Cassette change - chain length?

pfm401

Active Member
I'm planning to change from a 50-34 v 12-28 to 50-34 v 11-32. When I usually change chains I simply compare new v old to get the right length ( not changed gear ratios before!)

Is it a simple matter of adding 4 links to cope with the larger cog? Is an outer plate / inner plate set 2 links or 1? Or is it more complex than this?

Thanks, Paul.
 

Smokin Joe

Legendary Member
 
Is an outer plate / inner plate set 2 links or 1?
Strictly speaking, outer+inner constitutes just one link, but most folks seem to count them as two.

Is it a simple matter of adding 4 links to cope with the larger cog?
Not quite. You also need to take account of the derailleur's capacity to cope (or not) with the extra 5 links (or half-links) worth of slack that the wider range cassette is going to generate.
 
OP
P

pfm401

Active Member
Strictly speaking, outer+inner constitutes just one link, but most folks seem to count them as two.



Not quite. You also need to take account of the derailleur's capacity to cope (or not) with the extra 5 links (or half-links) worth of slack that the wider range cassette is going to generate.
Do you mean 4 links (or half links??)

The derailleur can definitely cope - the 32 and rear mech were supplied as part of a new group set. It's Tiagra 4700 medium cage which I understand can cope with up to 34
 

si_c

Veteran
Location
Wirral
It's only a 1 full link increase in necessary chain size, so I'd just leave the same chain on and avoid the large/large combination - the second sprocket in on the 11-32 tiagra is a 28t.

You can look at sizing the chain properly when you put a new one on.
 

robgul

Guru
As suggested, check the capacity of the rear mech - if you look on the flat plate on the wheel side you'll find the product code (if it's Shimano) - it'll be RD-nnn - you can then Google to find it and check the capacity spec (you can usually add 2 teeth to the max Shimano gives) - BUT it gets more confusing as there are SS and GS models of the same code product - GS is the longer one .... you'll need to eyeball and make a judgement.

And IME the calculator methods are confusing when compared with the simple"wrap round biggest sprocket and chaonwheel and add 2 links" (and I'm fitting new chains several times a day)

Rob
 
Do you mean 4 links (or half links??)

The derailleur can definitely cope - the 32 and rear mech were supplied as part of a new group set. It's Tiagra 4700 medium cage which I understand can cope with up to 34
No, I mean 5 half-links (normally expressed as 5 teeth or 5t).

Think of it as how much "slack" the derailleur has to be able to take up between the extremes of large chainwheel/smallest cog on one hand and small chainwheel/largest cog, on the other.

You get that value by adding up the teeth difference between the two chainrings and the difference between the largest and smallest cog.

For your original setup, that's (50-34) + (28-12) = 32t

For your planned setup, it's (50-34) + (32-11) = 37t

That's where the requirement for an extra 5t of derailleur capacity comes from. Derailleur capacity is related to the length of the cage, and is distinct from the issue of the largest cog that the derailleur can accommodate.
 

andrew_s

Guru
Location
Gloucester
You are changing 50/28 to 50/32 (large to large, which is what counts). That's 4 extra teeth.

That means you need 2 links of extra chain (1 inner, 1 outer), assuming the old chain was correct.

A tight chain is in contact with half of the teeth on the chainring, and half of the teeth on the sprocket, plus you need the chainstay length above and below. If you increase the sprocket by 4 teeth, the half of the sprocket that's in contact with the chain has 2 more teeth, and everything else is the same.
It's not dead on exact because the angles change, but the difference is a small fraction of a link.

If you think the old chain may not have been correct, use Cycleops' method.
 
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