Markjc

I'm thinking of changing my SRAM Force 1x 40T to 44T. Will I need to change the chain to a longer length? I can't find anything that references the diameter of the different chainrings. If the chain needs to be longer and idea of how many extra links?

oldschoolbike

Think of chainring circumference in links, not chainring diameter. The chain engages only half your chainring, so the new chainring will want only two more links, the smallest unit you can normally add or remove.

Will you need to add links? Maybe not. Try pinching a couple of links together with the bike in bottom gear, to see if there is two links worth of slack available without stressing the derailler. As for the advisability of just adding a link pair, that will depend on the wear state of the current chain. Also, two "quick link" master links just a link apart may not be a good idea - others will advise I am sure, I have no data/experience on that.

grumpus

Depends how it was set up - if it's already as short as it will go then you'll need to add an inch i.e. one inner and one outer link, to compensate for the approximate two teeth more that the chain wraps around the chainring. To check, split the quick link and wrap the chain around the largest sprocket and the 40t chainring, not through the derailleur, so the ends overlap on the chainring - if it only overlaps 4 or 5 links you'll need to add a couple more. Rear suspension and you may need another couple more - it's important that the chain can easily shift to the largest sprocket without binding up, even in the tightest position of the swing arm.

Bill Kapaun

Put it on the largest cog and see if you have an EXTRA inch of slack.

Markjc

Thank you for your thorough response. It's a gravel bike so I don't need to worry about rear suspension.
Even if I don't change the chainring, your explanation will come in handy anytime I replace a chain. It's a nice check rather than just match the old chain.
Thanks again. Mark

Markjc

I appreciate the visual. I appreciate all the members that share their knowledge.

Markjc

I really like your description of "Think of chainring circumference in links, not chainring diameter."
I got around to looking at the bike and chain and I now think there isn't space to add a bigger chainring. The bike is an Cervelo Aspero and the chain stay bows out leaving very little of any space. (I can't post a picture due to not having made 10+ post)
Oh well.. But I did learn a lot from everyone's advice and comments.

Bill Kapaun

Chain has 1/2" pitch so 4 teeth would add 2" to the circumference of the ring.
Divide by PI and you have the diameter increase. Divide that by 2 and you have the radius increase.

Assuming you have an 11T smallest cog, can you actually spin a 40T ring?

Duragrouch

Yes, thinking of only about 180 degrees of chain wrap around the chainring is completely accurate. Just realize that, in terms of the rear derailleur "maximum capacity", to make matters easier for mechanics, that rating is based on "whole" chainrings and cogs, not halves. So for example, my 50/34 crank (16 tooth difference) plus 11-30 cassette (19 tooth difference), total is 35 tooth difference, and requires a rear derailleur with a minimum of that capacity, if I use all gear combinations.

Also note that the above is not "maximum cog size", that is a different value and a function of how low the rear derailleur jockey pulley hangs below the cassette, and/or the "slope" of the derailleur pantograph linkage and how well that matches the slope of the cassette.

Bill Kapaun

Wrong, but it's certainly close enough. Think of a bike with extremely short chain stays, very large ring & tiny cog. The large ring will have more than 180 and the tiny cog will have less.
It would only be "completely accurate" if both sprockets are IDENTICAL in size.

The main question is where does your post show any connection to what the OP said?

PromptCritical 

Well, that wasn't the OP's question, but that isn't really that big of a gear (smaller than the 52x13 back in the day). I can easily spin out a 42x11, and I'm not a spring chicken.

Bill Kapaun

I was referring to the OP's statement-
"The bike is an Cervelo Aspero and the chain stay bows out leaving very little of any space."

I was showing how to tell how much larger the ring would be.
I don't give a rats if you can spin that big of a gear. I was asking the OP.

PromptCritical 

Don't get all b-hurt. I was just pointing out that 40x11 isn't that big of gear, which also had no connection to what the OP asked either.

Duragrouch

Yes, that's why I said "about" 180 degrees, I didn't think the difference of a few degrees was important. The reason I did mention the difference between that and RD ratings based on "whole" rings, is relevant to RD selection, if the OP needed to change that in the future; If they are considering changing chain length, verifying RD capacity is a prudent move, rather than rip off the cage or not taking up the slack, and a lot of factory systems come pushed to the edge of maximums these days. This is something I had to verify myself, when I was upgrading my bike from 1X to 2X crank, and I knew (from career in mechanical engineering and a chain/twin-sprocket device I designed to stabilize from swinging and maintain orientation of a moving platform) that the chain difference is based on an approximate half-wrap; I had to look on Sheldon Brown to see if rear derailleur capacity ratings were based on that, or "whole" chainrings and cogs, and it was the latter, though inaccurate, but that's the way they are rated, so that's what I needed to know, and it would be relevant to the OP if they change rear derailleurs. Based on the OP's question, it's clear this may be an "unknown-unknown", so it's appropriate, generous, and helpful to say, "Uh, hey, watch out for this..."