Crank makes a sudden jerky movement...

[Maz]

Hello,

Recently, I've been having some problems with the front crank (or at least, that's where the noise/movement seems to come from).

When I start off from stationary (at lights, in traffic, etc) , applying pressure on the pedals, occasionally the crank jerks foward maybe an 8th of a turn or less. It's a bit worrying.

The chain doesnt seem to be slippling (I changed it a few weeks ago, ensuring that the old chain was not too worn out - used a chain wear indicator - so cassette not changed).

I wonder if the BB is worn out (sealed unit on a Spesh Allez Elite?). Having said that, do sealed units get worn out?

Any ideas what might be wrong? Thanks

 

[potsy]

Have you checked the chainring Maz? Mine has been doing something similar and I eventually worked out it was the middle ring, £10 for a new one and it is now fine.

 

[Maz]

Have you checked the chainring Maz? Mine has been doing something similar and I eventually worked out it was the middle ring, £10 for a new one and it is now fine.

Hm, I'll have to take a close look at the teeth on the chainring (vast majority of my riding is on the bigger of the 2 rings).
I've never changed the chainring and it must be 3 y/o from new, used regularly in that time.

 

[potsy]

That's why I thought of it, mine is a triple and the middle gets used 95% of the time, 4000 miles had started to wear it out. Mine was also making a 'crack' sound when I set off or put a bit of power down.
New chain,cassette so knew it wasn't those.

 

[Gerry Attrick]

Ae you sure it's not the cassette Maz? If you changed the chain but not the cassette, I would reckon the cassette is your prime suspect.

One other thing. If the chain is Shimano, did you use a new connector pin? If you did, it may be a stiff link causing your problem.

 

[Fab Foodie]

Is the crank very slightly loose on the BB axle?
The old square-taper types would develop a little bit of play and it would give the effect you describe as the ally deformed.

 

[Maz]

Ae you sure it's not the cassette Maz? If you changed the chain but not the cassette, I would reckon the cassette is your prime suspect.

One other thing. If the chain is Shimano, did you use a new connector pin? If you did, it may be a stiff link causing your problem.

Gawd, I hope it's not the cassette, because I bought the chain wear indicator tool so I could change the chain early enough to not need to change the cassette (the current cassette was put on new when I did the last but one chain replacement).

Chain is an SRAM with powerlink.

 

[Gerry Attrick]

Could you post clear side on pictures of the cassette and chainset?

 

[Maz]

Is the crank very slightly loose on the BB axle?
The old square-taper types would develop a little bit of play and it would give the effect you describe as the ally deformed.

Hi, I'll check tomorrow to see if there's any looseness, but I don't know if the bike's old enough for the square-taper crank.
It is a Specialized Allez Elite 2008
http://www.evanscycl...d-bike-ec001387

 

[Fab Foodie]

Hi, I'll check tomorrow to see if there's any looseness, but I don't know if the bike's old enough for the square-taper crank.
It is a Specialized Allez Elite 2008
http://www.evanscycl...d-bike-ec001387

Other loose crank types can display the same behaviour.

 

[TheDoctor]

...and will be similarly buggered if ridden loose for any amount of time. I've killed a Hollowtech crank like that

 

[snailracer]

Freehub is probably slipping due to gummed-up pawls not locking reliably. You can tell if the clicking does not sound precise and loud when turning backwards. Flush with solvent, leave to dry then relube with oil.

 

[Maz]

Could you post clear side on pictures of the cassette and chainset?

 

[Maz]

Freehub is probably slipping due to gummed-up pawls not locking reliably. You can tell if the clicking does not sound precise and loud when turning backwards. Flush with solvent, leave to dry then relube with oil.

Hi, funnily enough, I did all this about 2/3 weeks ago, so probably not the problem in this case.

 

[rusty bearing]

You should change your cassette by now. the wear isn't obvious but its there nonetheless. Shows as problems changing gears. Your front outer is completely b******d, there's not enough of the teeth left to prevent the chain riding over them under pressure. Cant make out the inner but its probably not much better.