Changed Chainset Querry Regarding Chain Length

pb21

Hi,

I was running a 39/53 chainset with a 12/25 cassette.

I have just put on a 34/50 chainset, obviously the chain is a bit too long now so needs shortening. Previously the chain length was fine.

Would people recommend I take out two links, or four, or two to start with and see how it goes?

Thanks

svetty

Two should be OK assuming chain was OK to begin with

pb21

Thanks for the reply. i think I will go for two to start with.

The interesting thing is that previously there was a 14 tooth difference between the outer and the inner and now there is 16 so the rear mech has to cope with more variation.

I think whats is going to end up happening is that in order that the 50/23 gear is acceptable chain length wise (occasionally 50/25 :-/) the 34/14 or 34/13 is going to be pretty slack...

lesfirth

pb21 wrote:

Thanks for the reply. i think I will go for two to start with.

The interesting thing is that previously there was a 14 tooth difference between the outer and the inner and now there is 16 so the rear mech has to cope with more variation.

I think whats is going to end up happening is that in order that the 50/23 gear is acceptable chain length wise (occasionally 50/25 :-/) the 34/14 or 34/13 is going to be pretty slack...

Why would you want to use 34/13 or 14 or even 15 ? If you do not use them there is no problem and even if you use them by mistake there is still no problem.

buckmulligan

Chain tension at the extremities (i.e. big-big and small-small cog combos) is determined by the capacity of your rear mech to take up the slack chain. Using a 50-34 chainset with a 12-25 cassette means you need a rear mech with capacity of 29T ( (50-34) + (25-12) ); you don't say what rear mech you're using, but unless it's something quite old you should be fine. As an example, the short-cage version of the most recent 105 grouspet (5801) rear mech has a capacity of 33T, which was down from 34T on the older 5701 version.

The crucial part is getting your chain length correct. The one thing you DO NOT want to do is make it too short; if you do this and try to shift into the big-big combo (and let's be honest, we all accidentally do this from time-to-time when you're hammering it up a hill and can't remember what day of the week it is, let alone what gear you're in) you run the risk of shearing the rear mech off and it ending up in your rear wheel, busted spokes, cracked frame, total carnage.

My advice is to use the guide on Sheldon Brown, of running it over both big cogs (and not through either derailleur) and then add two links; or alternatively, just take out links until you can use the small-small combo without the rear mech completely folding back on itself (i.e. there's still some tension applied to the chain). Then check (before you go out for a ride!) that it'll comfortably go into the big-big combo without the rear mech going forward beyond its capacity.

Modern groupsets are very flexible so you shouldn't run into too many problems if you pay attention to what you're doing. Good luck!

pb21

lesfirth wrote:

Why would you want to use 34/13 or 14 or even 15 ? If you do not use them there is no problem and even if you use them by mistake there is still no problem.

Well being a bit facetious I would want to use them to cycle!

If I were riding in a hilly area with a short area of flat I may well use a gear of 34/15,14,13, why would you want use 50/23,21,19?

In addition I went out this morning and noted that when riding in 34/17 I was getting some 'noise' from the drive train, a regular rattle type noise that wasn't apparent when in 34/19 or 34/16. Is this likely to do with the chainring being exactly two times the sprocket?

pb21

BuckMulligan wrote:

Chain tension at the extremities (i.e. big-big and small-small cog combos) is determined by the capacity of your rear mech to take up the slack chain. Using a 50-34 chainset with a 12-25 cassette means you need a rear mech with capacity of 29T ( (50-34) + (25-12) ); you don't say what rear mech you're using, but unless it's something quite old you should be fine. As an example, the short-cage version of the most recent 105 grouspet (5801) rear mech has a capacity of 33T, which was down from 34T on the older 5701 version.

The crucial part is getting your chain length correct. The one thing you DO NOT want to do is make it too short; if you do this and try to shift into the big-big combo (and let's be honest, we all accidentally do this from time-to-time when you're hammering it up a hill and can't remember what day of the week it is, let alone what gear you're in) you run the risk of shearing the rear mech off and it ending up in your rear wheel, busted spokes, cracked frame, total carnage.

My advice is to use the guide on Sheldon Brown, of running it over both big cogs (and not through either derailleur) and then add two links; or alternatively, just take out links until you can use the small-small combo without the rear mech completely folding back on itself (i.e. there's still some tension applied to the chain). Then check (before you go out for a ride!) that it'll comfortably go into the big-big combo without the rear mech going forward beyond its capacity.

Modern groupsets are very flexible so you shouldn't run into too many problems if you pay attention to what you're doing. Good luck!

Thanks for your reply.

I took two links out last night and when running 34/13 (or even 34/12, very unlikely!) the rear mech (Campagnolo Record 10s) is able to take up the slack and running 50/25 is OK too.