Hi,

I am about to install a new transmission on my road bike, 53/29 and 12/25 and a new chain.

How should I size my chain?

Actually, I know how to size my chain for this very transmissions, but say I want to put a bigger cassette for a special ride in the mountains and put a 12/27-or-28, do I need a new chain for that? or can I afford to add two extra links to my regular chain length and always ride with a chain a-couple-of-links-longer-than-it-should most of the time to have it perfect when riding the bigger cassette? or should I size it perfectly for my regualr gearing and just be extra careful never to extreme-cross-chain when mounting the bigger cassette?

Or,... for those steeper rides I could change the chainrings and keep the cassette and put my old 50/34... or would that be a mess when cross-chaining small/small ??

I mean, I cannot be the first one to do that... what's the best practice?

If you want to be able to fit both cassettes, size using small/small. It gets you the longest chain possible for your configuration. I always use small/small

How does small/small give you the largest chain possible? Perhaps you should elucidate the other aspects of the method. Maybe I am thinking differently than you what small/small means. I have always liked largest front/smallest rear/RD perpendicular to the ground.

OP, any of the popular sizing methods (see Sheldon Brown' site) will work and give you some leeway to go a couple of teeth up or down either front or back. I wouldn't ride the wrong size (not optimum) chain 100% of the time just for the 5% situation. If you are going to be changing to a radically different cassette or pair of chainrings, you will surely need a differently sized chain. Keep a spare in that size around for the special case. You will be doing considerable modification anyway. How hard can it be to replace the chain as well? Just make sure to use chains with master links to keep everything simple and avoid the need for spare pins.

Oh, I see that is the Campy method with no slack in the RD, i.e. the RD is at full extension. Okay! I still like the Shimano method of large front and small rear with RD perpendicular to the ground as it is reputed to give the mid-range length. But all the methods seem to work well.

This is what I do. Small/small, longest chain that doesn't hang (means the rear derailleur has some tension on the chain). Usually if you add a pair of links the chain will touch the pulley guide or the chain on a pulley.

My theory is two fold:
1. If the chain is as long as possible for your particular rear derailleur, (small) chainring, and (small) cog, then you can go to a bigger cog in back if your derailleur can handle it. If the chain is shortened to fit, say, a 53x25, and then you use a wheel that has a 27T large cog, you will run a risk of breaking something out back when you ride (and inadvertently go into the big-big). Even pros aren't immune to this - there's one semi-recent classic where I think an Astana rider is stranded, his rear wheel locked, in the big/big, and his chain is obviously too short for that combo.

2. Even if you never use a larger cog a longer chain has less tension on it and a little less friction. You also get to spread wear out a bit more, over another 2 or 4 rollers. You won't notice the extra couple links in weight. They're free you might as well use them. You have some extra links in case you twist a link or for some reason need to shorten your chain to get home and still be able to use all your gears.

The small-small method works fine as long as you're not exceeding the RD's capacity.

Nobody counts links?

Or just mount the 12-28 and do big/big bypassing the RD. If you do big/big, you can always accommodate a tighter range cassette without worries. Also, if you set your "b" screw with this largest cassette, you don't have to worry about contact when you go tighhter. (If you forget to adjust).

Really. If you haven't changed your gearing and have your old chain at hand, just match them up and trim.

Shimano method worked for me, as well. As I side note, I learned to do this myself this after having lbs switch me over from a 53/39 to a 50/34 and they just left chain length as is . Details people, c'mon!

I was under the impression that chains stretch (very bad, and badly) over time and the main purpose in replacing a worn chain is to eliminate chain stretch, which your smarmy reply contradicts.

Yup. That's how I do it too.

Just adjust the chain for the biggest cog you have. The whole point of the RD is to maintain tension as different sized cogs are used. A 12/27 is the same as a 12/25 except the last two cogs so a chain that works on the former will by definition work with the latter.

If you hang the chains side by side (a pair of nails driven into a stud in the basement or garage works well), it's pretty obvious how you can match them up.

You're right about the stretch but not about the magnitude. A full link is 1" long. The full length chain consists of ABOUT 104 half links (52 of each type). A chain is considered too long when it is about 0.5-0.75% elongated. For a 52" chain that is about 0.25-0.38" over its entire length, i.e. less than even one of the half links. It is easy to see the correct length for the new chain from the old chain even if it is stretched. Just line up the first matching links of each and trim the new one to the last full link end (that matches the old chain end in type) that is just smaller than the old one. It really is as easy as John was suggesting.

Which is why small-small generally works. As long as the derailleur maintains tension on the small-small, it will maintain tension with any larger combination. The only difference is you don't need to make sure you have your largest cog on at the time. If you have a wide variance in your smaller cogs (not typical IMO among people who keep different cogsets around) you'll want to make sure that you have your smallest cog on though.

There's zero need to keep the old chain for reference too -- the sizing is trivially and quickly (faster than counting links or laying two chains out side by side) done on the bike and doesn't even require derailleurs to be cabled since small-small is the zero cable tension position.

How do you set-up a chain with small/small?

I know the two following methods:
- big front/rear small: make sure that cogs, pulleys are aligned vertically
- big/big without running chain through derailleur: make sure you have one full link overlap

But how to to small/small? run the chain on small/small? through rear derailleur? and cut it to the longer length that ensures an acceptable tension? I actually have no clue, this is just an educated guess...

google "Sheldon Brown chain length" for a rundown

I did, and he talks only about the big/big I mentionned...

The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that one complete link (one inch, -- one inner and one outer half-link) overlaps. In almost all cases, this will give the optimum length.

See my post above, sorry I don't have the number handy. Basically you put the chain on the small front and rear and through the derailleur. Then you let the derailleur relax all the way back with essentially minimum tension on it and bring the end of the chain to the link that doesn't cause any more pull on the derailleur. That is the size. Any bigger combination of gears will pull the RD forward as it should be. I have never used this but supposedly it give a good length. It is the Campagnolo-recommended method.

Having wrenched for years, ....pitted professionally for cross for years, etc.

small/small

....from a utilitarian perspective it is just how it's done. You put it on the small small and pull enough tension on the chain to bring the RD cage off of dragging on the chain. Cut. Join. Done.

This is done because when racing you get wheels from wherever you can. Nowadays there are so many different drivetrains and gearing that people are running that you have no idea what cassette is going in there and you need capacity to handle the largest cog the RD can swing.

If you're 1 guy riding 1 bike with 1 or 2 sets of wheels using the same size cassette and will never change anything then sure...go big-big and make the chain the shortest it can be.

Small-Small makes it as long as possible.
Big-Big makes the chain as short as possible - you can not run a larger cassette without causing damage to the RD.

Do you mean just to make the chain taught?

RD at rest will have the back pulley (lower) be sitting higher than the front (upper) pulley. If you just took the slack out then the chain would rub on the cage/front pulley when pedaling. You have to add enough tension that the rear pulley (lower) moves the chainline to below the front pulley. That way the chain isn't running on the derail as you are pedaling in a small small combo....if you were to pedal in a small small combo.

Thanks this helps!!

Just finished building up a bike last night and gave Small/Small method a try. Gotta admit that is even easier and faster and I think I will do it that way from now on. Gives same results but less fuss messing with chain on big ring trying to sight perpendicular line to floor.