Chain length again!

[Brucey]

FWIW I basically agree with slowster, however I would also note that any direct measurement of chainstay length will also be subject to unwanted parallax errors which could easily make the chainstays seem 5mm longer than the actual c-to-c distance.

[Cyclothesist]

Chainstays are 470mm (18½")

I don't think that can be right. If they are 470mm, you would need a 122 link chain for the 38 x 51 gear.

Is the bike your Boardman? If so, the current model's chainstays are 425mm, and I doubt that the chainstay length has changed significantly from your model.

https://www.boardmanbikes.com/gb_en/pro ... #technical

I do appreciate your patience and my drib dribble of relevant info slowster -

No, this bike is a Cube Reaction Hybrid Pro, 11 speed with 470mm chainstays

And I was therefore further confused (perfectly normal and default for me) at entering the relevant details into the link you posted

It has to be me, not seeing the wood for the trees

Added to which the as delivered Cube 470mm chainstay/38 chainring/11-51 cassette/120 link chain -

- does indeed have 122 links if one includes the 'magic' joining link!

Any road up, the 'cheap' KMC E11 116 link chain has been ordered - and all will be revealed/obvious soon

122 links on the original alters my previous calculation by 2 - so 118 links to be safe. If you're lucky and the original chain is generously sized 116 might just do it but I wouldn't fit it without measuring round big sprocket and chainring plus 4 links.

[slowster]

Here's the result of that, see screengrab = 59 links?

59 x 2 = 118!

I think a 116 link chain will only work if the chainstays are 467mm or less, which is possible depending upon the manufacturing tolerances, but I would not be too hopeful of that.

[Steve O'C]

Short Answer;
Yes it will be fine to use a 116 link chain on the new setup.
Reasoning:
OP has stated that 120 link chain works fine on current setup. The only change being made is reducing the largest rear sprocket from 51 to 42 so down by 9 teeth. Each reduction in tooth number allows for a chain that is half a link shorter, so in this case a chain that is 4 links shorter (116 links) should be slightly slacker than the current 120 link chain on the existing setup.
Steve

[andrew_s]

Roughly speaking, I think you can cope with N/4 less links for a reduction of N teeth in the big cog: the chain wraps round half of the teeth, and you get two teeth per link?

It depends what you call a link. A chain sold as 116 links has 115 pins, or maybe 117 if you count the quicklink.
Therefore both inner and outer links count 1, and the difference between 51T and 42T is 9, half of which is 4.5.
You can only change chain length in increments of 2, so a 120 link chain can be reduced to 116 links, but a 114 link chain would (probably) be too short unless the current chain is over-long.

[Cyclothesist]

Short Answer;
Yes it will be fine to use a 116 link chain on the new setup.
Reasoning:
OP has stated that 120 link chain works fine on current setup. The only change being made is reducing the largest rear sprocket from 51 to 42 so down by 9 teeth. Each reduction in tooth number allows for a chain that is half a link shorter, so in this case a chain that is 4 links shorter (116 links) should be slightly slacker than the current 120 link chain on the existing setup.
Steve

Except he later corrected that to 122 links for the original chain. That adds another 2 links to the new set up too (=118 links). It's only going to fit if the 122 links is a tad long.

[Steve O'C]

Thanks cyclothesist, I had missed the updated chain length measurement
Steve

[Chris Jeggo]

- does indeed have 122 links if one includes the 'magic' joining link!

Any road up, the 'cheap' KMC E11 116 link chain has been ordered - and all will be revealed/obvious soon

If 116 links turns out to be too short you can still use your KMC E11 chain using two quicklinks joined together with a single inner link between them.

[deliquium]

And the answer is . . .

. . . no.

116 links are 2 too short for 38/42 with ~470mm chainstays and a 1X system (according Park Tool method)