# What's the logic behind this chain length estimation method?

I know of several methods of sizing a chain for a bike:

1. As many links as the previous chain.
2. Small-small. The RD should be barely expanded, the chain nearly rubbing against itself roller-cogs. The logic here is to be able to have the longest possible chain, that still allows shifting into any gear combination.
3. Vertical rollers. Shift big front small rear. The rollers on the RD should be in a vertical line, below the cogs. In this position the RD is in a good working point, because it's spring is both tensioned (and thus stiff against vibrations) and not over-elongated (and suffering accelerated wear). This is the medium working point.
4. Big-big without the derailleur. Without threading the chain through the RD, measure big-big and add 2 links.

My question is what is the motivation of the last method. To me, not threading the chain through the RD just doesn't make any sense. What physical quantity is being measured by shifting to big-big and why are 2 links being added?

As a side note, I recently tested all of those, and no two returned the same result.

• Here is an example reference, where they say " It is not necessary to thread the chain through the rear derailleur", but I have seen the more powerful claim, don't thread through the RD. Commented Nov 7, 2014 at 10:32
• One thing is that a slightly too big chain doesn't matter too much. A slightly too small chain on the other hand is bad.Can you rank them by number of links? I'm partial to option 1 (size by old chain) but thats not the right way to do it if you do a chainring+cassette size change. Commented Nov 7, 2014 at 11:15
• A full suspension bike with a pivot design which has "chain growth" will require a longer chain. In that case measure after having removed all air or coil from the rear shock and fully compressed it. Commented Nov 7, 2014 at 12:42
• Needs to be at least as long as #4, unless you're intentionally "shorting" it for some odd reason. (A few riders of the old school never use the big-big combination, and sometimes the derailer simply lacks enough tooth capacity.) You also don't want it to hang loose when on small-small (with the same old-school caveat). If you've got room to work with you'd probably want to "optimize" for the situation you use most often. Commented Nov 7, 2014 at 13:00
• Incidentally, you're actually only supposed to add one full link. A single chain link is two pieces: two half links, an inner half link and an outer half link. See the illustration on the linked (pun not intended) page for a better visual of the difference between a half link and a full link. Commented Nov 7, 2014 at 14:23