pennpaul

I ride a gravel bike but it's pretty much just a road bike now since the "gravel" here is beyond my abilities.

Anyway, I have a 46/36 in the front and an 11-40 in the back. I decided to get a compact crank (50/34), but realized I probably need a longer chain now since I'm adding 4 teeth.

Is there a rule of thumb that I can use to determine how many links I need to add? When I setup the 11-40, I used the whole length of KMC chain I bought. Of course, now I'm worried that the chain is too long since I lost 2 teeth on the smaller ring.

krecik

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DiabloScott

You want a thumbrule based on experience?

"Probably zero, might be one... maaaaaaybe two."

If your derailleur is REALLY tight in the big big, you'll probably need a new, longer chain. Else just wait and see.

Andrew R Stewart 

Since I don't assume that the bikes I work on have had their chains set up lengthwise for the best shifting (as opposed to the least cost, after 112 bikes the company has a "free" chain if they shorten each chain by one link/pair) I CAREFULLY test before I do anything. But I agree with DiabloScott. Andy

HillRider

The "rule of thumb" for changing total teeth count is add or subtract 1/2 of the tooth difference. For example, going from a 46T to a 50T chainring you should add 4/2=2 half-links. If the difference is an odd number go to the next even number since you can't add 1/2 of a link. The reason for adding only 1/2 of the difference is that the chain only wraps around 1/2 of the chainring or cog.

Before anyone says anything, yes, there are 1/2-links available but they are unusual and not commonly used.

HerrKaLeun

To do it right, don't rely on the original chain being correct.

Read the RD manual on chainsizing and verify if the chain is the correct length. Typically it is big-big + 2 or 4 links.... but that differs for SRAM and Shimano and for FS bikes there is even more to consider. RTFM.

squirtdad

I used big big +2 for years, but am now of the opinion that small/small works better ....this is what my LBS guy recommends and is what shimano recommends

3alarmer 

...if your goal is to gear up a little for road use versus gravel, have you considered running a smaller cluster in the back ? You can usually do this easily and relatively cheaply. But I guess I don't really understand the goal here.

Retro Grouch 

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Smaller than 11t?

Retro Grouch 

To answer OP's original question, why guess? Since you're going to break the chain anyway just wrap it around the big/big combination and test it for sure.

squirtdad

OP should also check what the wrap capacity of his rear derailler is, may not have the capacity for the change

currently he is at 41 wrap ((46-36) + (42-11) (10 + 31)

with a 50/34 the wrap is 45 (50-34) +(42-11) (14+31)

ThermionicScott 

+1. I almost never count links on my chain. I take a new chain out of the box, loop it around the big ring and big cog, add 1" for good luck, and cut it.

fietsbob

bike chain is 1/2" pitch , a full link is 3 rollers, 2 on the inner and the 3rd one the 2nd pin into the next inner link
so an inch..
now you join 2 inner links with a quick link, functionally an outer link with pins fixed in it.. one on each side..









.....

caloso

Same. If it was good enough for Sheldon, it is good enough for me.

pennpaul

Looks like I need to adjust my plans after a big Doh! moment. My current cassette is actually an 11-40, not 42. Not that that matters in the grand scheme because I was planning to use the original 11-32 on my spare rear wheel for Zwift. Of course, I'm not going to be swapping chains each time I want to Zwift, too.

Looks like I need to decide whether I want to stay with the 11-40 or go back to 11-32, and then buy another cassette to match. Doh.

HerrKaLeun

SRAM recommends +4 on the 10 speed and higher. Shimano recommends +2. Probably works either way, but if I have a choice I do what the manufacturer recommends.

Phil_gretz

I think that he meant the 40T rear.

Iride01 

Shimano publishes the exact procedures to use for determining chain length. There are differences for things like 3x or 2x front. Long, medium or short rear DR and other things. Other mfr's probably publish how they want chains sized to their groupsets too.

Shimano Tech docs are here... https://si.shimano.com/#/

You need to find the DM's appropriate to your bikes groupset.

pennpaul

The new crank is installed and it doesn't seem like I need to add or subtract any links.

squirtdad

just double check that big/big works.. it is not fun if it does not and you move into that at speed

Kapusta

My rule of thumb is to measure it on the bike, thus taking out any guesswork or assumptions.

Big-big plus 2 or 4 (half links), depending on the application.

djb

Go to the park tools video series on chain sizing.
Clear, concise, no guess work from a reliable source.