How to Reduce Premature Wear on the Smallest Sprocket of Your E-Bike Cassette

Wear on the smallest sprocket—or the highest-gear sprocket—is a very common problem. Some cyclists even have to replace it every six months, and it rarely functions properly beyond 8,000 kilometers. However, there are ways to extend its lifespan.

The incredible pressure each tooth of the smallest sprocket must withstand

Let’s imagine a 38-tooth chainring paired with an 11-tooth sprocket, which is the most common setup on e-bikes. This means that each tooth of the smallest sprocket bears the pressure of 3.5 teeth of the chainring, which transmits power from the mid-drive motor and the pedals to the cassette via the chain. This means that each tooth on the sprocket receives an average of about 16.36 newton-meters of torque during any electric bike ride. Given that average leg power is 100 Nm to 120 Nm, and about 60 Nm to 80 Nm for mid-drive motors on e-bikes, dividing 180 by 11 gives 16.36. This figure drops to 9 Nm for hub motors, since in that case, the sprocket receives only the power from our legs. 16 Newton-meters is roughly equivalent to the force required to tighten a large bolt securely. By comparison, the largest 44-tooth sprocket on some cassettes only needs to withstand a torque of 4 Nm. No wonder it wears out much later, even though it is subjected to heavy pressure during steep climbs.

What causes sprocket wear, and the solutions

• Starting with the shifter set on the smallest sprocket. When you are a few meters from your destination—whether it’s your home or a red light—shift to a larger sprocket
• Starting with the highest assistance level. When you’re a few meters from your destination—whether it’s your home or a red light—set your assistance to medium or low.
• Start climbing in the smallest gear. Just like the two behaviors described above, this can also reduce the lifespan of your motor. You should select a larger gear before starting the climb.
• Shifting from the smallest sprocket to a larger one while pedaling. When the chain is transmitting power from your legs and/or the motor, shifting puts damaging pressure on both the sides and the tips of the teeth. Stop pedaling or, at the very least, release pressure on the pedals before shifting gears.
• Dirty chain. Chain oil or grease mixes with sand and dirt, which scratches the teeth. Clean and lubricate your chain, chainring, and cassette regularly. You can use an old toothbrush and a cloth.
• Chain stretch. Periodically measure the distance between the chain links using a caliper. If it exceeds 0.75 mm, the chain will stretch beyond the distance between the sprockets, causing damaging friction. Replace your chain; this will save you money by delaying the need to replace the chainring and cassette.
• Slow cadence. The slower the cadence, the greater the pressure on your cassette to maintain the same speed, since a slower cadence requires using a smaller sprocket to maintain the same speed. Use a higher cadence, ideally over 55 revolutions per minute.
• Frequent use of the smallest sprocket. If it doesn’t require an uncomfortably fast cadence, try to limit your use of the smallest sprocket as much as possible.
• Larger chainrings. Many cyclists replace their original 36- or 38-tooth chainrings with 40- or even 44-tooth chainrings. This puts greater pressure on each tooth of the cassette, which is particularly harmful to the smallest sprocket. Think twice before replacing your chainring with a larger one.
• Improperly adjusted derailleur. If it’s not adjusted properly, the derailleur doesn’t just shift gears with difficulty. It also reduces power transfer to the wheel and increases wear on the drivetrain (chainring, chain, and cassette) and the motor. The best approach is to start by adjusting the limit stops. It doesn’t matter whether you start with the inner or outer stop. At the outer limit, the chain runs on the smallest sprocket. The rear derailleur guide pulley must be perfectly aligned. To adjust, turn the limit screw marked with the letter “H” (for “high”), which is located above the other screw on the derailleur.
• When possible, mounting a larger chain ring is a good solution.

You don’t have to replace the entire cassette; you can replace just the smallest sprocket.

Riding faster isn’t a problem

Fast bikes don’t wear out the smallest sprockets any more than others. In fact, the higher the speed, the less torque the pedals and motor deliver.