Personally, I think the causes of gear sensation include:
1. Improper adjustment of the rear derailleur cable, causing friction between the chain and an adjacent tooth, resulting in gear sensation. I believe the way to diagnose this is to lift the bike, turn the crankset, and listen carefully to the sound of the cassette. If you hear intermittent "clicking" or "grinding" noises, it’s very likely the cause. To fix this, simply adjust the tension of the rear derailleur cable — either tighten or loosen it as needed.
2. The cassette isn’t properly secured, or the rear quick release isn’t tightened, leading to a loose rear wheel or freewheel. This situation can also easily cause gear sensation. Generally, if this is the issue, you’ll feel the gear sensation across most or all gears. In such cases, use the appropriate tools to tighten the cassette or quick release. Some people, when removing the cassette for the first time, might hear a "clicking" sound and assume they used too much force or turned it the wrong way, fearing they’ve damaged the wheel — and thus choose not to make adjustments. However, whether you’re removing or installing the cassette, a "clicking" sound is normal when it’s tightly secured. Don’t worry about the noise; it’s expected. When installing the cassette, use all your strength — don’t be afraid of the clicking sounds. Use your full force, or ask someone stronger to help, and tighten the cassette until it can’t be turned anymore. Remember: use all your strength to tighten it. The clicking sound comes from the internal component structure and is meant to secure the cassette. Therefore, always make sure the cassette is tightly installed. And don’t forget to properly secure the quick release afterward.
3. The third reason relates to the chain. Some cyclists may find that their newly bought or assembled bike has no gear sensation, but after cleaning the chain, the problem appears. This is usually caused by improper cleaning methods. Before I cleaned my chain, I, like many other riders, looked up information online. Many suggested using kerosene or dish soap to clean the chain for a more thorough clean. I opted for dish soap, but after cleaning, the gear sensation became overwhelmingly strong. This may be because fine particulate contaminants were washed into the chain’s inner workings, or the chain was cleaned too thoroughly — stripping away its original lubricant and causing poor chain transmission, which leads to gear sensation. This is a common issue among many cyclists. The reason why many find that gear sensation disappears after changing the chain or riding for dozens or hundreds of kilometers is likely due to this cause. Personally, I believe you don’t need to replace the chain. I thoroughly cleaned the chain again and then applied Prolink chain lube. After that, the ride became smooth again. For those with similar experiences, you might want to try this method. I’d also like to specifically mention that I’m not impressed with the so-called "chain cleaners." I think if you’re maintaining your chain initially, using a high-quality lubricant is sufficient and works very well. If you’ve had a failed chain-cleaning experience like mine, thoroughly clean the chain again, then apply a good-quality lubricant — the gear sensation should disappear.
4. The fourth possible reason is the lubricant itself. The first time I lubricated my chain, I used the cheapest oil available online (I won’t name it specifically — those who are curious can look it up themselves). That time, I applied oil without cleaning the chain first. When I rode afterward, the gear sensation appeared faster than "Cao Cao" himself (idiomatic expression meaning “very quickly”), and it annoyed me for a long time. Why do I think it was the cheap oil? Because after resolving my gear sensation issue, I wondered if the low-quality oil was the culprit. So I applied the same cheap oil again, and the gear sensation returned. I was truly speechless. However, it’s also possible that the oil I used was counterfeit. So, consider the fourth scenario with caution.
5. The fifth possible reason could be chain aging or issues with the drivetrain system itself. Here, I think it might be problems with the jockey wheels or guide plates that are causing the gear sensation. But generally, new bikes wouldn’t have this issue, so consider this as a reference point.
Personally, I believe the above five points cover the reasons for gear sensation. I can confirm the first four points from my own experience, as I indeed encountered all four. The main issue stemmed from not doing enough research before disassembling the bike, which led to these problems. So, I hope those planning to disassemble or maintain their bikes will pay attention to the points I’ve mentioned — that way, you won’t repeat my mistakes.