Your bike’s highest and lowest gears are reserved for very intense efforts, whether you’re pushing yourself as fast as you can or trying to conquer a steep climb. If you lose the ability to shift into these gears, it can really hamper your performance.
I seem to spend a lot of time in my mountain bike’s lowest gear since my endurance is…subpar. So a shifting issue is something I’d be anxious to fix as soon as possible.
There are a variety of causes of lackluster shifting. Fortunately, many of them can be solved with a little mechanical knowledge. So if your bike won’t shift into its lowest or highest gear, keep reading to find out what’s going wrong and how you can fix it. But before that…
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Double Check the Range
Before you go taking things apart or buying a whole new drivetrain, check to make sure there’s actually a problem. I know there have been times when I’m climbing a steep trail and I swear my cassette isn’t shifting into the lowest gear…when it turns out I’m already there and I’m just struggling harder than normal to make the climb that day!
Maybe your shifting is feeling clunky all the way through its range, not just at the high and low ends. This will require different solutions depending on the problem. So put your bike up on a stand and run it through all the gears. If you’ve confirmed you’re only having an issue at the high or low gear, read on to the next section.
Bike Won’t Shift into Lowest Gear (Troubleshooting)
Derailleur or Hanger is Bent
A derailleur is pretty vulnerable hanging off the frame by your rear wheel. If you have a crash or hit an obstacle on the trail, this component is a prime target for damage. A bent derailleur will lead to poor shifting.
On most modern bikes, the derailleur attaches to the frame via a hanger. A derailleur hanger is a cheap metal piece that is meant to bend and break in the event of a crash impacting the derailleur. The reason? A derailleur is expensive, but a hanger is cheap. The hanger will sacrifice its structural integrity to save the more expensive component.
Despite this selfless act, a bent derailleur hanger will also negatively affect shifting performance.
To check to see if your derailleur and/or hanger is bent, shift onto the smallest cog (highest gear). Visually inspect to see if the smallest cog, derailleur guide wheels, and chain line up vertically. If they don’t, something is bent out of shape.
Derailleur L Limit Not Adjusted Properly
The derailleur has a range of motion that it moves within to shift gears. The reason for this is to prevent the chain from going beyond the high or low gear and dropping off the cassette. If the chain were to drop off the low gear, it could get wedged in between the cassette and spokes, causing damage to the chain, cassette and wheel.
However, if that range is too short, it may prevent the derailleur from moving in toward the wheel enough to shift into the lowest gear.
Chain Tension is Too High
Of all the cogs on a bike’s drivetrain, there is usually the greatest difference in size between the largest cog (lowest gear) and the next one up. If your bike’s chain is under too much tension, it may have trouble making the jump up to the largest cog.
Dirty cables will cause poor shifting performance in general, but it is much more pronounced when shifting down. It’s much easier for a bike to jump from a larger to a smaller cog, even with a poorly operating shifter. Think about it: you can easily fall down a flight of stairs without much effort, but it would be tough to fall up.
How to Fix a Bike That Won’t Shift Into Lowest Gear
Repair or Replace Derailleur or Hanger
If either component is slightly bent, they can be realigned with an adjustable wrench and a little trial and error. Be sure to remove the derailleur from the hanger before doing this so you don’t cause further damage to anything else on your bike!
Take your time when realigning your derailleur, as you don’t want it to break. If only the hanger is bent, they’re relatively cheap (which is the point), so you can buy a new one and avoid the hassle of bending it back into shape.
If either component is severely damaged, replacing it will be your best option. It will be difficult to get them back into working order, and even then your shifting will never feel as crisp as it did before the damage occurred.
Adjust L Limit Screw
Look for two small screws right next to each other on the derailleur. These are the High (H) and Low (L) limit screws. They prevent the chain from dropping off the high and low gears respectively.
For this solution, we’ll focus on the L limit screw.
Use a screwdriver to tighten the L limit screw fully. While turning the crank arms, push the derailleur inward to manually shift into the lowest gear possible. With the screw fully tightened, it will probably only make it to the 2nd or 3rd lowest gear. Turn the L limit screw a quarter turn counter-clockwise, spin the cranks and push the derailleur inward again.
Repeat this process until the derailleur shifts into the lowest gear without skipping or making any noise while pedaling.
Adjust B Screw or Barrel Adjuster to Reduce Tension
The B screw is located by itself at the back of the derailleur. It adjusts the derailleur’s guide wheel position in relation to the cassette. Turning the B screw will move the guide wheels closer to or further away from the cassette, thereby increasing or decreasing chain tension.
If you’ve got the L limit screw adjusted properly and are still having issues, move to the B screw. Shift into the lowest gear possible and see how close the guide wheels are to that cog. If they’re almost touching, turn the B screw a half turn clockwise to increase the gap. Try shifting and see if you are able to shift into the lowest gear.
Repeat this process until you are. This usually only requires the smallest of changes, so make small turns at a time.
If the chain is struggling to shift because it’s under too much tension, locate the barrel adjuster: the rotating piece found on the shift cable either down at the derailleur or up at the shifter itself.
Turn the barrel adjuster clockwise a half turn at a time to relieve tension on the derailleur and chain. Try shifting into the lowest gear and repeat this process until smooth shifting is achieved!
Clean or Replace Cables
Remove your shift cable from the housing and visually inspect it. If it looks a little gross, it’s time to give it a good cleaning. If it looks rusty, corroded or otherwise damaged, you’re better off replacing it completely.
Bike Won’t Shift Into Highest Gear (Troubleshooting)
A bent derailleur or hanger can also cause problems shifting into the highest gear. The solution is the same as above, so I’ll avoid being redundant here and we’ll move onto the next potential issue.
Derailleur H Limit Not Adjusted Properly
A properly aligned H limit prevents the chain from jumping off the high gear. While this doesn’t have the potential to cause as much damage as a chain jumping off the lowest gear, it’s still something we want to avoid.
However, if the H limit doesn’t allow the derailleur to move through a full range of motion, it may prevent the chain from reaching the smallest cog.
Cable Tension is Too Low
As I mentioned above, the chain has an easier time rolling downhill to smaller and smaller cogs rather than uphill if the chain tension is too high. But if tension is too low, the chain can easily drop down multiple cogs at a time or fall off the cassette completely. It will have an especially hard time staying on the smaller cogs at the front of the cassette.
How to Fix a Bike That Won’t Shift Into Highest Gear
Adjust H Limit Screw
Find the H limit screw on your derailleur and tighten it fully. While turning the cranks, gently pull the derailleur outward to manually shift into the highest gear you can. With the screw fully tightened, it is unlikely to shift into the highest gear. Turn the H limit screw a quarter turn counter-clockwise, spin the cranks and pull the derailleur outward again.
Repeat this process until the derailleur shifts into the highest gear without skipping or making any noise while pedaling.
Adjust B Screw or Barrel Adjuster to Increase Tension
If you can’t shift into the highest gear, you may need to use the B screw to decrease the gap between the cogs and the derailleur guide wheels. With the chain on the lowest gear, turn the B screw a half turn counter-clockwise to decrease the gap. Attempt to shift into the highest gear, and repeat as necessary.
To increase tension on the shift cable, turn the barrel adjuster a half turn counter-clockwise. This will lead to a crisper shift, giving the chain enough tension to snap to the smaller cogs without dropping off the cassette.
Use All Those Gears!
The highest and lowest gears may be the least used, but they’ll be the most missed when you need them but don’t have them. Use the tips above to tune up that shifting and get the most out of your bike so you can push yourself harder on your rides!
Thinking if you can change gears on a bike trainer? Read our article to find out.