Loud clunk moving to smallest sprocket

Question

When I shift gears to the smallest sprocket it makes a very loud clunk sound. There's also a lot of movement in the chain, like a bounce. I've taken a video demonstrating this Video of clunk I'm moving off the smallest sprocket then back down.

Bike is about 8/9 months old, but only used regularly the last 5 months and has a GRX400 groupset.

Some observations:

• It occurs when I'm either on the big or small chainring (which I think would rule out the chain being too tight?)
• Gearing is otherwise fine apart from this, although there is a bit of clunking moving to the second and third smallest sprockets as well, but not as bad.
• It occurs with the clutch on or off.
• It also happens riding on the road and on a turbo trainer.
• There's no interference from the front derailleur.
• The derailleur is a direct mount and it doesn't look bent in any way.
• It has a 10 speed cassette and I can see a spacer behind the cassette as expected.

What I've done/checked:

• The cassette is torqued to at least 40nm. There is a tiny bit of movement/play with the cassette but I believe that's normal.
• Reindexed the gears and confirmed low and high limits are correct.
• The route of the chain through the rear derailleur is correct.

Answer 1

The clunk you demonstrate in your video is normal. Some frames, deep dish wheels, or especially a full disc wheel, can amplify the clunking sound of shifts to smaller cogs on a rear cassette under load.

As to why this clunk is louder going from the second-smallest to the smallest cog vs. other shifts, one possibility is how abrupt the chain meshes with the new cog it has been shifted to. A larger cog distributes this meshing of chain and cog more gradually, loading some teeth on the new gear selection while others have not been cleared of the larger cog being shifted out of yet.

When getting down to an 11-tooth cog, there is not enough teeth on the cog to distribute this "gradual" meshing effect. Let's assume that we even have five or six teeth engaged in an 11-tooth cog, there is not enough extra teeth to moderate this meshing and therefore when the chain engages the 11-tooth cog, it drops into it all at once -- BAM! And there is your clunk.

Ways to test this theory (subjective, and even so, minor differences if any):

• Your 15 --> 13 shift should also clunk a little more, but not as much as your 13 --> 11 shift.

• The clunk "may" be even a tiny bit louder when on the big chainring vs. the small chainring. When on the bigger chainring, there is slightly less wrap up of the chain on the rear cassette, which "may" enhance the clunk.

• Use no-chain loading while shifting to the 11 cog. You may still hear a clunk (albeit quieter) as the derailleur cage spring (the only load on the chain at this point) pops the chain into the 11 on the cassette. You can probably view this literally watching the chain drop in.

Another factor may be the percentage of gear change may be greater than other shifts and that is a factor. With your ten-speed drivetrain, if you are running an 11-34 or an 11-36 GRX 10-speed cassette, your three smallest cogs are 11-13-15.

The 13 --> 11 shift is a 15.4% change whereas the previous 15 --> 13 shift is a 13.3% change.

• 13 --> 11: 100 * (13-11) / 13 = 15.4%
• 15 --> 13: 100 * (15-13) / 15 = 13.3%