Description of The Problem. The drivetrain makes a “popping noise” which intensifies when pedaling hard.
Possible Sources Of The Issue
1. Improperly Adjusted Front or Rear Derailleur
The two derailleurs (front and rear) control the position of the chain over the chainring(s) and the rear cogs. If the derailleurs aren’t adjusted adequately, the chain could rub.
In the case of the front derailleur, it’s possible for the chain to rub against the outer part of the derailleur’s cage when riding in the largest chainring and against the inner part of the derailleur when operating in the lowest gear.
When that’s happening, the most likely culprit is the gear cable. It’s either partially torn (incapable of maintaining tension) or improperly tensioned (too much or too little tension).
If the chain is rubbing against the outer part of the derailleur cage, it will be pretty obvious. Shift to the largest chainring and spin the crank arms slowly. Observe the derailleur’s cage. If the chain rub is severe, you will see it. (To check for rubbing against the inner part of the cage, shift to the smallest chairing)
Observe the front gear cable. If it looks intact, adjust it via the barrel adjuster that’s on the shifter as shown in the video below.
Note: In some cases, the limit screws will have to be adjusted too. The limit screws determine the maximal lateral movement of the derailleur in both directions.
A Note On Friction Shifting
Modern drivetrains are indexed. This means that each click of the shifter pulls or releases a pre-determined amount of cable.
Some old-school bikes have friction shifters. Friction shifters move as much as the rider wants them to. This makes shifting slower and more difficult, especially for beginners.
However, there’s also an advantage – the user can “trim” the front derailleur by moving the shifter a little bit to the front or back to eliminate chain rubbing.
Some higher-end index shifters designed for road bikes have a trim function too that moves the front derailleur slightly after the initial shift is complete.
An improperly adjusted rear derailleur can also cause the chain to skip and create a popping sound.
Usually, the derailleur isn’t indexed properly and the chain finds itself between two cogs upon a shift. As a result, the chain starts skipping because it’s neither in the low nor in the higher gear.
In some situations, the chain may move to a certain gear, usually when the rider exerts more effort, only to return to “no man’s land” after the tension is reduced.
In this case, the solution is once again to re-index the gears.
2. Damaged Or Improperly Adjusted Bottom Bracket
The bottom bracket is a mechanism consisting of bearings and a spindle allowing the crank arms to rotate smoothly. The bottom bracket sits in the so-called bottom bracket shell of the frame.
If the bottom bracket is damaged or too tight, it can also be the source of a “pop” during pedaling.
To diagnose the bottom bracket, disconnect the chain and spin the crank arms. If they’re rotating smoothly, and there’s no weird noise, the bottom bracket is more than likely fine.
If the bottom bracket is not performing well, it has to be replaced or re-adjusted. Most modern brackets come as a cartridge unit and are meant to be replaced rather than serviced.
However, there are also old-school bottom brackets built on the cup & cone principle which can manifest the same problem when the pre-load adjustment is too tight to prevent the bearing from rotating freely.
In that case, the pre-load of the bottom bracket can be readjusted to eliminate the “popping sound”.
For more information on servicing a similar bottom bracket, watch the video below:
3. Pedal Issues
Another source of the issue could be a damaged pedal bearing. I experienced this issue a couple of years back. One of the bearings on my right pedal had disintegrated and a few ball bearings had fallen out.
As a result, the axle was spinning around a broken bearing with a gap. The gap itself caused some of the ball bearings to group together. When the axle would pass over them, it would push them away and create a snap/pop. In my case, the only solution was to replace the entire pedal because the plastic was damaged too, and there was no point to service the unit.
But in most scenarios, the damage is smaller and can be resolved by performing basic pedal servicing.
To learn how to service a pedal, check the next video:
5. Stiff Chain Links
The pop may also be the result of a stiff chain link. If you’ve recently shortened the chain, the links of the new connection point may be too stiff. The stiffness will prevent them from moving fluidly over the cog’s teeth. If this is the issue, moving the affected zone laterally ever so slightly will resolve the issue.
If the links are damaged, however, it will be necessary to replace them or the entire chain.
6. Damaged or Worn Chainrings/Rear Cogs
If the teeth of a chainring or a rear cog are worn, the chain will skip in that gear. The problem manifests very often when the rider is pedaling outside of the saddle. Sometimes the skip could be very abrupt and cause a sudden drop of the pedal causing the rider to lose balance.
In that scenario, it’s recommended to replace the rear cassette, the chainring(s), and the chain.
It’s also possible that one or a few chainring teeth are damaged as a result of a fall or hit. The crooked teeth would create an uneven spot and cause the chain to skip/pop in the area. If the chainring is made of steel, it’s possible to rebend the crooked teeth and improve the alignment. But if the damage is severe, a replacement is in order.