Rear Mech Cage Length Question

[TheDoctor]

This may be a terminology issue. You need to add an inner and outer link because that's how chains are made. Now, I'd probably say add a link but actually be thinking of a pair of links. I'd never get involved with halflink chains unless I absolutely had to.

 

[roubaixtuesday]

Is it a pair of outer plates (like a split link) or is it a pair of outer plates and a pair of inner plates as well?

I think of it as the latter, just because that's the smallest quantum of chain you can extend by.*

But to my other point, I think you only need 2 extra teethsworth because three chain will only wrap half of the rear cog.

And finally, I'll speculate that the chain may well be long enough anyway - I found mine was when I made an equivalent change. But that would need to be tested.

 

[roubaixtuesday]

*unless you use one of these

https://www.acycles.co.uk/kmc-single-speed-chain-half-link-15708.html

 

[Ajax Bay]

One of the ways is large to large (omitting the Derailleur) plus two links

Two half links - one inch long.

use the current length, which is suitable for 28T, and add 4 to make it suitable for 32T.

Chain is engaged with half the teeth of the sprocket, so 16-14 extra half links (as has been said above by @roubaixtuesday ).

when you are running big-big or a combo close to it, it surely takes more than 2 links to account for the amount of chain running through the mech and around the 2 jockey wheels?

Nope. One link (one inch) is enough to cater for the cage at about 45 degrees and still be comfortable (safe changing into inadvertently) large to large.

 

[greenmark]

What I'd do would be use the current length, which is suitable for 28T, and add 4 to make it suitable for 32T. No measuring required. It assumes that your current length is right.

Someone will probably be along in a minute to tell me I'm an idiot.

You're not quite an idiot because you said it assumes the current length is right.

But if it is wrong, then adding more links could end up with too much and the derailleur jamming when you go small x small. So it would not be wise to assume the initial set up is right.

My favourite method on chain length is:
- adjust b-screw so that top jockey wheel as has least 10mm clearance from each sprocket
- run the chain big x big through the derailleur so that the jockey wheels is no more than 45 degree angle to vertical
- then add one link pair (one outer + inner)
- then check small x small to make sure there is no chain wrap around

 

[EasyPeez]

Nope. One link (one inch) is enough to cater for the cage at about 45 degrees and still be comfortable (safe changing into inadvertently) large to large.

Fair enough.

And finally, I'll speculate that the chain may well be long enough anyway

My chain is currently a little noisy when on the big chain ring and towards the top end (lower gears) of the cassette, even though it is always immaculately cleaned and lubed, but perfectly quiet in all the other gear combos. I have wondered for a while if this might be because I cut it a link short last time around, so that it's a bit strained at the 'top end'. So I was going to give it an extra link anyway next time, to see if that would be better. As such, I'll probably try adding 2 extra links (4 half links) when I put the 32 cassette and new chain on.

Two half links - one inch long.

Good clarification - I wasn't sure what we were counting as a link so thanks for making that obvious.

So, given the above, and the fact that I'll be using a quick link to join the chain, I think this means that I actually need to add 1.5 links + a new quick link (or 3 half links + quick link).

 

[Kestevan]

Just moved Mrs kes from a 28 to 32 cassette (Ultegra 6800). It worked with the short cage, but if cross chained it was either very very tight or slack as a slack thing.

If you're mechanically sympathetic, and can guarantee not to cross chain it when tired/distracted then should be no real problems......

I decided to replace the short cage mech with a medium - sodds law says you'll forget half way through a long ride and end up with something getting broke...

 

[Ajax Bay]

If you . . . can guarantee not to cross chain it when tired/distracted then should be no real problems...... . . . - sodds law says you'll forget half way through a long ride and end up with something getting broke...

It's not worth the risk of fitting a chain the length of which does not allow large to large. Sooner or later you will inadvertently make that shift, and it'll either just make it and the RD cage will be flat forward and the return run of the chain tight, or it'll go 'bang'.
The risk of the (inadequate) RD cage not wrapping the 'long' chain is much less traumatic: in fact the symptom is almost helpful. When the rider in their small(est) chainring shifts down to a 'too small' sprocket, the chain will wrap onto itself and create an audible, recognisable noise which will prompt the rider to shift to a/the large(r) chainring (if necessary double shift ie one up on the back as well). Note that this circumstance is rarely when the drivetrain is under stress: unlike the large to large situation above. Worth recognising that a 'too long' chain is more likely generally to bounce around and may come off (chainring shift - even with FD limits correctly adjusted).

adding more links could end up with too much and the derailleur jamming when you go small x small.

Not really. The RD doesn't respond by jamming if the chain's too long (see description above).

then check small x small to make sure there is no chain wrap around

Having made sure it's long enough to go large/large plus one (inch) link, what is your solution if there IS chain wrap around? I assume 'new (longer cage) RD. I suggest one doesn't need to be as radical (see above).

 

[Ming the Merciless]

Replaced a chain yesterday. I go for the big / big (not passing through derailleur) and overlap by two chain links method. Works a treat as far as I am concerned.

 

[Blue Hills]

On chain length I always use the system outlined in the excellent Park Tools book.

Haven't read all through it but this looks like it, from point 4 onwards.

https://www.parktool.com/blog/repair-help/chain-length-sizing