Chain Ring/Cog impact on chain length.

bigW

Well-Known Member
#25
Any reason you didn't fit the old chain to the new chainring/cog to see how many to remove before breaking?
Coulda sworn that way back when that it was 1:1 teeth to links, had the wheel off for cog change and just zapped the four links. Since I knew that 42x15 was dead center of my adjustment range without use of a half link there were only two possible answers.

remove 4 links.
remove 2 links.

I did not stop to think it through. I’ll do the math but I expect that the unequal size of chain ring vs cog does not result in a material length difference whether you remove four teeth from the chain ring vs removing four from the cog.

That being said I usually alter the chain length once a year, whereas those saddled with a derailleur have to make shifting decisions all the time. Hmmm should I go up with RD and down with FD? Or the opposite? Oh shit my DI2 battery has crapped out. 😬😬😬

SS4Ever..... ( or at least until my foot pain issues become unmanageable )
 

1speed

Incredibly profound yet fantastically flawed
#26
I have 2 questions:

a) what measuring unit do you use for the chainstay lenght ? Is it links?
b) why F/4 and not F/2, R/4 and not f/2 ? Is that to compensate the chain ring and cog radius that would be otherwise overlapping the chainstay lenght?

Typically, inches for length because that's the unit of measure on most chains when you buy them, but it's all convertible - chain links have to be uniform in length for a given chain, and cogs and rings have to have the same distance between teeth, and both of them have to be relative to the length of a link. On the F/4, R/4 instead of F/2, R/2 basically, yes to your question. Think of it this way - the chainstay length is center of BB to center of rear axle. If you draw a vertical line through these two points, they'd cut the chain ring and cog in half, respectively - so to get the added distance needed for your chain as it wraps the ring & cog, you start with an arc length of 1/2 of each circle: in our case, expressed as F/2 and R/2. But now you need to think about how a chain ring or cog is constructed - half of it is peak (or "tooth") and half of it is valley (or "groove".) Since our unit of measure is "tooth", we divide our arc length in half to get it all back to the "tooth scale." Thus, F/4 and R/4. If the ring and cog size were expressed as actual measures of distance (like inches or cm or whatever) they'd need to account for more than the number of teeth - they'd need the valley portion of the ring or cog as well.

BTW way, no nerd war here: first, it's not my equation so I have no investment in it - it's just something I found online when I was building my first SS and cared about things like getting every measurement exact. But more importantly, @Patrick is right about this not working perfectly for significantly large ratios. But as he also points out those ratios have to get pretty far out of line before they jump outside the tolerances of whatever your adjustment range is. Even if you used this for a 53:11 SS (in which case, you should build in some time to plan a fittingly lovely memorial for your knees), you're not going to be off by that much, perhaps even within the range of your chainstay adjustment. But it's definitely something to remember and a good enough reason to always check that your length will work BEFORE you cut the chain.
 

bigW

Well-Known Member
#27
Typically, inches for length because that's the unit of measure on most chains when you buy them, but it's all convertible - chain links have to be uniform in length for a given chain, and cogs and rings have to have the same distance between teeth, and both of them have to be relative to the length of a link. On the F/4, R/4 instead of F/2, R/2 basically, yes to your question. Think of it this way - the chainstay length is center of BB to center of rear axle. If you draw a vertical line through these two points, they'd cut the chain ring and cog in half, respectively - so to get the added distance needed for your chain as it wraps the ring & cog, you start with an arc length of 1/2 of each circle: in our case, expressed as F/2 and R/2. But now you need to think about how a chain ring or cog is constructed - half of it is peak (or "tooth") and half of it is valley (or "groove".) Since our unit of measure is "tooth", we divide our arc length in half to get it all back to the "tooth scale." Thus, F/4 and R/4. If the ring and cog size were expressed as actual measures of distance (like inches or cm or whatever) they'd need to account for more than the number of teeth - they'd need the valley portion of the ring or cog as well.

BTW way, no nerd war here: first, it's not my equation so I have no investment in it - it's just something I found online when I was building my first SS and cared about things like getting every measurement exact. But more importantly, @Patrick is right about this not working perfectly for significantly large ratios. But as he also points out those ratios have to get pretty far out of line before they jump outside the tolerances of whatever your adjustment range is. Even if you used this for a 53:11 SS (in which case, you should build in some time to plan a fittingly lovely memorial for your knees), you're not going to be off by that much, perhaps even within the range of your chainstay adjustment. But it's definitely something to remember and a good enough reason to always check that your length will work BEFORE you cut the chain.
Yup, that’s the dealio.

Thanks
W

Measure twice cut once.