Diagnosing "clonk" in drive train.

[Brucey]

Again b. obvious, but when a new fault appears immediately after you have 'done something' then the 'something' is most likely to be the cause of it.

In this case the 'something' was not only the fitment of a new small chainring as originally stated but also (we later find through questioning) the fitment of a new chain.

The OP's description of when/how the clunk occurs actually makes sense as a (new chain/old sprocket type) skip when using a gear near to 1:1, so that ought to be eliminated from the list of possible causes.

cheers

[pwa]

there are all manner of things that might be loose and move, but if the clonk is (not quite as you describe) really a slip as well and it occurs when you are mid power stroke, under load, it just might be a new chain slipping on older sprockets....?

cheers

Despite reading all the sage advice here I still struggle with the concept of the sprockets wearing out in a year and somewhere over 1,000 miles on the road.[ Hmmm....20 miles a week for a year is around 1,000 miles. Possibly a bit more, then? Ah. Odometer says 1874 miles.]

It isn't that long since I put another new chain on, following the advice about changing chains on a fairly regular basis.

I am (old MTB) used to running a chain until, well, forever.
Still on the original chain rings and cassette on my 90's MTB, although I'm sure I've changed the chain at least once.
I know I replaced the rear derailleur a few years back.
Possibly the difference between a 7 speed and 9 speed?

If it is new chain/worn sprockets can I assume that the chain will bed in after a while?
I will still check the other suggestions.

From what you say, I doubt the sprockets are significantly worn. 9 speed sprockets don't wear down quickly.

Another thing that can produce odd sensations in the drivetrain is a malfunctioning jockey (pulley) wheel on the rear mech. And they are easy to change if that were it. Lift the chain off each one and check the jockey wheel goes round with no discernable friction. It is a twenty second check so worth doing.

I wonder how the smaller front ring might have affected the position of the rear mech if the chain is the original length. In bottom gear, does the top pulley wheel come very close to touching the big sprocket? That is another easy fix if that were it.

The symptom does sound like the chain hanging up on top of a chain ring tooth then suddenly dropping down to the proper position. Could it be that the cranks have been off the bike and have been put back on but not quite tight enough. I would be putting the bike on a stand and running it very slowly in top gear and looking for signs that the chain is not running centrally on the new chain ring, and brushing against the inner edge of the ring more than the outer. Worth a look at least.

[Brucey]

I have occasionally had skipping with new chains on new sprockets; anything is possible and the potential effects of recent changes are the first things to investigate.

cheers

[pwa]

A stiff chain link is another thing to check for.

[LittleGreyCat]

More information: the problem did not manifest straight after the fitting of the new chain and smaller front chain ring.
I have ridden a few times without problems, and the clunk only started on the last ride.
Winter riding and wet conditions so there could be crud in somewhere, I suppose.
Today is not a nice day for on bike diagnostics.

[Brucey]

in which case it might not be related to the recent work. I'd still double- check those things first though.

cheers

[531colin]

......….The symptoms seem to match some play in the chain between the front rings and the rear cassette and an uneven pedalling style, where my leg lets the chain slacken just before TDC on the right...... after a brief pause in the pressure on the freewheel allowing the wheel to go faster than the cassette, .........Does any of this ring any bells with anyone?

I ride with a couple of people who do exactly this.
It seems to go like this; they set the saddle so high that to reach the pedal at the bottom they are on tip-toe and/or rocking their hips.
"Just tapping along" all is well, they can pedal smoothly.
However, looking for high cadence (and/or high power) their pedalling gets a bit ragged, so that the cranks slow down when the pedals are at top/bottom centre, enough so that there is no drive for a fraction of a second. When the drive is re-instated, I hear the freewheel snatch.
I think my riding companions do it with both legs rather than one leg, (ie. twice per revolution, not once) but I'm not certain of that.

You might find lowering the saddle 5 or 10mm fixes it (mark, measure, photo before you change anything)
Alternatively, if you use cleats, its possible to get a short boost in power by pushing your foot forwards over the top and pulling the other foot back at the bottom. This works for me at a middling cadence I can't co-ordinate it at a fast cadence.

[Debs]

My Ultegra transmission has developed an occasion clonk, the chain is a 105, 4 months old with about 1000 miles.

Everything looks to be in good order, tightened to spec, clean regularly and properly lubed.

My fickle finger of suspicion is presently pointing at the sram connector link i used (?)
although this is a fairly recent fault, only the occasionally little chain-clonk over past 100 miles....

[mig]

is the clonk in the OP on every pedal rev when it occurs?

[Brucey]

My Ultegra transmission has developed an occasion clonk, the chain is a 105, 4 months old with about 1000 miles.

Everything looks to be in good order, tightened to spec, clean regularly and properly lubed.

My fickle finger of suspicion is presently pointing at the sram connector link i used (?)
although this is a fairly recent fault, only the occasionally little chain-clonk over past 100 miles....

.

that does sound more like a stiff link than the OP's. Could be the connecting link; there are tiny differences between chain widths and sometimes this matters. Ultegra exists in 6s to 11s forms; which do you have?

cheers

[Debs]

My Ultegra transmission has developed an occasion clonk, the chain is a 105, 4 months old with about 1000 miles.

Everything looks to be in good order, tightened to spec, clean regularly and properly lubed.

My fickle finger of suspicion is presently pointing at the sram connector link i used (?)
although this is a fairly recent fault, only the occasionally little chain-clonk over past 100 miles....

.

that does sound more like a stiff link than the OP's. Could be the connecting link; there are tiny differences between chain widths and sometimes this matters. Ultegra exists in 6s to 11s forms; which do you have?

cheers

11 speed Ultegra though-out, tho' chain replaced with a brand new 105 last October. The Sram link connector is also 11sp.
It ran perfectly from first fit, must of left the previous ultegra chain on too long, the silence that followed was exceptional

The price difference between 105 chain and a Ultegra chain is only £1.00, i do wonder if there is a difference?

I have some brand new KMC connectors, so could swop it out. I usually use the KMCs and never had a problem with them.

[peetee]

Was the bike cleaned before the parts were replaced and if so, how?

[Brucey]

I have some brand new KMC connectors, so could swop it out. I usually use the KMCs and never had a problem with them.

to check for a stiff link, get the bike in the workstand and hold the RD so that the tension on the chain is greatly reduced (i.e. so that the chain sags plenty). Then backpedal slowly, listening carefully and keeping an eye on the chain as it runs through the RD. Under these conditions a stiff link will normally be visible and/or make a noise at it passes through the RD. I don't know if there is a clearance problem with 11s quicklinks but then I don't know there isn't one either.

Another popular failure at this time of year is a freehub body that is on the way out. If there is water inside they tend to misbehave in cold weather. If the water is there for long, they tend to corrode inside and become increasingly prone to intermittent problems.

If you catch it soon enough a good flush with oil in the freehub body normally sorts it out. However if the oil comes out brown then you have probably left it too long.

Also check the RD; the pivots can start to bind and the chain isn't properly tensioned any more. This can lead to skipping too.

cheers

[LittleGreyCat]

is the clonk in the OP on every pedal rev when it occurs?

Yes.
RHS only.

[Brucey]

is the clonk in the OP on every pedal rev when it occurs?

Yes.
RHS only.

If you are strongly right-legged then it still could be anything. But if you are not, this is suggestive of a problem in or near the RHS of the crank; loose chainring bolts, loose pedal, loose BB cup, that kind of thing.

cheers