How much does chain length matter? Inch here, inch there.

[Manc33]

I am not asking how much chain length matters in the usual way, I know the chain needs to be long enough to go around the 2 biggest cogs while being short enough to not cause the rear mech to fold up completely on the smallest chainring and smallest sprocket. I am asking a slightly more finicky question...

What I want to know is... my chain right now is at 54 inches and it works on big to big (with easily enough margin for it to be safe, its nowhere near fully stretching out the rear mech on big>big) but using a chain length calculator with my exact chainstay length in millimetres put in, it says the chain needs to be 54.95 inches - in other words 55 inches.

Since the chain already fits around it on big>big with it being 54 inches, why is it "better" if it is 55 inches?

The calculator (with chainstay length and "oversized derailleur" accounted for) is probably right, because on the middle ring and biggest sprocket, the swingarm of the rear mech is not quite vertical and I know it should be. So I know the chain needs 1 more inch adding for the "best" setup, but why does that improve anything?

I have noticed in the lowest gear, the upper jockey wheel is rubbing slightly and I guess adding more chain will bring that slightly away? Already got the b-tension in, its a mech that can cope with the low sprocket's size etc etc.

You wouldn't think adding or taking an inch would matter but I have no idea. In reality the chain could probably go to 53 (when the calc says 55) and still fit, but it would be right on the limit.

I keep trying different cassettes, splitting chains, adding links back, worrying about it coming loose one day, buying a new chain and so on and so on. The thing is if I had not had access to a chain length calc, my chain would be 54 inches not 55 inches (because 54 fits easily!) and I would be breaking the chain "too short" when the point of a new chain was to have it as a complete and un-split chain.

I can tell if I add a link the rear mech swingarm will be vertical (if it goes back by 0.5 inches).

[Brucey]

I'm not quite sure what the question is; who knows how the chain calculator you (randomly) used works, or how it allows for 'an oversized derailleur'...?

cheers

[Manc33]

I'm not quite sure what the question is; who knows how the chain calculator you (randomly) used works, or how it allows for 'an oversized derailleur'...?

cheers

Its the first main calc anyone would come across on Google after typing "chain length calculator" in, however the question isn't about the calculator, its about whether the inch of seemingly unnecessary chain matters or not.

I tried to find a calc that lets you put in the distance between the jockey wheel bolts but couldn't find one accommodating for that.

We need Sheldon Brown back again.

[Cyril Haearn]

Maybe you are thinking too much

You could just try different chain lengths

I guess a longer chain might last a bit longer but it weighs more of course

[cycleruk]

If you have a "quick link" joining the chain you could just add another QL and 1/2 link and see what happens.

[landsurfer]

I just set the lower derailleur cage roughly vertical when on the smallest sprocket and ensure there is a smooth chain flow when on the largest sprocket of the cassette .... it works ....

[Brucey]

I'm not quite sure what the question is; who knows how the chain calculator you (randomly) used works, or how it allows for 'an oversized derailleur'...?

cheers

Its the first main calc anyone would come across on Google after typing "chain length calculator" in,

I'm tempted to say

a) it depends how google works for you (it most certainly doesn't do the same thing for everyone...) and also
b) 'so what?' it still doesn't mean it is going to be any less opaque about how it works for derailleurs...?

however the question isn't about the calculator, its about whether the inch of seemingly unnecessary chain matters or not.

I would suggest that you try it and see; you have previously indicated that you are somewhat fussy about how your shifts feel, so I'd assume that even if others think it doesn't matter, you might think it does...?

FWIW a rule of thumb is to determine the exact length to go round both the largest sprocket and the largest chainwheel, then add two links. I might add nearer three links if it is a long arm mech and there is capacity to spare; that way I can repair the chain (by shortening it) if the need arises. No-one needs a calculator for that.

BTW If your shift quality varies with small changes in chain length it isn't going to work very well on different chainring sizes either.

cheers

[531colin]

.................
I have noticed in the lowest gear, the upper jockey wheel is rubbing slightly and I guess adding more chain will bring that slightly away? Already got the b-tension in, its a mech that can cope with the low sprocket's size etc etc..............

In the real world, I USE the lowest gear....ie smallest chainring, biggest sprocket.
So I want it to work, properly......if the top jockey wheel just touches the sprocket, its OK, but if its really clashing, I don't like it.
i think you are wrong, i think shortening the chain will reduce the jockey wheel/sprocket clash, not lengthening it.....you can see by just folding the chain back on itself and watching the mech.
OR you could take the B screw out and put it in from the other side, so the screw head does the business.....or fit a longer screw.
But you don't really USE big/big, so you only need enough chain so it won't break if you accidentally go into big/big
Set it up so it works, not by some rule of thumb in a website somewhere.

[Dave W]

I set mine up small to small.

[pwa]

Using a chain with a quick link I usually just put the full chain on and try it on the smallest sprocket and the smallest chain ring, which is a combination you would not use for real. If that results in the derailleur folding up too much I just take a link out, and keep on doing that until it works. When it does, and if I can be bothered, I try big sprocket with big ring. That should stretch the derailleur arm to almost but not quite straight forward. Or you can just make sure the chain has the same number of links as the old one.

[meic]

Hopefully all of these methods will give a similar chain length.

Personally I would much rather live with a chain that is too long and runs back on itself in forbidden gears than one which is too short and gives a crunching sound when you try and engage forbidden gears at the other extreme.
Never considered leaving a "spare" link in the chain though.

[PJ520]

Whatever you do don't shorten the chain and go into big-big without trying it on a stand e.g. before riding. I did this and the chain jammed so tight it wouldn't budge. The only way I could get any slack to get it onto a smaller cog and get going again was by removing a derailer pulley.

[Samuel D]

Personally I would much rather live with a chain that is too long and runs back on itself in forbidden gears than one which is too short and gives a crunching sound when you try and engage forbidden gears at the other extreme.

Me too, not least because I rarely use gears anywhere near small-small, whereas I often use the gear beside big-big and even big-big on occasion (cresting a short hill of the appropriate steepness).

When putting together a new drivetrain, I use the “rigorous equation” in section 6 of this Park Tool article on chain length sizing. There’s utterly no need for it (or the “simple equation” that gives the same result!), but it’s fun to see practice match theory.

Manc33: how do Park Tool’s equations compare to your online calculator and current chain length?

[NATURAL ANKLING]

Hi,
There are some set ups that that won't accept big - big as small small means the chain is too slack, shimano don't recommend extreme chain line either even when it goes.
My old problem of finding the chain the wrong side of the derailleur tab probably happened when the chain snagged up front because I hadn't noticed the front derailleur was bent forward .
The bent front derailleur was giving poor shifting, locked up the chain forcing rear derailleur like this -

[Mick F]

Chain length:
Chains are Imperial.
Measure your chainstays in inches.

Chainstay Length X 2 + ( F/4 + R/4 +1 ) = Length of chain in inches.
Where F is the biggest front ring and R is the biggest rear sprocket.

My Mercian, for instance has 16.5inch chainstays, with 53t big ring and 30t big sprocket.
Therefore:
16.5 x 2 + ( 53/4 + 30/4 + 1 ) = 54.75 = 55inches = 110 links