Description of the Problem: When the rider is exerting a lot of effort uphill (usually out of the saddle), the chain slips involuntarily and switches to a different gear even though the shifter isn’t triggered.
The outcome is loss of balance, reduced control, and inefficient gearing.
Possible Sources Of The Problem
- Poorly Adjusted Derailleur
If the gear cable controlling the rear derailleur isn’t adequately adjusted, the derailleur will move to the wrong location (between the cogs) after the user initiates a shift. As a result, the chain might slip and skip.
The problem is more likely to manifest when the rider is exerting more effort.
Modern gear systems are indexed. Each click of the shifter moves a pre-determined amount of cable which depends on the number of speeds that the bike has as well as the brand of the components.
The derailleur itself has a property known as rear shift ratio. The rear shift ratio indicates how much the derailleur moves per 1mm of cable pulled or released by the shifter.
For example, if the derailleur has a 1.7:1 rear shift ratio, it moves 1.7mm per 1mm of cable pull or release.
Accurate shifting requires strict indexing of the system. When the derailleur and its gear cable are adequately set, each shift moves the chain from one cog to the next.
Setting up a rear derailleur is not incredibly complicated, but it’s a tricky process that requires a dedicated post and preferably a video explanation. You can read a detailed guide here.
Note: There are also old-school friction shifters that aren’t indexed and are essentially free to move as much as the user wants them to. The benefit of friction shifters is that they allow the user to mix all kinds of parts (e.g., an MTB cassette with a road derailleur).
If you have a set of those, but you are unaware, you may be moving the shifter inaccurately and causing a poor shift. It’s also possible that the shifter is somewhat loose and thus moving a bit by itself when you pedal hard.
It’s very easy to know if you have a friction shifter. If it’s moving freely without making clicks and clearly transitioning from one segment to another, it’s a friction unit. That said, friction shifters are old and rare. One example would be the downtube shifters found on retro road bikes.
- Bent Derailleur Hanger
The derailleur attaches to the frame via a unit known as a derailleur hanger. Modern derailleur hangers are made out of soft aluminum. The choice of material is strategic. In case of a fall, the hanger will bend without transferring stress to the frame.
If the hanger was as strong or stronger than the frame, it may damage the frame. (On older bikes, however, the hanger is part of the frame.)
If the hanger is even slightly bent, it will cause poor shifting. There are two options. Option 1: Realign the hanger with a hanger alignment tool. Option 2: Get a new one.
- Worn Cassette and Chain
With time the teeth of the rear cogs become pointier due to the friction against the chain. The worn-out teeth have a harder time biting against the chain. Consequently, the rider may experience a sudden chain slip.
Most of the time, this occurs when starting from a dead stop in fairly high gear or when pedaling uphill.
If the cassette is worn, the chain is more than likely up for replacement too due to extra stretch.
The only solution is to replace the cassette and the chain. Sometimes the chainrings will have to be replaced too.
- Worn Chainrings
It’s also possible that the problem is coming from the front rather than the back. Just like the cogs on the cassette, the teeth of the chainrings wear down too. A worn chainring can slip and trigger an immediate loss of balance.
- Worn or Contaminated Gear Cable
If the gear cable is old, there’s a chance that it has a small tear somewhere. The most vulnerable parts are, of course, the exposed ones.
If the cable is torn, then it will have lower tension and the shifter will consistently fail to pull the cable to the needed location.
It’s also possible that the cable’s housing (enclosure) is dirty and preventing the cable from moving freely. Replacing both will fix the issue.
- Cracked Frame
Unfortunately, a cracked frame may also be behind this issue. The crack is usually near the bottom bracket. During riding, the weakened area flexes and changes the frame’s length. As a result, the derailleur cannot move accurately and creates slippage.
To rule out this issue, it’s necessary to carefully examine the entire frame for a crack. If the crack is small and the bike is painted vividly it will be even more difficult to see it.
A cracked frame is usually not worth the repair unless it’s a special unit.
Thankfully, a cracked frame is not the most common source of a slipping cassette and chain.
- Stiff Chain
If the chain has been recently shortened, the connection link may be too stiff and inflexible. The solution is to examine the link and see if it’s installed properly. Then, bending the chain at the affected location laterally ever so slightly should alleviate the issue.
Note: A stiff chain link is likely to cause slippage even when the rider isn’t producing a lot of power.
- Broken Shifter
The shifter is the commanding unit/brain controlling the derailleur. If the shifter is broken or contaminated, it may be failing to move and release as much cable as necessary. The outcome will be a partial, insufficient shift.
- Brocken Freehub Mechanism or Freewheel
Bikes can coast thanks to a ratcheting mechanism in the rear hub (if the bike is using a cassette) or in the freewheel. To learn how this mechanism works, consider reading this post.
If the pawls (the units that engage against a ratcheting ring inside the hub or the freewheel) are damaged, then the cassette/freewheel will spin and fail to transfer power to the wheel.
If the bike is using a cassette, it will be necessary to replace the entire rear hub or at least the driver’s body.
If the bike is using a freewheel, a new one will fix the problem because in this case, the mechanism is inside the freewheel rather than the hub.