Bent derailleur hanger problems? I feel you. I remember back in the day when I used to ride cheap mountain bikes that had no derailleur hanger, only a built-in dropout on the frame. It was fine if the frame was made from steel. You could bend it back and forth countless times and it never broke apart. However, when aluminum frames became more common, there were certain problems with this kind of design. You see, aluminum stretches easily and each and every time this frame dropout would bend it would slightly weaken it. Until it eventually broke and you would need a new frame.
That’s why the derailleur hangers became a standard for any aluminum bike, and eventually for any frame. Why? Because they allowed any type of frame to last a very long time. And, since mountain biking became more and more technical, this allowed the frame to survive much harder hits to the derailleur.
Customers at my bike shop often come to me confused about how their derailleur hangers bend so easily. It’s usually really difficult to explain to them that it’s supposed to happen in order to prevent frame damage or derailleur itself from being bent. Derailleur hangers are made from slightly weaker material because they are the cheapest and easiest part to change in case your derailleur gets hit.
How a Derailleur Hanger Gets Bent
We already know that any fall on the trail or the road can cause a derailleur hanger, or even worse a derailleur itself, to get bent. However, most of the time you won’t even know it happened. Especially if you didn’t ride your bike on trails recently.
There are many ways in which it can happen. The common one is placing your bike in a type of bike stand that is on the floor. If you place your bike with your rear wheel inside the rack you should be careful to adjust your rear derailleur so it doesn’t bend on the rack. Depending on the rack usually the best position is your highest gear or, in other words, your smallest cog.
It’s best to always put your bike in place in a way that the derailleur won’t get squeezed by anything else. That way you will always be safe from getting it damaged.
How to Know If There Is Something Wrong With A Derailleur
Usually, when there is something wrong with a derailleur other than the cable being stretched you will notice some lousy stuff happening. Like for example, chain dropping between the cassette and wheel spokes. Gear changing can be a tell as well, especially if the gears suddenly get messed up completely. Cable won’t stretch that quickly, it would be more of a gradual change.
All in all, these things can’t be known for certain until you actually look at the derailleur and its hanger from the back and see if it’s bent or not. A good rule of thumb is that if you can’t adjust it properly that means something somewhere is either bent or a cable is getting stuck.
How to Know Which Part Got Bent?
Most of the time, derailleur itself won’t get damaged. The part which is the most exposed on it is actually the joint which absorbs the hits until it gets close to the spokes. In other words, until it reaches the position where it’s blocked by the low limit screw. At that point, if you didn’t hit it too hard only the derailleur hanger will get bent. I can usually guess what has been damaged from experience by which way it got hit.
Same as before you won’t actually know until you take a deeper look into it. First visually and then by trying to adjust it properly. You can then easily troubleshoot exactly what and where is causing the problem.
Problems Caused By Bent Derailleur Hanger
If you continue riding with a bent derailleur, no matter how good you try to adjust it, eventually the chain will get stuck somewhere. It will either shift wrong and get damaged under load or it will fall off the cassette and get stuck. I’ve had my chains twisted several times and it’s not a welcomed event. Chains on 12-speed cassettes are quite thin and sensitive to this. Even a mild force, when applied, can bend the chain.
You are lucky if it only breaks on one chain link because you can always replace that link with a quick link.
Wheel spoke damage
It’s easy to guess what happens when the chain falls off the cassette. If it falls off on the side of a bigger cassette cog, it gets stuck in the spokes. Most of the time it will only scrape them and ruin their looks. Unless you were going fast enough to pull on them and break them. Both situations can mess up your wheel, sometimes even permanently. It causes stress on the rim, or in other words, it’s the same as hitting the rim directly.
Most of the time this happens, the only option you have left is to change the rim or even the wheel, and those can get quite costly on better bikes.
Logically, if the same thing happens on the other side of the cassette, the chain will get stuck between the frame and the small cog. It’s fine if you have an aluminum frame, the only damage you will have is sort of a scar on the frame. Never yet seen an aluminum frame break from a case like this. However, carbon frames tend to get doomed by this. Usually, it’s just a small crack that you don’t even notice until it breaks completely.
The even worse thing is, warranty claims go down the drain if there is even a suspicion that this was the cause of breaking.
Derailleur cage getting completely bent
Any time your chain gets stuck in the spokes at speed faster than average walking speed, you can say goodbye to your derailleur. The wheel keeps spinning while the crank isn’t. The lower part of the chain will pull on the derailleur and destroy it completely.
On an average day, I change 2-3 derailleurs from this cause. And it’s difficult explaining to customers why and how this happens and how they need to check for this problem from time to time.
How To Fix Bent Derailleur Hanger
How to make derailleur hanger straight
When it’s just a small bend in one direction I usually straighten it out on the bicycle, sometimes even with a derailleur on. I like to use Olympia Tools Adjustable Wrench 15 Inches, but other tools might work as well.
If it’s a bit twisted then you really need to take it off the bike and place it on a straight surface. Usually, I use a hammer to straighten it this way but if it’s a good way or not really depends on lot of factors. It’s wise to consider that aluminum and especially this specifically weaker aluminum is really sensitive to stretching and breaking. Once it got bent you really can’t tell how much weaker it became so any bigger bent I would advise replacing the whole derailleur hanger.
Why Is A Park Tool DAG-2.2 Derailleur Hanger Alignment Gauge so great to have?
Sometimes the derailleur gets bent a little as well. What usually happens even a perfectly straight derailleur hanger won’t fix things with a bent derailleur. Since it’s not recommended to try and straighten out the derailleur itself, it means that you need to bend the derailleur hanger the way to fit with a bent derailleur.
In other words, you need some kind of alignment tool in order to align it with the wheel’s rotation. This is where Park Tool DAG-2.2 Derailleur Hanger Alignment Gauge comes in handy. It’s not really a cheap tool but it’s worth every penny. Especially if you are mountain biking a lot. I used to replace several derailleur hangers a month for each little bang it took. The beauty of it, you don’t need t concern yourself with making derailleur hanger weaker since you can just straighten it anytime in a minute.
Is It Better to Have Spare Derailleur Hangers?
I would say for sure it is, as long as your derailleur is in perfect condition. Once it isn’t new derailleur hangers won’t fix the problem of bad shifting, you will need to replace the derailleur as well. It’s kind of a can’t have it all situation. You need to have both new derailleur hangers and an alignment tool available if you want to avoid any headaches in the future.