"Half" shifting -- what and why?

[zia]

Excuse my beginner terminology as I work my way up the road biking learning curve...

I have a Shimano 105/Ultragra mix on my bike. To shift the front derailer, I have to push in twice. (But not for the rear.) My husband says this is for "half" shifting... or somethinglikethat. I barely notice it anymore, but I don't understand why. Why would I only want to shift halfway?

When I first got the bike, I get doing it *incorrectly* (only pushing in once) and kept dropping the chain. I thought something was wrong with the derailer!

[tctrek]

I used to have this problem sporadically on my 105/Ultegra setup on my old bike. My DH says this happens to him about 1/3 of the time. I'm definitely not a component guru and know very little about the mechanics of the bike, but I know it's not a "half shift". It's something to do with the position of the chainring when you shift. Your LBS might be able to tune it up for you. I don't think it's a bad thing, but I also don't think it should happen all the time.

On my new bike with all Ultegra, it doesn't happen at all... one tap and I move into the large or small ring.

[Eden]

It's not a problem - its for "trimming" your front derailleur. While you don't want to cross chain, sometimes you can be in a gear combination that is perfectly fine, but the chain is still rubbing - you can use the "1/2 shift" to move the derailleur just enough to stop the rubbing, but not shift to a different ring.

[tctrek]

Should you have to shift twice all the time? I could see if it was sometimes, but OP is stating it is all the time?

[OakLeaf]

You just need to give it a harder, longer shove (longer in distance, not time), similar to shifting two or three cogs at once with the RD. It will always trim ("half shift") if you stop at that first stop, and it will then be possible to do it again to accomplish the shift, as she's doing now.

[VeloVT]

This is actually a really nice feature. My new bike (ultegra) has it but my older bike (older ultegra) didn't -- and I find it really helpful. I have a bad habit of riding in 39-12, and if I couldn't trim the derailleur I really couldn't use that combination on the new bike without serious chain rub. I actually have to trim around 39-14 I think.

(I know somebody's gonna tell me I shouldn't be riding in that combination anyway ).

Are your shifters/fd 105 or Ultegra? I find it's quite easy to do a full shift all at once on my bike (2007 Ultegra, I think). I have another bike with older 105 that requires a serious, hard shove to shift into the big ring that I think would be difficult for those with small hands (it's pre-"double shifting", no trim available), but I've heard newer 105 is better in that regard.