Rear Derailleur Issue
 

Hi,

I wonder if someone can suggest what my issue is. I have eight gears on the rear sprocket. When I turn the pedals by hand, gears one to seven feel fine, very smooth. Gear 8, the smallest in the cassette, feels ... it's hard to describe. Lumpy is the best word I can think of. Like something is not aligned or engaged.

The 48T crank is new, the chain is new. The jockey wheel appears to be right over gear 8. Gear 8 looks to be in good condition. The chain appears to have tension.

Any suggestions on what I should check please? Thanks.

If you ride in that gear a lot, you've worn it out and need a new cog or cassette. This is very common with a new chain. If will commonly skip under load. The good news is the smallest cog is also the lock ring for the cassette and they're pretty easy to find salvaged at a bike co-op. But you may want to check the rest of the gears too, under load.

Is this an ebike by any chance?

Hi Andrew, thanks for the quick reply.

The answer is yes, it is a standard bike that I just converted with a Bafang mid-mount. However, I have only done literally a couple of miles since the conversion, and I noticed this before I even rode it the first time. The odd thing is, that whilst the bike is a few years old, I hardly ever used Gear 8 conventionally.

So are you suggesting that there is something about the new Bafang crank that is causing the issue? As I say, the other gears are fine.

For an ebike, I wouldn't spend much time worrying with this. Since it doesn't much matter what gear you are in, just use a different one. That said, the smaller cogs can be noisier because the chain has to take a tighter path.

Thanks Andrew R Stewart and shelbyv. Actually, replacement cassettes don't appear to be horrendously expensive. But as shelby suggests, I simply will avoid gear 8, and use one of the lower gears. For all I know, that will give better range anyway, plus, it will put less strain on the drive chain anyway.

It just BUGS me that gear 8 is not smooth. But unless it needs some adjustment I am unaware of, I will probably just work around it.

The thing is that the smaller the cog, the less round it is. Cogs smaller than 13 teeth force the chain into a very tight turning path and don't engage the teeth of the cog as smoothly. Also, as noted earlier in this thread, small cogs wear much faster, 11 tooth cogs have fewer teeth engaging the chain than a 13 tooth small gog. This wears the chain faster as well. All this wear is further accelerated by a mid drive motor assist
The issue has nothing to do with the derailleur

Chordal Action: 2.2.1 Chordal Action

Sometimes there is a stiff link on the chain that causes problems on the smaller cogs. The chain works fine until it must go around the smaller radius of the smallest cog. If the chain was subjected to a splash of water on one particular section, the links can develop a spot where this partial stiffness can occur. Watch the chain as it travels through the derailleur pulleys and it might be more noticeable.

That makes sense, thanks.

Thanks, but that doesn't link to a valid site.

@paulcurtis - watch the lower jockey wheel. I had an issue with mine. I resolved it by twisting the cage ever so slightly. the issue was that the chain was derailing.

that’s great - I was trying to explain to someone a while back why the chain on the smallest sprocket “thrums” a bit, even when everything is set up correctly. I knew there was a specific term for this phenomenon, but I couldn’t remember it. Thanks!

Thanks for all the replies, I think I have the measure of this now. ('Thrums' - yes, that's it exactly)