Just curious. I have a stock Miyata 110 (12 speed) that runs perfectly. I can go big/big or small/small with no chain rub at all and I never have to trim the FD. It's flawless.

Then I have my Mandaric (14 speed) that I tinker with and tinker with and just can't get it right. Any big/big or small/small (even when not at extremes) causes some chain rub and I have to trim the FD going up and coming back down. I love the ride and don't really mind its quirks, I'm just trying to figure out if the issues are mine (not as mechanically inclined as I like to think) or if even the LBS would struggle to set it up to perform the way my Miyata does.

What's your experience?

I have two bikes that rub front small, rear small. The rub is on the inboard side of the big front chainring. I haven't tried messing around with spacers. When I posted a thread about it here a few months ago most replies were "why would you be in small/small anyway, learn how to shift." So last few rides I've just been getting used to shifting to the big ring sooner when shifting up.

Having a bike that doesn't rub in big/big and small/small is nice but those combinations put alot of side stress on the chain. I was always taught that while a bike should be able to shift into every combo you that doesn't mean you should use them.

I have found that on some of my bikes there is noting I can do prevent the chain from rubbing the inside of the big ring when on the smaller cogs in the back. I suspect this is simply due to the uber narrow BB and low profile cranks in use since the late '80s, abd the increasing number of cogs in the rear.

My 87 Bridgestone 550 need a little trim, but it refuses Big-Big combo, I can put it in that gear, but if I torque down, it will skip and try to jump off....so I don't go there. I usually cruise in the big ring (Flatland Florida), so have to be careful when I stop at a light to NOT shift all the way up the freewheel. Suntour Alpha 5000 indexed setup. It is probably chainline

All drivetrain parts being equal it can come down to chainstay length, longer is better when you want to use all those gears.

I'll admit to cross-chaining small-small, but I don't have any issue as long as I CAN trim the FD to avert chain rub.
(I can appreciate that it might be more of a challenge with those wide 126mm OLDs.)

I consider having to trim the front derailleur as completely normal.

Of course Auchencrow doesn't have issues with friction only!

Is there something else?

Short chainstays on some bikes will cause rub no matter what. I also ran into chain rub with the side of the large chainring. I solved the problem, along with a host of noisy shifting and clunking by swapping on a 9 speed chain on my Suntour 6spd Accushift set up. Dramatic improvement.

Having a dead-on chain line greatly minimize the chance of rubbing. I switched the stock BB of my Centurion Ironman with Shimano's BB-UN55 to achieve near-perfect chain line. No more rubbing.

https://sheldonbrown.com/chainline.html

Perfect mechanics? - always possible, just not in my shop.

A bike isn't that complicated, but I seem to do a much better job on other people's bikes.

Quiet = good. I always like another set of eyes and hands to go over my stuff.

Sometimes I surprise myself with how good I am.

Most of the time, I'm not that good and not that surprised.

"Sometimes you just get lucky, sometimes you knock on wood...
ain't it good to go out on a limb and find out that it holds?"-Rossington Collins Band.

Agreed. I really like the 18+" chainstays on a Peugeot UO-8. All gears are usable with little chain warp.

My bike is set up so that I can run Small-Small. I never run big-big but when I was setting things up I don't remember it rubbing in big-big.