Long cage rear derailleur required?

[Deefex]

I bought a Cannondale CAADX Tiagra recently with a standard cyclocross 46/36 crankset and a 12-28 cassette, but due to persistent creaking problems that repeated trips to the LBS didn't solve (Long storty which I won't bore you with here), I opted to swap out the BB using a BB30 english adaptor for a standard Shimano BB and replace the FSA crankset for a Tiagra 50/34.

This worked well, and after some tinkering with the front mech it seems ok except the large chainring-large cassette combo which grates something terrible. The front mech seems to be clear so the grating noise seems to be coming from the rear mech.

Now, I know this isn't advised as it's bad for the drivetrain, etc. but it got me thinking whether I need to consider a 'long' cage rear derailleur to cater for the larger chainring on the new crankset (50 teeth as opposed to 46)? The rear mech I have is simply badged as a 4660 Tiagra - no mention of whether it's long/medium/short, etc.

How do you know whether you need a long cage derailleur or not (ignoring triple cranksets for now)?
How can you tell what size of rear mech you currently have?
Are there any rules of thumb that can be applied for non-triple crankset setups?

Thanks in advance

 

[fossala]

My guess is your chain is too small. Long cage (medium) are for triple shimano chainsets.

 

[Jon2]

Long cage (gs) are needed for compacts sometimes, especially if you have a wide range of ratios. You need to find the tooth capacity of the rear mech you have online and see if it's enough for your setup.

Agree with fossala that the chain is probably too short. Does the chain still have tension on it when on small and small combo? If it does then it could just be the chain, if not then it could be that you need a larger cage mech, and a new chain to go with it.

 

[accountantpete]

You just need to calculate the chainwrap - which is dead easy

Difference of chainset 50 - 34 = 16

add

Range of cassette 28 - 12 = 16

Total 32.

So your chainwrap is 32 - now check the Chainwrap Capacity for your existing mech which will be around 29-31.

See this ad http://www.chainreactioncycles.com/Models.aspx?ModelID=19461



That is borderline but should be ok so I'd try a new longer chain first and if you still have problems get a long cage (actually it's a medium called GS in Shimano speak).

 

[Deefex]

Thanks for the replies.

I'm not sure I understand what you mean by 'chainwrap' though.
Does this tie in with chain length? (Number of links?)

Also not sure how to interpret the stats in your link. E.g. "Total capacity: SS 31T, GS 37T"

 

[mcshroom]

Its about how big a difference you can have between the most tensioned your chain would need to be (big front and big rear) and the least tension it would need (little front and little rear). Basically the mech needs to be able to change shape far enough to tension at both extremes. The longer the mech, the greater range of tensions it can cope with.

Another way of calculating this is to subtract the total number of teath used at lowest tension from that at the highest:-

capacity = highest - lowest = (50+28)-(34+12) = 78 - 46 = 32t.

As you quote the maximum range you should be able to do with an SS is 31t, but Shimano are a bit conservative so there's a good chance that a 32t difference will work ok.

 

[Deefex]

How does this translate into something meaningful though, like say chain length (No of links?)

 

[PpPete]

How does this translate into something meaningful though, like say chain length (No of links?)

Depends on the length of your chainstays.
There is software on Machinehead to calculate it for you, but I CBA to measure and count links
Just wrap the chain round big-big without going through derailleur, add two links, remove any surplus, run it through the derailleur, join it up with the quick link and you are done.

As per mcshroom's comment above, I'd expect a short cage mech with a nominal capacity of 31T should, in practice, be able to cope with a maximum difference of 32T.
Don't hold me to that though, I run a triple and push the GS mech (medium cage) far, far beyond it's nominal limits.

 

[RecordAceFromNew]

How does this translate into something meaningful though, like say chain length (No of links?)

The minimum allowable length of a chain is governed by the length of the chainstay (from centre of bottom bracket to rear hub axle) AND the size of the large chainring AND the size of the large rear sprocket. If the bike has a rear derailleur then it also needs to cater for that to allow the chain to run around the two jockey wheels properly. You can either use a chain length calculator (which asks you for the chainstay length and chainring and large rear sprocket size) or use the "wrap around large large" +2 links rule as PpPete said.

Rear mech chain wrap capacity is determined primarily by the length of its sprung cage, the objective of which is to ensure the slack of a chain (at minimum allowable length determined per above based on large large) can be taken up when the chain is on small front and back.

As others have said it is most likely that the size of your rear mech is fine for your set up. Horrible noise at large large can be caused by all sorts of things. When you changed the chainset did you get a new longer chain? If not it might be too short as fossala said (since you used to have a 46T large front ring but it is now 50T, so a pair of extra links are probably needed). See this for a visual check.

 

[Deefex]

Ok, fitted a longer chain this morning and it looks to have done the trick.
The drivetrain sounds fine, and the only thing that creaks is me - normal service has been resumed.

Thanks for the elaborations above. They've been very helpful.