What does trim (or no trim) on a Front der. mean?

I have a SRAM fd, and I've read that it has no trim capability. But to be honest, I don't even know what that means. What can I not do because it doesn't have trim? Thanks

It is not the FD but the shifter that provides trim capability. Trim is provided by small pulls or releases of the cable that lift the outer FD plate off a rubbing chain in higher gears on the cluster, or drop the inner plate off a rubbing chain in lower gears on the cluster. Many/ all Shimano brifters have trim capability, but only to trim the FD outboard, not inboard. Campagnolo brifters have almost continuous trim in either direction. I guess SRAM doesn't have it.
I ride Shimano and rarely use the trim. I'd sooner shift at the front.

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I have Campy Record and the trim feature is very nice. When I borrowed a Shimano bike on a recent trip I really missed the front d trim...and the thumb shifters.. and the chain dump..and the more comfy hoods...and the sleek cable routing and....

I hate the Campag thumb shifters. Whatever other virtues Campag has, the thumb shifters keep me away.

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"All that we see and seem is but a dream, within a dream..."

The Ultegra front shifter/front-derailleur on my 2006 and 2008 Giant TCR bikes do have inboard (when on the big chainring) and outboard (when on the small chainring) trimming which I use all the time to minimize chain rub.

I prefer to use up all the gears on the rear derailleur (except the cross chain combo) instead of shifting the front-chain ring as this is faster especially when on group rides. This is the reason I rely on the trim to prevent chain rub.

The inboard trim is engaged by a slight inward press of the inner shifter while the outboard trim is engaged by a slight inward press of the outer (main) shifter.

TRIM is a good thing to have ... but, a lot of MTB shifters (for example) don't have trim for the front derailleur.

The term TRIMMING for a front derailleur is undoubtedly a borrowed Nautical term -- trimming the sails (i.e., adjusting the sails) to maximize the wind ... keep the sails from fluttering/whatever.

So, trimming the front derailleur (as you may have inferred from what others have said) is done when the chain is rubbing against either the inner or outer plate.

If everything about your bike is set up as SRAM's engineers want it to be set up, then SRAM believes that the chain can be on either chainring AND on any cog in the rear WITHOUT the chain rubbing as it passes through the front derailleur. Maybe yes, maybe no IF you aren't using the right SRAM chain that SRAM has spec'd to be used with the rest of your SRAM group and/or your front derailleur's stops are/aren't adjusted properly ...

For example, when I use the correct BB spindle length (109.5mm) with my Octalink double (53/39) crankset instead of the one I normally keep on the bike (112.5mm), I don't have to "trim" the front derailleur (6503) ... but, with the 112.5mm spindle, I do "trim" the front derailleur. AND, if I were to use a triple crank & the 118.5mm BB, I would also have to trim the front derailleur.

I don't remember if I have to "trim" a 6500 front derailleur with the correct combination of components ...

It makes no sense not to have trim for the front derailleur. Some useful gears are bound to end up with the chain slightly rubbing against the derailleur cage. Nor does it make any sense to only use the rear and never the front when shifting to maintain cadence. This would mean that you could never make effective use of a half-step system on a triple, plus more immediately, it means that you are never using some of the useful gears that you have. Sometimes the best gear ends up being on that you get by shifting the front rather than the rear.

FWIW. I once had the SAME reservations because another rider who said that he had one of the first generation ERGO shifters had lamented the SAME thing ... and certainly, the first time I casually put my hands on a pair, I thought "well, yes, (he) was right" ... but, in actual use, unless you place the levers in a peculiar place on the handlebars OR have extraordinarily large hands, the thumb shifters are NOT in-the-way ... rather, IMO, they are THERE when you want to use them.

When I switched to campy I thought they might be a hinderance, but after the first mile, I realized the same thing.

Couldn't a person using a SRAM drivetrain simple add a barrel adjuster for trim?

Unless you use the SECOND (?) generation SHIMANO barrel adjuster which has the toggle AND route the front derailleur cable on the right side of the downtube (and, the rear on the left) as some people do to create a wider arc, it would probably be pretty tedious to reach down & twiddle the barrel adjuster that is on the downtube on-the-fly.

Still...if you really felt you needed such a thing, it could be done. Of course, there are always friction shifters for the ultimate in trim.

My commuter bike has a left Campag brifter ('98 Veloce, I believe), so my opinion is informed by regular usage. The right brifter and both brifters on my road bike are all Shimano.
The Campag thumb lever sits under the base of my thumb when I'm on the hoods, meaning that I have to slide my whole hand up the brifter to reach it. I don't appreciate that. The Shimano ones simply need a flick of the fingers.
I do appreciate the Campag trim for the FD - it is superior to Shimano's system.

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"All that we see and seem is but a dream, within a dream..."

That's good to know. The 9spd gear that I use doesn't have the inboard trimming on the big ring.

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"All that we see and seem is but a dream, within a dream..."

Ditto - I had 8sp sora many years ago with thumb shift - I just hated it..... it makes so much sense that shimano doesnt have this on their higher level groups, yet i cant understand why Campy has this on any of their groups??? its quite archaic!