... how much extra chain I'll need?

General cycling advice ( NOT technical ! )
Post Reply
User avatar
RickH
Posts: 5859
Joined: 5 Mar 2012, 6:39pm
Location: Horwich, Lancs.

... how much extra chain I'll need?

Post by RickH »

I'm planning an up/down/cross grade of my gearing* that will include the ability to use a larger 1st gear sprocket.

My first thought was "add 10 teeth, add 10 (half) links". But then I wondered will I only need to add half that as the chain only goes, approximately, half the way round. What caused me to wonder is my current chain comes with 118 links & I removed 6 to fit it (big rear to 1x front + 2).

Looking at chains for the new setup the chain I looked at comes in 118 or 126 link options but one local supplier charges an extra £9 for the 126 link version. I therefore wondered if the 118 link version was likely to fit the extra 10 teeth, given that I'm currently taking off 6 links.

(*FWIW I'm switching a 1x 11 SRAM mechanical setup for 1x 12 SRAM AXS electronic gears which will, effectively, add a 52 tooth 1st gear to my existing 10-42, giving ~18.5" bottom gear with a 36T chainring, & also make shifting the gears easier with my increasingly wonky fingers - probably arthritis, but not formally diagnosed yet - while still having combined braking & shifting on drop bars, which I don't want to give up.)
Former member of the Cult of the Polystyrene Head Carbuncle.
rareposter
Posts: 2689
Joined: 27 Aug 2014, 2:40pm

Re: ... how much extra chain I'll need?

Post by rareposter »

https://www.alpinetrek.co.uk/chain-length-calculator/

You need to know the chainstay length (as a direct line, centre of BB to centre of rear dropout) and also the number of teeth on the jockey wheels.

Even using the above, I'd still measure the chain properly on fitting - the calculator is very much a guide for buying the right chain.

And don't skimp on chains, it's one area where you really do get what you pay for.
User avatar
Chris Jeggo
Posts: 634
Joined: 3 Jul 2010, 9:44am
Location: Surrey

Re: ... how much extra chain I'll need?

Post by Chris Jeggo »

If your present set-up works with the 112-link chain on the largest, 42t sprocket, then your proposed set-up should also work on the new, 52t sprocket with a 118-link chain, i.e. your calculation that you need to add approximately 5 links is correct.
If you're a `belt and braces` person like me, try the chain length calculator too.
User avatar
RickH
Posts: 5859
Joined: 5 Mar 2012, 6:39pm
Location: Horwich, Lancs.

Re: ... how much extra chain I'll need?

Post by RickH »

Thanks for the replies. The calculator goes for 116 links, plus the note about jockey wheel sizes. The instructions for the new GX mech seem to be the same as for the current Rival 1 mech (refering to 2x & 3x, despite them both being 1x only) even though the jockey wheels are different sizes (both 12 on the Rival & 12 top/14 bottom on the GX. I presume the offset top jockey wheel in both mechs, much bigger on the GX, comes into play too.

Image

I'm not sure the reason for the different chain lengths for "Hardtail" & FS. A little extra chain tension with suspension perhaps?
Screenshot_20241031-091706.png
SRAM wrote:Wrap the chain around the large chainring (for 2x
and 3x systems) and largest cassette cog.
For full suspension (FS) and idler pulley (IP)
bicycles, add one inner link and one outer link
where the chain starts to overlap.
For hardtail (HT) bicycles, add two inner links and
two outer links where the chain starts to overlap.
I've been running big (only), big +2 & it seems to works OK. Plus it works on the Rival setup running a 30T chainring instead of the usual 36 just by adjusting the B screw. I think with an extra 2 links I might have run out of swing on the mech & had the chain rubbing on itself on the smallest sprocket.

I'm currently using SRAM X1 chains & was considering using the GX chain for the 12 speed setup. They appear to be similar, both visually & description wise.
Former member of the Cult of the Polystyrene Head Carbuncle.
rareposter
Posts: 2689
Joined: 27 Aug 2014, 2:40pm

Re: ... how much extra chain I'll need?

Post by rareposter »

RickH wrote: 31 Oct 2024, 9:45am I'm not sure the reason for the different chain lengths for "Hardtail" & FS. A little extra chain tension with suspension perhaps?
Depending on the suspension design (and this was much worse on older bikes running triples than modern suspension bikes on 1x), the compression and extension of the suspension can have the effect of lengthing or shortening the rear triangle quite dramatically. Pedal-related feedback is a big challenge in some suspension designs as the chain slackens or extends with the suspension action sometimes quicker than the mech can cope, especially if pedal forces are also a factor.

Worst case scenario, if you were in a big/big chainring/sprocket combination and the suspension compressed hard, it would "stretch" the whole chain system more than the already fully extended rear mech could cope with and rip the thing to bits.

1x with clutch derailleurs have gone a long way towards eliminating that issue. One of the massive benefits of a 1x system on a full-suspension MTB is that you can engineer out most of the pedal feedback by designing the frame without the incredibly restrictive need to accommodate a front derailleur.
User avatar
andrew_s
Posts: 5842
Joined: 7 Jan 2007, 9:29pm
Location: Gloucestershire

Re: ... how much extra chain I'll need?

Post by andrew_s »

RickH wrote: 29 Oct 2024, 12:52pm My first thought was "add 10 teeth, add 10 (half) links". But then I wondered will I only need to add half that as the chain only goes, approximately, half the way round.
Half that is correct - i.e. 10 teeth gives 5 (half) links.

However...
a) That assumes the current chain length is correct
b) You can only add links in even numbers, so it's either 4 or 6 (half) links.

What is important is that the chain is long enough to shift into big/big.
If it's too short, and you inadvertently change to big/big in the dark one evening, it is possible to break the bike badly enough to require calling for a ride home.
The normal rule is big/big (without passing through the rear mech), plus 2 (half) links
User avatar
freiston
Posts: 1608
Joined: 6 Oct 2013, 10:20am
Location: Coventry

Re: ... how much extra chain I'll need?

Post by freiston »

My tuppence worth . . .

You need the chain to manage big/big plus the RD cage without it being "tight". The cage on full stretch should hardly require any extra links than if the chain was connected without the RD, irrespective of jockey wheel size. So wrap the chain around big/big and give an inch/inch and half extra.

Mathematically, if you accept that the extra requirement due to the cage should be negligible, then you need the tangent line from top of the big chainwheel to top of the big cassette sprocket times 2 plus half the circumference of the chainwheel and half the circumference of the cassette sprocket. Then add an inch/inch and a half for comfort.

I tested this with one of my bikes - 48T & 34T with 440mm chainstays, treating one tooth (also one half-link or link, whichever way you think of a link as) as 12.7mm and rounding all calculations to nearest mm, adding 25mm for comfort and got 112 links - same as the Alpinetrek calculator. I didn't bother counting what I've actually got on the bike. For what it's worth, the difference between the calculated length of the tangent line and the chainstay length, in my example at least, was negligible (c.1mm)
Disclaimer: Treat what I say with caution and if possible, wait for someone with more knowledge and experience to contribute. ;)
Post Reply