I am planning on changing my 5700 triple set-up to a compact double.
I don't want to change the shift levers if I don't have to.

Can I use the triple levers, using the two outer positions for the double? Both outer positions have trim options.

I've seen posts about doing this, and I think that I'll be OK.
Is the spacing the same on the double as it is on the triple?

Does anyone have step-by-step instructions, or tips on how to do this?

Thanks,

Peter

In campagnolo u can, in shimano no clue. sorry.

Shimano's 5700 series STI brifters are double or triple specific and Shimano's tech documents specifically say you can't use a ST-5703 brifter with a double crank.

That said, it will probably work as you surmise by using the two outer chainring positions and locking out the granny ring position with the low limit screw. AFAIK, the spacing between the outer two chainrings is the same for both the double and triple cranks.

It will work. Shimano says that the 5703 is triple only because there have been some problems with people using previous generation triple shifters on doubles, using the lower two positions and blocking off the third with the limit screws. When you accidentally try to shift into that non-existent and blocked-off outer position, a small part in the shifter has been known to break (even when using a surprisingly small amount of pressure on the lever). That is why their shifters are now double or triple specific. However, as I said it will work, and to avoid the possibility of breaking something then you set up the cable so that you use the outer two positions of the shifter. You'll then still be able to click the shifter into the non-existent inner position, but the cable will just go very slack and the derailleur won't move.

I own two bicycles that use Ultegra 9-speed shifters. One is a compact double, the other is a triple. The shifters have 4 shifting positions. On the triple I have one position for the big ring, two for the middle ring and one for the granny. On the double I have 2 shifting positions for each chainring. I don't have dropped chain or other shifting issues with either bike. I'm pretty sure that's the way Shimano intends for it to work because I also have Flight Deck computers on both bikes and they always indicate the appropriate chainring.

Would the same set up work with other triple shifters? I can't say because I've never tried it. If it was my bike and I already owned a triple shifter I'd definitely give it a try to see if I could make it work. If I was buying new parts, I'd hold out for matching parts.