Loose chain?

[DWiggy]

I have recently got myself a new bike, a BMC Street racer 2011 with 105 group set.
I was messing around cleaning and oiling the chain when I noticed that when I select the small cogs front and back the chain is very slack with hardly no tension and it was resting against the stay? Looks like the chain might be too long as the derailleur is as far back as it can go (Apparently the old owner replaced the chain not long before I purchased it) (BTW its a compact)

Thanks in advanced

 

[Lee_M]

they arent designed to go small cog to small cog.

 

[DWiggy]

they arent designed to go small cog to small cog.

Did think that but just doesn't look right

 

[Lee_M]

I'd be more concerned with how tight it is on the biggest cog rather than how loose on the smallest. My ultegra setup leaves a mess on the chainstay if I end up on small/small

 

[RecordAceFromNew]

Did think that but just doesn't look right

Use this to check how many links are needed and how many are there. But if you are planning to remove any make sure you check it is long enough for big big first.

 

[Cubist]

they arent designed to go small cog to small cog.

Did think that but just doesn't look right

However, sometimes you can shift there inadvertently, and if the chain's too long it'll unship or break. If the mech is folded back on itself with no tension in this combo, then yes, the chain's too long. As a check, try big/big, where the mech should still have some spring left in it, but if it still hangs anywhere below 5 O'Clock, could do with a shorter chain. The mech can be almost horizontal in big big, as long as there's a recognisable S shape to the chain running through the cage it's OK.

 

[Rezillo]

I've still got tension on the smallest cog settings and have always used the following from the Shimano rear derailleur instructions. I've never been that clear, though, about how to set up the B tension screw or just what the second bit is about.

 

[Cletus Van Damme]

Use this to check how many links are needed and how many are there. But if you are planning to remove any make sure you check it is long enough for big big first.

Thanks for that link mate that is of real use. I was just wondering when it states over-sized derailleur does it mean a long cage mountain bike rear mech?

 

[RecordAceFromNew]

I was just wondering when it states over-sized derailleur does it mean a long cage mountain bike rear mech?

He is referring to rear mechs with large (15T/13T) jockey wheels. You find them on some Shimano Megarange rear mechs.

 

[DWiggy]

Just some photos showing gearing in different positions, hope this helps...or am I just being paranoid:

Big / Big


Small / Small


Small / Big

 

[GrasB]

You've got a lot of capacity left in the big-big combo. I'd pinch 2 inner links together on big-big to see if you can remove a pair of links like the top section chain below.

 

[DWiggy]

You've got a lot of capacity left in the big-big combo. I'd pinch 2 inner links together on big-big to see if you can remove a pair of links like the top section chain below.
View attachment 22425

Will give it a go, thanks (did think the chain might be a tad too long)

 

[GrasB]

For non-racing use I prefer my chai to be as long as possible, so I'd only take out the links you need to maintain tension on small/small. The reason of this is so you have as much chain as possible for sorting out a broken chain in-ride.

For race day bikes I run the chain as short as possible, this improves the mech's chain tension control & thus shifting in non-optimal conditions.

 

[Cletus Van Damme]

I was just wondering if anybody could give me some advice regarding my chain etc. I needed to lower the gearing on my bike, so put a 28t chainring on and a recently a 28t cassette. The existing Tiagra rear mech just could not deal with this, I could of tried shortening the chain but I am sure it was a lost cause as the rear mech apperared to be touching the cassette in just about every combination of chainring/cassette. So I got a Deore MTB rear mech that's specification is cassette large sprocket size 28t-34t. Things are way better but when setting up the B screw tension on the smallest chainring and largest cassette sprocket I cannot get the jockey wheel as close to cassette as I could with my old Tiagra setup. The screw is just about backed right out. However when I move to the smallest cassette sprocket as listed in the documentation they are much as I would expect. I do not think my chain length is too long as I used the calculator as listed above and adjusted the length accordingly. I just wondered is this a characteristic of a MTB rear mech? I have not had chance to test it yet on the road as I need another cassette spacer. I have attached some pictures below at various chainring/sprocket combinations to see what you guys reckon of the chain length. Thanks for any adivce.

 

[RecordAceFromNew]

I was just wondering if anybody could give me some advice regarding my chain etc. I needed to lower the gearing on my bike, so put a 28t chainring on and a recently a 28t cassette. The existing Tiagra rear mech just could not deal with this, I could of tried shortening the chain but I am sure it was a lost cause as the rear mech apperared to be touching the cassette in just about every combination of chainring/cassette. So I got a Deore MTB rear mech that's specification is cassette large sprocket size 28t-34t. Things are way better but when setting up the B screw tension on the smallest chainring and largest cassette sprocket I cannot get the jockey wheel as close to cassette as I could with my old Tiagra setup. The screw is just about backed right out. However when I move to the smallest cassette sprocket as listed in the documentation they are much as I would expect. I do not think my chain length is too long as I used the calculator as listed above and adjusted the length accordingly. I just wondered is this a characteristic of a MTB rear mech? I have not had chance to test it yet on the road as I need another cassette spacer. I have attached some pictures below at various chainring/sprocket combinations to see what you guys reckon of the chain length. Thanks for any adivce.

Firstly the Deore's angle looks a little odd - are you sure your B screw is catching the tab on the hanger like pic below?

Secondly I see absolutely no reason why the top jockey of a Tiagra rear mech would touch a 28T rear sprocket. Something is definitely not right.