I have always done big/big and add 2 links chain sizing.

with my recent build with 5800 105 with a compact crank (50/34) and 13/32 cluster, the result was not as good as I wanted in terms of shifting and rear derailler position

My mechanic suggested trying low/low saying that it is shimano's current technical reccomendattion

it worked great for me, not sure how it would work with vintage deraillers and crank/cluster sizing, but will try soon

this method if you are not familiar with it (and I wasn't) is:

put deraillers in low/low combination
thread the chain thru the deraillers and have on the low rings and teeth
size the chain so that it does not rub on rear derailer

here is a video link


ymmv

I can see how this would work. But I can't yet see the advantage over my current method: big (front) - small (rear) and vertical cage. This works for me, as long as I respect the chain wrap limits of the RD.

I've used the low/low for years on about any derailleur - had no idea Shimano spec'd it (maybe I should abandon it )

Putting the chain on the smallest chainring and smallest cog to size the chain is how I learned to do it, I've never done it any other way. For many years I didn't know some people did it differently. It works for me on my vintage bikes with 42T small rings and modern bikes that have compact cranks.

This should work, unless you're intentionally exceeding the recommended chain wrap capacity. Some derailleurs will allow their stated capacity to be exceeded, others don't.

I would just caution you to shift into the big chainring, and carefully work your way up to the big cog in the back, checking that the chain length allows that combination without exploding.

+1 except I always check that not only can I carefully get into the big-big, I can do it aggressively like I might when I hit the top of the big climb and I am completely stupid (until I recover a little oxygen). (Wrecked RDs run much less satisfactorily than slightly too-loose chain.)

Ben

the long accepted general best practice (at least as explained to me) was - as short as possible but no shorter.

so - use the small small cog combination, make the chain the shortest possible length that it is not sagging (that is, the jockey cage is just starting to take tension) and then set the chain at that length. Run thru the various cog / chainwheel combinations and confirm that you have enough chain and that shifting is good. Sometimes too much chain tension will inhibit shifting to a smaller cog. On triple chainrings you may not be able to use the combinations of large ring / large cog.

No one rule covers all possibilities, but this approach yields good results in most cases.

Mark Petry
Bainbridge Island, WA USA

I don't agree with this, provided that your RD has sufficient chain wrap capacity. With proper capacity and chain length, the RD will handle any possible combination of front chainrings and rear cogs. There may be cross-chaining issues, but those can avoided by choosing different gears. Cross-chaining should be avoided, but it should not be catastrophic if you do it enroute to a different gear.

The real risk is when the chain is actually too short to fit around the chainring and the rear cog, and the results can be severe. Trashed RD, bent/broken derailleur hanger, and broken rear spokes - and worse - you winding up on the ground.

agree.....and this is where i was with the classic big/big sizing the derailler was almost parallel with the seat stay...it would get into big big, but not really rotate....I got lucky, hit this one riding where I was able to recognize it, stop pedalling and put a fut down all in about a millisecond. any other situation I probably would have gone down....went a got a new chain to install before the next ride

yes, you are right, with a long enough cage and careful selection of chain length, you MAY be able to get the chain to wrap the large / large or small / small combination on a triple, but why ? Both are at the extremes of chain tension and slack, and also friction and wear associated with cross-chaining are at a maximum, but more importantly, the gear ratios of both combinations can be approximated by less extreme selections of ring and cog.

I have only one bike with long derailleur cage, it can easily shift the large large, but I never ever use that combination, or the small small either. Sort of a department store bike anyway as you can see :- )




Mark Petry
Bainbridge Island, WA USA

If using your small-small method, I'd carefully check the big chainring - biggest cog combo before riding it. The chain needs to be long enough to not jam when shifting into the big-big.

I sized using the usual "big-big + 1 inch" method, and it was fine. Like previous comments mentioned, this might be best if you are exceeding the recommended cassette sizes.

A rider might never want to use the big-big, especially on a triple, but it's easy to lose track of the current gear, and shift to the big-big by accident.

This is how I usually do it as well. BIG/small and vertical jockey cage. It was the Shimano way until recently. Works for me too. I always check the BIG/big and SMALL/small for chain length limit too.

The big/big method works most of the time, but on occasion it's less than optimal.

BTDT

Now I have run setups that had the RD completely folded back and the chain rubbing on itself running the small-small. (Triples) That combo only gets used for when long hills level out and I do not want to do a double shift, then have to reverse that double shift as soon as the hill steepens again. Never had an issue beyond the noise. Now, with more modern cranksets with their pegs and pickup teeth, usually I cannot even run the small-small without the bigger ring picking up the chain so it isn't even an issue.

Ben

Makes sense. When I was running my bike's original 52/42 chainring and 13-24 freewheel, I cut the chain to suit.

After switching to a 50/39 chainring and 13-25 freewheel, the chain rubs the rear derailleur cage in the small/small combo -- 39T chainring, 13T freewheel cog. I suppose I could cut a link or two but haven't bothered. I just avoid that gear combo.

Main problem with the chain being a link or two too long is the chain slop-slap on downshifts -- occasionally the chain drops, but instead of fixing it I just jockey the front derailleur lever to pick up the chain again while riding. So far, so good, although it can be unnerving on hill climbs in traffic -- I have only a couple of seconds to reset the chain before I run out of momentum.

totally get that in this case doing big big plus 2 links gave me a chain that jammed (yeah could be user error but I checked 3 times, marked the chain with a marker, etc) using the smalll small method above gave me a lot longer chain

Oh, yes. That department store with only one department: the nice bike department.

Large/large is a safety issue. It's not an issue of being able to use it, it's an issue of not having the drivetrain implode if you accidentally do. Small/small, well I run cassettes with small gaps and 11t at the top end. It's useful for stiff headwinds and trying to stay in the goldilocks gear.

I think one needs to review both, plus the shifting performance, with B screw rear mechs, that can come into play too.
With other when expanding the range, a few rear derailleurs allow multiple positions for the jockey cage, that can sometimes help or be needed.
I just did that to a Nuovo Record rear mech when I went beyond the limits of the front chainring differential, 50/34

The little/little method will work with doubles or triples and gives the longest possible chain within the RD's wrap capacity. I've used the same chain length on doubles and triples, because the longer cage on the triple takes up the slack in the little/little position. There is no advantage to using a shorter chain. The only time the big/big method is a must, is when the largest cog being used is larger than recommend for the RD cage length. Then you better add length and know that the chain may hang loose in the little ring and several of the smaller cogs. It takes 4 teeth larger to use 1 inch of additional chain length.