Bikes are supposed to be fairly quiet. When your bike starts making an unusual noise, it’s often a sign that there’s a mechanical issue in need of attention. The faster you take action, the more likely you are to fix the problem while it’s small, preventing your parts from wearing out quickly and saving you lots of time and money on repair and replacements.
Clunking while shifting is a common problem with many bikes. Just like with other problems, you’ll want to run down a quick checklist in order to figure out exactly where the issue is coming from.
In most cases, the bicycle clunks when shifting due to a loose part in the cassette, freewheel, or rear derailleur, it can be due to loose cables, and sometimes the front derailleur might be at fault.
Here’s exactly what you’ll want to do to figure out the core cause of your issue and some steps you can take to fix it at home.
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Before we start, it’s worth noting that it’s totally normal for there to be some noise when the ratchet on your rear wheel is first engaged as you start to pedal.
If this noise is loud or otherwise different than normal, it’s definitely worth checking out, but the presence of a soft noise here should not be a cause for major alarm. When in doubt, ask an experienced cyclist or a bike mechanic for a second opinion.
Checking Your Cassette / Freewheel
The rear wheel of your bike is attached to your chain via a ratcheting mechanism. This allows the wheel to spin faster than the gears when you coast or go downhill while ensuring that your chain delivers power to your wheel when you pedal.
In many cases, a clunking noise when you shift is caused by the engagement of this ratcheting system. If the noise is louder than usual or has an odd quality to it, it points to something being loose within your cassette freehub or freewheel hub.
To diagnose this, you’ll want to try wiggling different parts of your rear gearing stack with your hands. Try gripping your first and last set of cogs on the rear wheel and firmly tilting them back and forth in different directions.
If you can move any of the rear cogs indepe3ndently of each other, you’ve found the cause of your problems. Your cassette stack should be tight enough that no cog is able to wiggle on its own. It’s okay if the entire stack has a small amount of play in relation to the bicycle, but if any cogs have play relative to each other you’ve got a problem.
If you’re catching this problem relatively early, the fix is simple: you want to remove your rear wheel and tighten your cassette stack.
You’ll need a special tool to do this, but if you can get your hands on one for a few minutes, you can simply crank your cassette down quite tightly and see if that fixes the problem. Most cassettes have the torque spec printed on the unit itself.
In most cases, you’re shooting for somewhere between 30 and 50-newton meters, which works out to be about a 60 lb curl worth of force if you’ve got a 6″ wrench. In other words, make it fairly tight.
If you’ve got a cassette and tightening your stack doesn’t fix your problem, you probably are missing a spacer. This usually occurs when someone takes apart your cassette to service it and forgets to put a spacer back on.
Cassettes tend to be sized by gearing, but there are a number of circumstances when it’s a good idea to run a 10-speed stack with a spacer on an 11-speed cassette. Without this spacer, it’ll be tough to get your stack tight enough to prevent the cogs from wiggling.
Cog wiggle is a big problem due to the way force is applied to your cassette hub. If your cogs are tight against each other, they’ll distribute the force from your chain more evenly against your freehub’s splines.
With a loose stack, the cogs can’t share force with each other, causing the torque from your chain to have an incredibly narrow footprint that can wear out your freehub quite quickly.
This is often the cause of the clunking noise as well. With a tight stack, the cog corresponding to your next gear will already be tensioned against the freehub’s splines. With a loose stack, you’ll take that cog from a totally resting position to full engagement instantly, causing a clunk.
Freewheel Problems
Freewheels and cassettes differ in one important way: you can easily disassemble a cassette with the right tools. Cassettes have a ratcheting mechanism that’s independent of the cogs, enabling you to adjust your cog stack quite easily.
Freewheel hubs, on the other hand, have the ratcheting mechanism and the cogs in one assembly, making them much harder (if not impossible) to adjust.
If you have a freewheel and not a cassette, fixing things becomes a lot more complicated. Try to narrow down the source of your clunking noise to the freewheel itself.
If you can replicate the clunk via wiggling parts of the freewheel in isolation, you’ll probably want a new freewheel.
While an experienced mechanic can fix some freewheel problems without replacement, it’s a complex, tedious process that often reveals issues that require new parts.
Adjust Your Rear Derailleur
A clunking noise might be caused by issues with your shifting. If this is the case, you’ll likely have odd shifts or a chain that acts oddly at either end of your shifting spectrum in addition to your clunking problem.
Here’s a quick rundown on how to adjust your rear derailleur at home to alleviate these issues.
First, visually check to see that your derailleur hanger is straight. Your derailleur should drop down in a straight line. If it’s at a jaunty angle, you’ll need to adjust your derailleur hanger.
This is probably something you should leave to a professional, as derailleur hangers are quite delicate and can break while being adjusted by the most experienced professional.
If a bike shop breaks your derailleur hanger they’ll have no problems replacing it. If you break your derailleur hanger, you’ll have to buy and install a new part.
In order to adjust your derailleur, you’ll need to locate a few things. First, look for a pair of small screws, often with an L and H next to them. These are called limit screws.
You’ll know you have the right ones if you can see the ends of the screws positioned in a way that they impede the derailleur from extreme movement. Your limit screws prevent your derailleur from going too far in either direction.
