Deep ‘thrumming’ vibe from chain set when in top gear?

[Sloth]

I have had a GRX 1x11 fitted in place of the 105 2x that the bike came with.
The bikes a Giant Fastroad Advanced 1 (2022).
We all know the annoying clicking of chain rub when in the largest rear gear and at one extreme of chain angle.
We’ll, imaging the other extreme when in the smallest rear gear.
Instead of a clicking or typical chain rub sound, I get a very low, thrumming sensation at the opposite end of the sound spectrum to typical chain rub.
It only happens when I’m in the smallest gear (remember it’s a 1x)
It’s also almost like I feel some drag or resistance when pedalling in that gear, as if the front chain ring is somehow not aligned and causing this weird low thrumming vibration with every pedal stroke, but only in the highest/smallest rear gear.
It’s not ruining my rides but it just doesn’t feel ‘right’.

 

[C R]

I have had a GRX 1x11 fitted in place of the 105 2x that the bike came with.
The bikes a Giant Fastroad Advanced 1 (2022).
We all know the annoying clicking of chain rub when in the largest rear gear and at one extreme of chain angle.
We’ll, imaging the other extreme when in the smallest rear gear.
Instead of a clicking or typical chain rub sound, I get a very low, thrumming sensation at the opposite end of the sound spectrum to typical chain rub.
It only happens when I’m in the smallest gear (remember it’s a 1x)
It’s also almost like I feel some drag or resistance when pedalling in that gear, as if the front chain ring is somehow not aligned and causing this weird low thrumming vibration with every pedal stroke, but only in the highest/smallest rear gear.
It’s not ruining my rides but it just doesn’t feel ‘right’.

That sounds more like the rear wheel bearings to me.

 

[fossyant]

Potentially it's the chain angle. GRX chain line is a little further out than standard. I assume nothing is 'worn' ?

 

[wafter]

Potentially it's the chain angle. GRX chain line is a little further out than standard. I assume nothing is 'worn' ?

IIRC the GRX chainline is a whole 2.5mm further out than the road groupsets; however limitations of 1x notwithstanding this should improve the chainline in the smallest sprocket.

In the absence of any better ideas I'd be inclined to chalk this up to the 1x setup; potentially exacerbated by the small sprocket and extreme chainline. That said with the addtional offset I suppose the chainline shouldn't be that much worse than the with 2x on the largest chainring.

 

[fossyant]

To be honest, small sprockets can run a little grumbly unless you are spinning - I rarely use the smallest sprockets on any of my bikes unless I'm going down a dirty big hill, even with a 46x11 on the CX bike (when with road tyres).

By Smallest we are assuming sprocket. Big gear @(referring to the ratio) to me means small sprocket, small gear means biggest sprocket. Had OP said smallest sprocket, the first paragraph applies.

 

[C R]

To be honest, small sprockets can run a little grumbly unless you are spinning - I rarely use the smallest sprockets on any of my bikes unless I'm going down a dirty big hill, even with a 46x11 on the CX bike (when with road tyres).

By Smallest we are assuming sprocket. Big gear @(referring to the ratio) to me means small sprocket, small gear means biggest sprocket. Had OP said smallest sprocket, the first paragraph applies.

I assumed smallest sprocket and that's why I suggested wheel bearings. The higher rotation speed under load makes dodgy bearing rumble louder.

 

[ColinJ]

My bike in Devon is a 1x11. When backpedalling it has a little more friction in the transmission than I am used to, and it makes a subtle noise. I suspect that the tighter fit between the chain and the narrow/wide chainring is the cause. It feels perfectly okay when riding though.

 

[wonderloaf]

I had this recently, disappeared after changing the freehub bearings.

 

[Sloth]

Thanks all.
It's a fairly new bike with low mileage so I'd be surprised if it was bearings, although it's possible I suppose.
Yes, I mean the smallest rear sprocket.
As I said, it only does it in the smallest rear sprocket, in time with pedal strokes, and I doubt I'm fast enough to overwork wheel bearings.
Doesn't do it when freewheeling, only when pedalling.
It never did it with the 105 2x11 set up, even in the extreme angles, usual chain rub noise yes, but not this deep rumbling noise/sensation.

 

[wonderloaf]

I had this recently, disappeared after changing the freehub bearings.

Just to add a bit more detail, I noticed that there was a slight play in the cassette sprockets, initially I thought the locknut wasn't tight (but it was) which led me to find I had play in the freehub bearings. Have you checked for play in the cassette?

 

[wafter]

Since it's with every pedal stroke, it's not the sound of the sound of the tyres on the road perchance..? Does it change with the amount of force you're applying to the pedals?

 

[Sloth]

Just to add a bit more detail, I noticed that there was a slight play in the cassette sprockets, initially I thought the locknut wasn't tight (but it was) which led me to find I had play in the freehub bearings. Have you checked for play in the cassette?

Yes there is some wiggle in the rear cassette (back and forth, not away from the wheel etc.)
I got the LBS to check it and they said it was ‘normal’ and even if they fitted a new free hub it would ‘be the same after a short while so why bother’. I admit I was t overly convinced but thought they knew better than me.

 

[Ajax Bay]

Potentially it's the chain angle. GRX chain line is a little further out than standard.

Lots of ideas upthread.
I'd have thought it unlikely that the OP is putting significant (ie to cause this compared to large sprocket) torque in their 'top' gear.
I just thought I'd bring facts to the 'chain angle' suggestion ("chain line" is irrelevant init) (and @wafter 's spot on comment).
With a compact (or double) and an 11-28 the chain angle in large large (or small small) is 2.9 degrees.
With a 1x 12sp the 'extreme' chain angle for the longest and shortest gears is 2.6 degrees (ie less, not more).

IIRC the GRX chainline is a whole 2.5mm further out than the road groupsets; . . . this should improve the chainline in the smallest sprocket.
. . . That said with the additional offset I suppose the chainline shouldn't be that much worse than the with 2x on the largest chainring.

 

[gbb]

This 'thrumming ' used to happen to me when fitting a new chain, it usually lasted a few rides then passed. I've had it 3 or 4 times, always when fitting a new chain.
The logic I applied ?, perhaps incorrectly, new chain, old cassette teeth caused a temporary mismatch and vibration. This would pass as the two meshed with a little time and use.

So perhaps because your smallest sprocket has rarely ever been used, similar is occurring ?

 

[fossyant]

Is the limit screw set properly as it's the smallest sprocket ?