Bicycle Mechanics - Using 3x STI shifter on a compact double crank?

I'm extracting one question from a multiple-question post of mine elsewhere. What are the downsides to using a 3x STI on a 2x (compact double) setup? I'm currently a downtube shift man (loud and proud, since I bought this bike in 1976), but I'm switching to STIs, without ever having used them myself. Wondering if I can use a shifter meant for a triple on a double, and if it will bug me for years for doing so instead of just getting one meant for a double. Great deal on a Shimano 3x is tempting me. I could get a Nashbar pair meant for a double for the same price, but I'm tempted by the big discount on the Shimano 3x, but only if the downside isn't too great. Please advise. Thanks.

You can use a triple front shifter no problem for a double. When the limit screws are set properly for the front chainrings, it will operate just fine.

Ironically I am looking for a set of STI brake shfiters for a 3x7/3x8/or 3x9 since all I have currently is a couple of sets of 2x8 STI shifters (dura ace and 600). With my weight, I really want to have the bail out bottom chainring if I can't make it up a hill in the middle ring.

For 9-speed cassettes, Shimano spec'ed the same part number STI for either double or triple use. I own an Ultegra equipped tandem with a triple crankset and an Ultegra equipped road bike with a double crankset. They both use the same shifters. On the triple I get 1 shift position for the granny, 2 shift positions for the middle ring, and 1 for the big ring. On the double I get two shift positions for each chainring. Both bikes have Flight Deck computers and both computers always indicate the correct chainring so I'm sure that mine are set up the way that Shimano intended.

If I already owned the parts, I'd try the triple shifter with a double crankset and see how well I could make it work. If I were buying parts, I'd get the correct shifter to match the crankset.

I'll throw in an information tidbit here, the original RSX brifters came with the same left shifter for both doubles and triples.. so in theory a triple can always be a double.

I'll throw in an information tidbit here, the original RSX brifters came with the same left shifter for both doubles and triples.. so in theory a triple can always be a double.
For the record, 2 clicks means its for a double and 3 clicks means its for a triple, right? It seems to me that the first shift say from smallest ring to the middle (triple) or big ring (double) has 2 clicks in quick succession if you gently shift the lever.

If the shifter has that first 2 quick clicks on a full throw of the lever and then you throw the lever again, you get another full click then that is for a triple.

For the record, 2 clicks means its for a double and 3 clicks means its for a triple, right? It seems to me that the first shift say from smallest ring to the middle (triple) or big ring (double) has 2 clicks in quick succession if you gently shift the lever.

If the shifter has that first 2 quick clicks on a full throw of the lever and then you throw the lever again, you get another full click then that is for a triple.
Why would you have 3 clicks for a triple? >.< You have your starting gear and then 2 clicks. I did notice something funny with my RSX's anyways everyone said they didn't have trim positions but the shifters do indeed have trim pull or I'd be SOL.

I think you're refering to the trim positions on newer brifters, the tiny 2nd click you hear with each shift.

So it's sounding like I'm fine using the 3x on my compact double. So, relating to my other post, if the Nashbar 2x STI pair costs the same as the Shimano 3x STI pair, but the Nashbar pair is 90 grams lighter than the Shimano STIs, do I chase the gram savings and buy Nashbar, or chase brand name and buy Shimano (assuming the 2x versus 3x is completely inconsequential)?

So it's sounding like I'm fine using the 3x on my compact double. So, relating to my other post, if the Nashbar 2x STI pair costs the same as the Shimano 3x STI pair, but the Nashbar pair is 90 grams lighter than the Shimano STIs, do I chase the gram savings and buy Nashbar, or chase brand name and buy Shimano (assuming the 2x versus 3x is completely inconsequential)?
Litmus test. Are you going to race on it? Your answer is your answer for chasing gram savings.

Nope. No racing. But if people said to me that the quality is precisely the same between the Nashbar and the Shimano shifters, then grams would be the tie breaker. If there is a quality difference, that beats the gram advantage. Or if it would be even a minor pain dealing with a 3x pair instead of a 2x pair, then the convenience factor would outweigh grams and brand name.

I think you're refering to the trim positions on newer brifters, the tiny 2nd click you hear with each shift.
That is exactly what I'm talking about. That tiny 2nd click on the small to large ring shift. If you gently downshift after you have upshifted, you can feel the first click release then you keep pushing the shifter and it then clicks again.

I wanted to make sure that was normal for double shifters. So what exactly does the trim position actually do?

For 9-speed cassettes, Shimano spec'ed the same part number STI for either double or triple use.

Except for Dura-Ace. Nine speed D-A shifters are specific to either double (7700) or triple (7703). I suppose the 7703 will work on a double but the 7700 will not work on a triple.

For the record, 2 clicks means its for a double and 3 clicks means its for a triple, right? It seems to me that the first shift say from smallest ring to the middle (triple) or big ring (double) has 2 clicks in quick succession if you gently shift the lever.

If the shifter has that first 2 quick clicks on a full throw of the lever and then you throw the lever again, you get another full click then that is for a triple.

Depending on the year model and the group model all Shimano shifters do not function the same way. What Retro Grouch says is typical for most shimano triple shifters set up on a triple crankset. There are actually 4 main positions plus a soft click for trimming the smallest chainring. Remember that the first position is simply all of the cable out of the shifter, then the "soft" trim position, then three solid clicks. The middle ring has 2 positions and the big ring has the last position. After shifting to the big ring the 3rd position can be used for trimming. This allows 2 positions for each of the 3 chainrings. Shifting from the smallest to the middle normally requires shoving the brake lever to the 3rd main position and then trimming back to the 2nd if the chain is running on the extreme left side of the cassette. Shifting from the middle to the big or big to middle requires only one click.

Some of the earlier triple shifters, especially in the lower groups have only 3 positions.

Depending on the year model and the group model all Shimano shifters do not function the same way. What Retro Grouch says is typical for most shimano triple shifters set up on a triple crankset. There are actually 4 main positions plus a soft click for trimming the smallest chainring. Remember that the first position is simply all of the cable out of the shifter, then the "soft" trim position, then three solid clicks. The middle ring has 2 positions and the big ring has the last position. After shifting to the big ring the 3rd position can be used for trimming. This allows 2 positions for each of the 3 chainrings. Shifting from the smallest to the middle normally requires shoving the brake lever to the 3rd main position and then trimming back to the 2nd if the chain is running on the extreme left side of the cassette. Shifting from the middle to the big or big to middle requires only one click.

Some of the earlier triple shifters, especially in the lower groups have only 3 positions.
What he said :D

One thing nobody's mentioned is that it's possible to damage a triple lever shifting a double, if you manage to over-shift it, putting extreme tension on the cable while the derailleur's hard against its stop.

Not a very likely scenario, since you generally don't want a bigger ring than your big one... but it's a possibility. Or, you could set the shifter up so the cable goes slack on the full downshift. Not sure which way works better.