OK, I used the big chainring-largest cog +1", bypassing the DR to determine chain length. But this doesn't seem to look right- it's on the largest cog and 42T chainring (I'm running 45, 42, 30...). Should the DR be extending this far forward, or do I need to add a link or 2? (It is shifting fine, BTW...). Thx

Will the bottom pulley swing forward another inch? If so, I'd say you dialed this in perfectly. Looks isn't the issue. You want the chain long enough to allow for a hiccup on the freewheel/cassette and not tear up the derailleur. (That torn up derailleur leads to all sorts of other bad consequences.) But a chain longer than that leads to sloppier shifting on the smaller cogs. Now, if this derailleur is at its limit for swing forward, add links!

If to your eyes, this doesn't look right, well good. A reminder this is a gear you do not want to be in.

If that photo is on the 42T ring, and you’re running 45-42-30 rings as stated, then I’d say the the chain is too short! Can you safely shift on and off the 45 when on the biggest rear cog? Test very carefully on the stand, and don’t try to force anything.

If I understand correctly, that's the position on the 42, NOT the 45? If so, and you know not to try that same cog with the 45, I say go for it. To paraphrase Detective Callahan, " a drivetrain's got to know its limitations".
You know what you've set up, just be cautious.

We're it me I would make sure it was long enough for big-big but I am a forgetful old man who may choose the wrong gear. If you don't have that issue pedal on!

+1 to that ^^^.

I ride often at night with steep hills all over Seattle. It’s easy to forget which ring I’m on when i can’t see it clearly, so I always adjust the chain for safe big-big shifts. There’s no penalty beyond a bit of noise for a slightly too long chain, but as [MENTION=392125]79pmooney[/MENTION] said, there are BIG consequences for a chain that’s too short.

Yeah, in the photo the front is on the 42t, and I do know that big-big is a no-no, though that doesn't mean at some point if I'm not paying attention it might not happen... But the RD does have an additional 1" of forward pivot from where it is in the photo. But I may just add another link to play it safe....

Yeah, the big-big problem with a too short chain is just too dangerous of a beast to mess with. I did it once...once! You're riding along, going up a hill, feeling like you need a lower gear, shift to a bigger cog without realizing you're in the big chainring, and WHAMMY! Your drive train just exploded -- chain into the spokes, probably cutting a few of them, bent hanger, and your derailleur has twisted itself into a pretzel. It doesn't even necessarily feel like you're forcing the shift -- it didn't for me -- the forces at work are just much more powerful than you'd imagine.

FWIW

Ability to do big/big is hard requirement....learned that, but got lucky was going slow and was able to fall gently and not hurt me or drive train

I have grown to really like the small/small chain sizing method.... use it for vintage gear when i am pushing limits and for modern gear

I am with the others -- large-large cross-chaining should not lock anything up or bend chain, chainrings, or derailleur. I ran a 48-45-34 / 13-15-17-19-21-24 half-step-plus granny on the PKN-10, with a short cage SunTour Cyclone II rear derailleur, and I accepted a small amount of chain slack in small-small to protect myself in large-large.


Aside: I love half-step-plus-granny gearing, and your 3-tooth drop up front is great with the 2-tooth progression among your smaller cogs.

You are lucky your chain doesn't catch between the chainrings. I did lots of half-steps, 1/3-steps (50-47-44), and half-step-plus-grannies on older cranks, but as Sheldon Brown presciently cautioned me, I could not make a 53-50-39 work with a modern Campag. spider. I settled for a 50-42-39 half-step-plus-overdrive / 1.5-step-half-step, which actually worked out pretty decently.

Yep, that's how it happens, all right. Murphy's Law is not a joke, but a design principle that should be taken seriously.

The half-step+granny is new to me, set up this way mainly to use what I had on hand. I'm still getting used to it, but I like it. Did a ride yesterday with some pretty steep climbs and found myself wishing I had one more lower gear.... Ideally I'd like a 7sp freewheel that has a 32t or 34t large cog that isn't a 'mega-range' with the giant jump between the two biggest cogs, but the only ones I've found like that are the ($$) ones from IRD. I think I might have a 28t chainring, so may try that. It's also early in the season, so another week or so of regular riding and maybe the low I've got will be fine.
The FD shifting is taking a little getting used to. Going from the small ring to the middle I often end up over-shifting to the big ring, and a couple times I've had the chain come off shifting from the middle to the big. Seems like the limit screw has to be set just right so I've got enough trimming to stop rubbing when in big ring/smallest cog, but tight enough to keep it from coming off when making a fast shift from the middle to the big ring. Seems like it's only happened when I've shifted without a lot of load on the pedals.. The 45t ring was originally the inner ring, so it's lacking the little tab to prevent the chain from falling between the arm & the ring- thinking about drilling & tapping so I can put it on. And I just need to get better at finessing my shifting. The Criterium shifters need to be pretty well tightened down to keep from slipping and aren't exactly the lightest shifting. Looking forward to (eventual) Retrofriction levers....

I always cut my chains to accommodate big-big. Failure to do this could lead to absolute destruction of the rear derailleur if someone, say a friend that I loaned my bike to, does not understand the ring to cog relationship.

This.

The rule of thumb is large/large+2. Why did you use +1, if I may ask?

1" or 2 half-links is the rule I've always heard/used. It's enough.

OP's picture looks fine to me. Yes, it's unusual to see the tension pulley pointing forward, but if you are able to push the cage up further with your finger while it is in the big-big combination before it hits any stops or pulls tight, then you are good.

What's the derailleur look like on small/small? if you can add another link and still have it under tension, I would do so.

Your setup on the PKN-10 cries out for the Suntour VX-S rd. It maxes out at maybe 26t capacity, but is wonderful at 24ish and handles a 30t rear cog easily.

Yes, I misread the original post and thought he meant 1 link, not 1 inch which is the same as 2 links. You're right - if he's in big-big and there is still spring in the derailleur, he should be fine.