situated on the top end of the crank. Then using a hex tool, tighten it while holding the crank down to prevent it from moving.

After you are done tightening, test the crank; if it still wobbles, then tighten the bolt even more. But if the crank does not wobble, then your work is done. Some bikes have plastic dust caps covering the bolt. For those cases, you need to first remove the cap using a flathead screwdriver before you can tighten the bolt. And after the tightening process, just press the cap back into its initial position.

Another way through which you can fix a loose crank is by tightening the bottom bracket bearings. These bearings are located between the bottom bracket and the crank. And we will show you how to tighten them without using any special tools. First, you need to slightly loosen the bearing by rotating the outside counterclockwise. You can just use your fingers for this step and. For the next step, you need a screwdriver and a small mallet.

Adjust the screwdriver in an angled position in the grooves inside the bearing and, using an upwards force, rotate the inside clockwise. If it starts to feel a little hard and becomes difficult to move, tap the back of the screwdriver with a mallet for extra force. Once the inside of the bearing is tight enough, test the crank. If it does not wobble, then rotate the outside of the bearing clockwise and tighten it back up.

Some Precautions to Take When Going Out Riding

Even if you have fixed the crank of your bike and stopped it from wobbling, here are a few precautions that we believe you should take before taking out your bike for a ride. First, we recommend that you carry a portable multi-tool with you when you go out riding in case of emergencies. This is in case you are out on a deserted trail and the crank starts to feel loose again. And while a small tool like that might not fix it as well as a heavy-duty tool would do, it can at least tighten it enough to allow you a safe journey back home.

Next, we encourage you to check the cranks and other parts of the bike before going out to ride. This is especially important if you have had some parts of the bike malfunction before. Because if they start to malfunction again, then it is better that you find it out at your home rather than at the trail. Therefore, while it may seem bothersome, it is totally worth it to check your bike thoroughly before a long ride.

Related MTB Guides: How to fix gears on MTB | How do MTB’s work


How do you tighten a mountain bike crank?

You can either tighten a mountain bike crank by tightening the hex bolt using a hex key. This bolt is situated at the top end of the crank, and it connects it to the bottom bracket of the bike. Or you can tighten the inside of the bottom bracket bearing using a screwdriver and a small mallet. This bearing is located in between the crank and bottom bracket.

Why does my crank arm keep coming loose?

Sometimes even after you have tightened up a crank, it still keeps on coming loose. This happens when you frequently use your bike with a loose crank without fixing it. And so it gets so damaged that even when you fix it, it will come back loose pretty soon. This is why in cases like these, it is recommended that you take your bike to a mechanic and get it professionally fixed.

How tight should crank arm bolts be?

One of the ways you can fix your crank arms that come start wobbling is by tightening the hex bolt. But while it is important that you tighten it enough to prevent it from loosening up again, you shouldn’t over tighten it. This is because if you overtighten it, you might experience some difficulty pedaling backward or in something else. And so, that’s why we recommend that you tighten the bolt to 35Nm – 40Nm. Because it is tight enough for the crank to work properly without being overly tight.

Wrapping Up

Having a loose crank on your mountain bike can get not only extremely annoying but also quite dangerous. This is why knowing how to fix it can be quite useful for those who go on frequent bike rides. And so, this article shows you a few ways how to fix a loose crank on a mountain bike so that the next time the crank comes loose, you can tighten it yourself through any problem.

Photo of author
Sonia Seale loves to take her bike to the mountains near her place, the bike track has provided her the opportunity to go mountain biking since she was a child. That's where she developed a great deal of interest in cycling, no matter if it's electric or mountain bike, she's all over the place and loves to fix her own bikes along with exploring the new features coming along in the industry. That's why she's here, sharing her experience and learnings with you guys.