Englishtourer

This one's got me scratching my noggin somewhat.

Recently towards the end of my ride i noticed an odd sensation, under pressure my crank seemed to slide forward slightly.
Only a little each time but it happened about 3 or 4 times.

I thought the issue may lie in the freewheel (which I rebuilt a while ago)
I noticed that the bearings could probably do with replacing so just in case I tore it down again, cleaned re-greased new bearings and reassembled.
First ride it happened again (only once this time).

From what i've read of similar issues it can be worn ratchet pawls in the freewheel, however these seemed alright on inspection.
I've also heard this can happen from worn chainrings however these were replaced around 6 months ago and are in good order, as are the teeth on the rear. As far as i can tell anyway... they're certainly not sharks teeth and anyway it definitely felt like more of a slip than a the chain jumping.

The shram chain is also fairly new (less than a year).

The only thing i can think is if the freewheel itself has somehow come slightly loose from the hub?

The freewheel is a Suntour New Winner, Used but teeth look in good order to me.
The rear derailleur is a Suntour Cyclone GT
The front chainrings are a bit of a mix but all new about a year ago.

Does anyone have any suggestions as to what might be going one?
'm at a loss!

Iride01 

If you certain the cogs aren't worn on the front or back and the chain isn't worn, then the freewheel is a suspect. Did you use grease in it that might not be letting the pawls engage fully?

Grease is fine for the bearings, but my preference would be nothing but a light coat of some oil or petroleum based preservative on the pawls just for corrosion control. Since they should not be doing anything until you coast, there isn't much need for actual lube for them. IMO.

And does this happen a lot? Or maybe just a couple times? If just a couple times, maybe you didn't have the freewheel tightened all the way.

Englishtourer

Thnaks for the reply, the freewheel was my first thought too but I've just rebuilt the freewheel for the second time to make sure. The pawls weren't gummed up in any way (i've run light oil through it regularly).

I've been running the bike with this set up for a good few months with no major issues so its odd that its happening now if it is the freewheel being loose from the hub itself.
the only issue i've noticed up till now is occasional chain jump on the smallest sprocket, but this recent issue fees very different, like a smooth slip.

Iride01 

If the freewheel was loose, I can only barely imagine it being loose enough on the hub for it to slip enough for you to really notice it. And certainly it would be tight now if it did slip once or twice.

Is it a special FW that a new or NOS Shimano FW wouldn't be worth the 20 or so bucks to replace it with? Though if it's your chain or rings on the front, then you'd wind up with a spare FW. <grin>

Maybe someone else will come up with some better ideas what else to look at. That's about it for me.

Englishtourer

Just had another close look found the culprit!

The front chainrings are not very worn, but one of them is missing a tooth!

Can't believe i missed this (bit embarrassed really)

ClydeClydeson

Your description sounds like what happens when you put a new chain on worn cogs. The wear on cogs is seldom visible by eye, but the teeth will wear down to match whatever chain you were using, and a new chain won't quite mesh properly,. THis often happens on only the most used cogs.

ClydeClydeson

Also, the 'missing tooth' on the chainring is likely just a 'gate' - a tooth or teeth with different profiles to make it easier for the chain to move between gears. A single missing tooth would not cause any problems because you have at least 30-odd other teeth left to do the job.

Englishtourer

Thanks for your advice,

I know there can be issues with mismatched chain and teeth however the chain was new at the same time as the chainrings so they should have meshed together.
I suppose it could be possible for this to be happening on the rear sprockets of course as they are older but i'd be surprised as they appear in very good condition and there has been no issue with them for several months.

The missing tooth is definitely not a 'gate' I'm aware of these, however they don't usually come accompanied with a jagged uneven edge and fracture lines!
I agree it's strange that this is happening as the other teeth should take the strain.
I'm guessing that this may be happening at the points when the (missing) tooth should be engaging or disengaging with the chain. (just before 12 of 6 o clock)

its certainly also odd that the tooth has fractured, this must have been a manufacturing fault, as (being the middle ring) its unlikely to have had any strong imact.

Anyway a replacement chainring is on its way so fingers crossed it helps...time will tell!

dabac

How is the chainring attached to the crank arm?
I’ve encountered a few bikes with the chainring basically peened or swaged onto the base of the crank arm. On those, the chain ring can slip under pedal pressure. If you keep at it, it’ll eventually spin freely or even break loose entirely.

bikebikebike

Great thread.
I have a 2016 low mileage Cannondale Quick Disc 5 that I bought with a Bafang BBSHD mid drive.
Same kind of skip under load. Intermittent with throttle of this overpowered motor in middle gears.
I did a piecemeal replacement of the usual suspects with the answer being at then end (of course)
Nothing looked bad until I isolated the problem by swapping another wheel in. Freewheel pawls were just not up to the task.
Cannondale appears to use some weak Forum OEM freehubs that are not branded. Prolly work OK for most situations.
Checklist needs to include, Chain, Chainwheels, cassette, RD: hanger alignment cable and idlers, and freewheel/freehub.
Cassette cogs can be tricky to clear and probably best checked by substitution,as are most of the other components if they look generally OK .
It really helps to have extra parts, or extra friends.