Crank makes a sudden jerky movement...

[Cubist]

Rusty Bearing has spotted your problem Maz. If you look at your top pic of the chainring , the teeth at one to two o'clock, with the downtube behind them, are virtually non-existent. If you were to push hard where they are taking the load from the chain, they'll slip giving the effect you describe. The fine detail pic of the tooth over the chainstay is a classic "sharksfin" shape of a worn chainring. Finally, the biggest giveaway is the excessively scalloped or scooped area between the teeth, most visible at 12 o'clock in your top photo. Using these as markers, check your inner ring as well

Can't comment on the cassette. It looks OK, but would need an expert eye.

 

[Maz]

Other loose crank types can display the same behaviour.

Hi - Checked the crank. Seems nice and tight.

 

[byegad]

With outboard BB bearings they do wear. I got 4500 miles out of mine on my Kettwiesel and being a recumbent that lives out of range of wheel spray. I agree the chainrings are gone.
I'd also check the BB for slog. If they have any movement then fitting new ones is a piece of cake with the right tools, which will cost as much as most LBS's charge to do the job, so next time you just buy the parts.

 

[Maz]

Rusty Bearing has spotted your problem Maz. If you look at your top pic of the chainring , the teeth at one to two o'clock, with the downtube behind them, are virtually non-existent. If you were to push hard where they are taking the load from the chain, they'll slip giving the effect you describe. The fine detail pic of the tooth over the chainstay is a classic "sharksfin" shape of a worn chainring. Finally, the biggest giveaway is the excessively scalloped or scooped area between the teeth, most visible at 12 o'clock in your top photo. Using these as markers, check your inner ring as well

Can't comment on the cassette. It looks OK, but would need an expert eye.

I was hoping you wouldn't say that.
Is it an easy job to replace the chainring(s)? Do they come as a pair or separate? Special tools?

 

[4F]

Here you go maz http://bicycletutor.com/replace-chainrings/

 

[HLaB]

I was hoping you wouldn't sat that.
Is it an easy job to replace the chainring(s)? Do they come as a pair or separate? Special tools?

I've never done it beforebut I have investigated it; you can buy separate chain rings (the ones I looked at if iirc reanged between £30 for a small ring and £60 for a large ring). You only thing you have to watch out for is to get ones of the correct BCD (110mm for a compact) and the same bolt patern (5 bolt is common). Ive only ever tightened chain ring bolts in theory you need a special tool to hold the bolt while you tightening them with an allen key but in practice I've never used a special tool bolts either hold them self or can be restrained by a large bladed flat screwdriver or similar.
There was a good thread about it a while ago iirc you can unscrew the old rings and slip them over the cranks, its the same for installation.

 

[Cubist]

£35 quid, in stock at Chain reaction

http://www.chainreactioncycles.com/Models.aspx?ModelID=38169


About fifteen to twenty minutes to swap them over, you don't even need to take the pedals or cranks off. Your chainring bolts come apart either using an allen key and a broad blade screwdriver, or if you're lucky two allen keys, one from the back and one from the front.

 

[Maz]

£35 quid, in stock at Chain reaction

http://www.chainreac...x?ModelID=38169


About fifteen to twenty minutes to swap them over, you don't even need to take the pedals or cranks off. Your chainring bolts come apart either using an allen key and a broad blade screwdriver, or if you're lucky two allen keys, one from the back and one from the front.

Thanks, Cubist.
Will this one fit my bike? How did you figure this out?
What does PCD mean? Is it the diameter of the ring...edit...I just googled this for the answer.

 

[Broadside]

Potsy - my bike (Secteur Sport) is also making a cracking sound as I set off/put down the power, it sounds like you have sorted it out on yours. What chainring did you buy as a replacement? I am wary of putting an incompatible chainring on because I know the teeth have to line up or you will get chain suck, this happened once when I put the chainrings back on without getting the markers lined up properly.

 

[potsy]

Potsy - my bike (Secteur Sport) is also making a cracking sound as I set off/put down the power, it sounds like you have sorted it out on yours. What chainring did you buy as a replacement? I am wary of putting an incompatible chainring on because I know the teeth have to line up or you will get chain suck, this happened once when I put the chainrings back on without getting the markers lined up properly.

Hi- I got a Sora 39t middle ring from CRC I think it was, £10.99 anyway, quite straight forward to fit if you mark up where the rings should be before you take it all apart
Just make sure you get a 130BCD in whichever one you go for, the Sora one looked identical to the one that was on there anyway.
Done nearly 80 miles since fitting and the cracking noise hasn't been there at all.

 

[rusty bearing]

Its BCD.... Bolt Circle Diameter....if you had a four bolt chainring it wouldnt matter but you dont so it does. Take chain ring to your LBS then you can be sure of getting the right ones. Yes easy enough to change but do dab a spot of Loctite on the bolt when you refit it.
If you haven't got a convenient LBS try
http://www.cyclefix.net/business_directory.html

 

[Broadside]

Thanks all, will try replacing my middle chainring and hope that sorts it.

 

[Cubist]

Thanks, Cubist.
Will this one fit my bike? How did you figure this out?
What does PCD mean? Is it the diameter of the ring...edit...I just googled this for the answer.

Maz

Bit of detective work. The link to your bike showed that it runs a 50-34 compact Tiagra chainset. It's 5 bolt, 110 BCD, confirmed by the photos you posted. On the CRC link I sent you look to the right of the picture. The picture is actually of the 34T inner ring, but the link to the right shows two types in stock, and the lower of the two is the 50T outer for £34.99

 

[Broadside]

I think for about £50/60 you could get a complete crankset which I would do rather thank shelling out £35 for just the outer chainring. The crank is heavily scuffed already, a new one would look much nicer!

 

[Ivan Ardon]

What about turning the existing rings around to use the unworn face of the teeth? It's worth a try before shelling out, if you're trying to do this on a budget.