Next, look for your barrel adjuster. This is usually a twistable bit around a cable that leads into your derailleur. The barrel adjuster changes the space between each gear, or how far your derailleur moves each time you shift.
With these located, it’s time to put your bike into gear. Put the chain on the largest cog in the front and the smallest cog on the rear. Be sure to make sure that you shift as far down as you can go in the back.
If it’s out of alignment, your shifter might have unused gears past the point where the chain is on the smallest cog. If the chain doesn’t go to the smallest cog, adjust the barrel adjuster and the H-limit screw as detailed below until you can shift it there, then follow the next set of instructions.
We’re now going to throw our shifter out of alignment in order to adjust our H-limit screw. Add slack to the cable by turning the barrel adjuster clockwise a few turns. This should push the location of your smallest gear farther away from the bike wheel, allowing us to independently adjust our limit screws.
If your limit screws aren’t marked, the H-limit screw is the one that makes your derailleur wiggle in this position. Our goal here is to adjust this screw until it’s slightly too tight, then back it out.
Tighten the screw by half-turn increments until you can hear the chain start to rub on the next cog. Once that happens, loosen it until the noise disappears and you’re all set.
If your limit screw is very tight, it might prevent you from accessing the smallest cog. If this happens, simply loosen and follow the procedure above.
Next, tighten your barrel adjuster by turning it counterclockwise a couple of turns.
Pedal, then try shifting up exactly one cog. If the chain does not shift, tighten the barrel adjuster counterclockwise for about one turn. If it shifts two cogs, or shifts and then rubs, loosen the barrel adjuster clockwise about one turn.
Once it’s shifting one cog per shift, throw the barrel adjuster slightly out of alignment until it makes noise, and then slowly dial it back in again until the shifts are smooth and silent. Be sure to check each shift on the bike and make fine adjustments until each shift is right.
Don’t worry about the largest cog, however, as we’ll be addressing this with the L limit screw.
You might find yourself running out of cable during this adjustment process. If this happens, you’ll want to find your cable’s pinch point, loosen the screw, pull the slack out, and tighten the pinch point again.
To adjust the L limit screw, put your chain on the smallest cog in front and the second to largest cog in the back. We’ll want to go through the same procedure we used for our H limit screw.
Adjust the L limit screw until the chain makes noise when it’s shifted to the largest cog in the back, then loosen it until the shift occurs silently.
When your L limit screw is adjusted properly, you shouldn’t be able to make the derailleur cage move inward past the largest cog.
This procedure should fix any adjustment issues with your rear derailleur. If it does not, it’s likely that you have a cable issue and should replace your cables.
Cable Issues
The cables on your bike are responsible for relaying tension from your shifters to your derailleur. If your cables are old and elastic, they lose some of this tension, causing you to have inconsistent shifts and other issues.
Both diagnosing and fixing this problem are fairly simple.
If you find yourself constantly adjusting your derailleur, either there’s something interacting with your bike when it’s stored or you have stretchy cables.
Try to eliminate the influence of any pets, children, housemates, or outdoor critters and see if your bike is leaning on any components that would affect the shift. If the problem persists, you’ll want to replace some cables.
The good news is that bike cables are very inexpensive. The bad news is that the replacement can get complicated, especially on bikes with internal cable routing. On top of that, you’ll have to re-adjust your derailleur after you’ve replaced your cables.
Personally, I would strongly consider asking your local bike shop to help with this process, as you’ll get a professional derailleur adjustment and a quick check of other bike components while skipping the hassle of replacing your cable.
In most cases, you’ll want to replace both your inner and outer cable at the same time.
Replacing your cable is mechanically simple: you unhook the pinch points, remove the cable, put a new cable in, and secure everything.
The process is a bit more involved, however, as your cable needs to run against (or inside of) your frame in particular spots, and you’ll have to cut your cables down to the correct size.
Again, it’s something you can totally do at home, but it’s also something that a bike shop will do a lot faster while being able to check your bike for other issues.
Adjusting Your Front Derailleur
Your front derailleur is usually not a cause of clunking when you shift, but it’s worth going over how to adjust it just in case. First, visually inspect the cage and make sure it’s straight and at a reasonable height. If it’s not, you’ll want to adjust it first.
You want your derailleur cage to be about 2 to 3 mm above the big parts of the cogs in order to keep the chain in place, and you’ll want it to be very straight.
The derailleur cage will be mounted via a clamp with some ability for adjustment with an alan key. If it’s clamped to your frame, be sure to mark the initial position with some tape before you make changes.
Loosen the key, move your cage to the right position, and tighten it again before proceeding.
Next, we’ll adjust our derailleur in very much the same way that we adjusted the rear derailleur. Shift to the innermost cog on both the front and rear shifters, loosen the barrel adjuster to add slack to the cable, and then adjust the inner limit screw until the chain is as close to the frame as it can get with no noise or chain rub.
Next, tighten the barrel adjuster until the shift happens smoothly and without noise, then adjust the outer limit screw until it prevents the chain from popping off of the cog without any chain rub.
To adjust the outer limit screw, you’ll need to apply constant pressure to your shifter to counteract the internal spring. Just like with the rear derailleur, be sure to throw things a little bit out of alignment on purpose and use your ears to tell when you’ve gotten the adjustment just right.
