I am considering converting my 53/39 shimano ultegra cranks( 6400 seris) to compact gearing (ie 50/34).
what is the smallest chaining I can get? or is 39 the smallest I can go.

thanks for any input.

Generally a 38 but, there use to be an obscure source for a 37.

"Compact" cranks have a 110 mm bolt circle diameter (BCD) that permits using a 34T chainring. You have a 130 MM BCD crank that, as Sydney said, takes a 38T ring as the smallest (I've never seen a 37 offered). Don't pay for a 38T for your current crank as the improvement over your 39 will be minimal.

Just curious...

But why do you want to change to a compact crank. Weight savings, or better gear ratios or are there another reason?

I want to swap it out mainly for better gear ratios, since I hardly find myself pushing the 53 (for city riding anyway). The main factor was the compact crank I install on my bro's bike a 50/34.
I find the 34 kinda wimpy but I can accelerate with the 50 with out too much effort.
after that I am pretty much sold.

You might consider a 50/36 compact. If I were looking to buy a compact that's what I'd want. It really depends on your needs, get what you need.

Al

I've yet to find a 50/36 being sold anywhere. They are normally, it seems, being sold as 50/34 but you can buy a 36 ring to replace the 34. You may be able to get it done at point of purchase if you buy at LBS as opposed to Internet purchase.

We have lots of guys liking the compact road setup. Some of them are running the outboard bearing two-piece crank setup and switch back and forth, quickly, depending on the type of riding or race. Lots of guys using it for cyclocross as well.