Cycling UK Forum

I have a 1 x 11 bike, with a 38 tooth chainring, it came with an 11-51 cassette. The chain has 120 links. The rear mech is Shimano Deore RD-M5100-SGS. All works very well.

Except I hate the big jumps in the cassette ratios so have replaced it with an 11-42, which makes me and my legs much happier + gives me a suitable bottom gear

Without yet having a 116 link chain to try - is it possible to calculate whether 116 links (or less) would be suitable for a 38 tooth chainring and an 11-42 cassette combination? Knowing that 120 links works with a 38 tooth chainring and 11-51 cassette and the rear derailleur is the same?

As per usual, my Google fu is more than sub optimal

deliquium wrote: ↑23 Nov 2023, 3:54pm I have a 1 x 11 bike, with a 38 tooth chainring, it came with an 11-51 cassette. The chain has 120 links. The rear mech is Shimano Deore RD-M5100-SGS. All works very well.

Except I hate the big jumps in the cassette ratios so have replaced it with an 11-42, which makes me and my legs much happier + gives me a suitable bottom gear

Without yet having a 114 link chain to try - is it possible to calculate whether 114 links (or less) would be suitable for a 38 tooth chainring and an 11-42 cassette combination? Knowing that 120 links works with a 38 tooth chainring and 11-51 cassette and the rear derailleur is the same?

As per usual, my Google fu is more than sub optimal

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?

But can you just put it in big/big and pull a loop of chain to see how many can be removed before there's a problem?

roubaixtuesday wrote: ↑23 Nov 2023, 3:59pm 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?

But can you just put it in big/big and pull a loop of chain to see how many can be removed before there's a problem?

Many thanks roubaixtuesday for a speedy reply

But you've lost me with the N/4 thing

deliquium wrote: ↑23 Nov 2023, 4:06pm

roubaixtuesday wrote: ↑23 Nov 2023, 3:59pm 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?

But can you just put it in big/big and pull a loop of chain to see how many can be removed before there's a problem?

Many thanks roubaixtuesday for a speedy reply

But you've lost me with the N/4 thing

On N/4: the chain only wraps around about half of the teeth on the rear cog. So if you reduce the number of teeth on the cog by N, you only reduce the required chain length by N/2. Eg the chain engages 16 teeth on a 32 tooth cog, but 8 on a 16 tooth cog. So in that case, you'd need a chain sized for 8 fewer teeth.

But each link engages *two* teeth, so you only need to reduce chain length by half the number of teeth engaged reduction, which itself is only half the number of total teeth reduction.

If that makes any more sense!

But like I say, I'd just test how many you need by putting it in big/big then making a loop in the current chain.

Chain length calculator:

http://www.machinehead-software.co.uk/b ... hcalc.html

I take a more pragmatic and less arithmetical approach.
I run the unjoined chain through its course on the big to big gear. That is where most chain is needed. Hold the links next to each other so that the top pulley puts at least some bend into the chain. Note how many links are left over.
Now try small to small. Is the derailleur wrapping up the slack without fouling itself, when the chain is held with the same number of links left redundant?
This will give a practical test of possibilities. The length which fulfills these two criteria may be variable over a small range, so you can fine tune it, bearing in mind that you do not want to push either limit too closely.
I hope this is clear, because it seems to me to ensure things will run well.

Much appreciate all your replies folks

I'm embarrassed to say roubaixtuesday, I'm even more thick or ignorant than I previously thought, as your much appreciated further reply, didn't help

I don't have the 116 link chain yet - otherwise it would have been dead simple to work out the answer.

Was just hoping there was simple Secondary Modern (failed Maths O level) 'calculation'. (which I think roubaixtuesday has proved?)

This is all because of there's a very tempting price on a KMC E11 chain at 116 links and I was hoping to understand if it would work before ordering

deliquium wrote: ↑23 Nov 2023, 4:43pm I don't have the 116 link chain yet - otherwise it would have been dead simple to work out the answer.

You don't need maths.
(also note that it's not just chainring / cassette size, the length of the chainstay is a rather integral part of it too!)

Put the existing set-up into biggest cog (42T).
Is the mech at / near its limit of stretch or is there a fair bit of leeway left in it?

If the chain has a quicklink in it, undo that and just pull the chain tighter by a link at a time until the mech is a max stretch. If it doesn't have a quicklink, you can still do that, just "fold" a link in your fingers as you pull the mech tighter.

Keep doing that until the mech is at max stretch then add a link back in (to stop it ripping the mech off as the chain bounces).
If you've taken out 4 or more links then a 116L will do.

TL:DR:
There will be enough links in your 116 link chain for the 11-42 cassette. It might be spot on or 2 links too long.

To check sizing before fitting:
For 1x to size the chain put it on the biggest sprocket and the chainring but not through the derailleur. The correct size is the length of the chain to form a complete loop plus 4 links.

the maths is not trivial,but I think that 116 links will be just fine, provided your chainstays are shorter than 18.5"

deliquium wrote: ↑23 Nov 2023, 4:43pm otherwise it would have been dead simple to work out the answer.

Using the chain length calculator to which I linked, it could not be more dead simple. The answer is 116 will be enough.

You can confirm this yourself. Enter your current chainring and sprocket sizes, and compare the results with 465mm vs. 466mm chainstays. The two results tell you that your bike must have chainstays of 465mm or less (assuming your 120 link chain is sized for 'big/big').

Now enter the smaller 42t sprocket size and use 465mm as the chainstay measurement. The result tells you that a 116 link chain is the length needed.

deliquium wrote: ↑23 Nov 2023, 4:43pm
I'm embarrassed to say roubaixtuesday, I'm even more thick or ignorant than I previously thought, as your much appreciated further reply, didn't help

I'm just bad at explaining! Anyway, I think you have your answers

Brucey wrote: ↑23 Nov 2023, 5:19pm the maths is not trivial,but I think that 116 links will be just fine, provided your chainstays are shorter than 18.5"

Chainstays are 470mm (18½")

slowster wrote: ↑23 Nov 2023, 5:43pm
Using the chain length calculator to which I linked, it could not be more dead simple. The answer is 116 will be enough.

You can confirm this yourself. Enter your current chainring and sprocket sizes, and compare the results with 465mm vs. 466mm chainstays. The two results tell you that your bike must have chainstays of 465mm or less (assuming your 120 link chain is sized for 'big/big').

Now enter the smaller 42t sprocket size and use 465mm as the chainstay measurement. The result tells you that a 116 link chain is the length needed.

Thanks slowster, missed your linked post Here's the result of that, see screengrab = 59 links?

Any road up, I apologise to all for being too idle to figure it out meself by playing around with the existing 120 link chain, to decide the answer. The current 120 link chain still works superbly with the replacement cassette (having lost 9 teeth).

Thank you everybody for your responses, sorry again for being twp and lazy.

New 116 link KMC E11 chain for £24.95 ordered - as a spare/backup

deliquium wrote: ↑23 Nov 2023, 6:11pm 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

slowster wrote: ↑23 Nov 2023, 6:27pm

deliquium wrote: ↑23 Nov 2023, 6:11pm 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