Why Does My Mountain Bike Skip Gears?

Have you ever had when you tried to go from second gear to third but the bike just feels like going right to third?  I had it happen to me enough that the other day I just decided to dig into it to see what causes this and how hard is it to fix.

Mountain bikes skip gears for one of three reasons.  Either the rear derailleur hangar is out of alignment, the rear cassette is worn, or chain wear is responsible for erratic gear shifting.

What Causes Mountain Bikes to Skip Gears?

Gears skip in mountain bikes happen for one of three main reasons.  These are common causes to all mountain bikes.

The Rear Derailleur Hangar

The most common cause of gear skipping is that the rear derailleur hangar is either damaged or out of alignment.  Either one can have the chain going a little sideways across the rear gears or cassette.

a picture of the rear cassette gears

As you can see from the picture looking at the gears, they are all in parallel.  The rear derailleur  lines up the chain with the gear you set it for.

a rear derailleur

If the rear derailleur is bent or not adjusted correctly, the chain will not entirely engage with the gear and maybe only line up with a couple of teeth on a gear.  It won’t grab correctly and it’ll usually drop down to the next gear from the one you chose.

There are many ways in which the rear derailleur can be the cause.

  • The bike fell over and damaged the rear derailleur
  • The rear derailleur is just worn
  • The springs in the rear derailleur don’t have enough tension to guide the chain

There are many ways it can be the cause and to fix it usually is either align it or replace it.  Many times it just needs adjustment as the cable tension changes after initial break-in of a few hours of riding.

The Rear Cassette is Worn

Another possibility is that the rear gears are worn in your mountain bike.  This can happen once a bike has a lot of miles on it.

When some teeth are worn or damaged on a gear, the teeth won’t hold onto the chain which can allow it to jump to another gear, especially if the chain has a little slack in it.

Worn or Damaged Chain

If your chain has seen better days, then it’s possible it’s the real cause.

Chains can stretch over time and unless they are holding tight to the gears, they will slip and jump when you’re pedaling hard on a trail.

How Do I Fix Gear Skipping?

The fix is usually to replace a part or two on your bike, but it really depends on the cause.

The Rear Derailleur is the Cause

There are a few ways to narrow down the issue.

If everything is fine, but under load like going up a hill it happens, then it’s definitely the derailleur as the cause.  If you can move the derailleur by hand and the spring tension brings it right back, then there’s a good chance it needs to be aligned with a DAG-1 to bring it to spec.  It’s hard to see that it’s off without using the right tools to measure it accurately.

If the spring is shot then it’s a simple replacement of the rear derailleur.

Many times the gear cable just needs adjustment.  Here is how to do it in a nutshell:

  1.  Put the bike into the smallest gear.
  2. While riding, press your shifter once.
  3. If it doesn’t shift exactly once, loosen the right barrel adjuster by turning it away from you.
  4. Rinse and repeat.

Sometimes it is called re-indexing gears, and here a video to show you how.

If those are ok, then it’s time to see it in action.  Put the back of the bike up and pedal the bike with your hands while changing gears and watch what the chain is doing.  If it kind of jumps around, then it’s likely the chain tension is wrong.

Worn Rear Gears

This one you can generally tell by inspection.

Look closely at the cassette and see if any of the gears has some worn nubs on them.  If so, it’s time to replace them.

Worn Chain

This one is really common.  Bike chains stretch over time from use.

The best way to see how much slack is in the chain is picking the halfway point of the chain between the front and rear gears and pushing down and then up to see how much the max flex is.  Generally you want less than an inch of play.

What Maintenance Can Prevent Gear Skipping?

Many times damage to things like a derailleur happen through using a mountain bike like it should.

There are a few things you can do to help keep an eye on the gear cable tension.  After only a few hours of initial use, it breaks in and the cable should get an initial adjustment and period checking of it.

When it needs adjustment, it’s time to re-index your gear.  The guys over at Bike Radar have a great video they put together on it.

Other that that, just keep your chain lubed up and keep an eye on the tension and when it gets too far, just replace it.

Can a New Bike Skip Gears?

Actually yes.  New bikes can encounter this gear skipping issue and it’s not always due to a manufacturing defect.

Sometimes it is broken from the factory, and even more commonly it’s the bike fell off of something onto the right side and bent the rear derailleur.

The bike may be great new (as it should), and after riding it for 10 hours you notice it feels a little weird.  Most of the time this is the gear cable stretching and breaking in.

It’s time to re-index your gears and you’re back in business.

Conclusion

Once you know the causes, you can narrow down and diagnose the issue.

Most of the time the rear derailleur hangar is the problem.

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