Bicycle Mechanics - Which chainring to change, front or back?

I recently picked up a nice used road bike from eBay. The gearing is kicking my rear. It is a compact double with 39/50 front and 25/12 back. On a climb the slowest i can comfortably pedal is 9mph. Way to much for me. I would like to get something around 5.5-6 for steeper climbs.

First the Group set is a 2006 Dura Ace. Should I drop the front inside ring or change a back ring. Is it possible to change the rings on a cassette? Secondly, what is a good source for buying the appropriate rings?

Thanks in advance.

What speed? 9 or 10

If your inside ring is a 39 my question is are you sure it's a compact crankset? Compacts have a 110 mm BCD and can take a ring as small as a 33. Standard Shimano cranksets have a BCD of 130 mm, normally have a 39 inner ring, and may be able to take a ring as small as a 38.
If your cassette is a 9-speed you could convert it to a 13-28 if you can find the needed parts, a first position 13 with built-in spacer and a single 28. I have done this with good results but single cogs are not always easy to find.
There several sources for Shimano compatible cassettes. One option is to buy a 12-27. Another is a custom cassette from Harris Cyclery (Sheldon Brown site): http://sheldonbrown.com/harris/k7.html#9
You can probably run a 13-30 but this may require adjusting or replacing the rear derailleur's "B" screw. Your derailleur is rated for a maximum of 27 teeth on the biggest cog but it will surely take at least a 28.

I have a compact double (10 Speed) with 50-34 chain rings and was able put on a 12-30 cassette for hills without changing the rear dérailleur.

http://www.bikepedia.com/quickbike/BikeSpecs.aspx?Year=2006&Brand=Orbea&Model=Orca+Dura+Ace&Type=bike#.UD6e1dZlQig

This is the bike, it has a 10 speed rear cassette. Everything is stock except the seat.

That is showing a standard crankset. It sounds like a triple would probably be ideal for you but that would be expensive. I'd start by swapping to a compact double and a cassette with at least a 28T big cog. A cassette with a 30T big cog would be a bit better but it may not work with your RD. A low gear of 34/28 will put you around 6.5 MPH at a cadence of 70.

My apologies on the crankset, I thought a double was a compact, my BAD assumption. I appreciate everyone's input. Sounds like I need to make changes to both front and rear cogs.

Front >Rings
Back >Cogs

How about sprockets?

Most often for the gearing, swapping the cassette is cheaper and may solve your problem. But from 25 to 28 is not that much of a solve. If a 30t cassette may be fitted with current RD then it's the best and cheapest fix.
A triple crankest is great for hilly terrain but can get very expensive (also the brifter may not pull the cable enough for triple)

Messing around with http://sheldonbrown.com/gears/

It looks like if you're going about 9 MPH in 39/25, then to get to 5.8 MPH at the same cadence, you'd need a 1:1 ratio between your smallest chainring and largest cog. To get this, you'd either want:

1) A triple crank with a 30T inner chainring and a cassette with a 30T largest cog (it's also possible to replace the 30T with a smaller chainring like a 28T and match to an 11-28 or something). This would require a new crank, new cassette, new rear shifter (if it's currently double-only), a triple front derailleur, a long/mid-cage road rear derailleur (or MTB RD), and likely a new/longer chain, or

2) A compact crank with a 34T inner chainring and a MTB cassette with a 34T largest cog. This would require a new crank, new cassette, a MTB RD* (both short/mid- or long-cage might work), and a new/longer chain.

Option 2 would probably be cheapest since it definitely does not require a new shifter (the most expensive part) or a new FD.

*If using a MTB RD with a road shifter, you cannot use the 10-speed Dyna-Sys; you must use a 8- or 9-speed version.

By the way, to see the numbers I was looking at, go to the Sheldon site above and input 26 x 1.25 for wheel size and MPH @ 80 RPM for gear units. These were inputted after trial-and-error so that a 39x25 gear outputs 9.1 MPH. You can add other chainrings and cogs to compare what your speed would be; this takes into account whatever your actual wheel size and cadence are.

I know I'm going to be a party pooper here, but are you sure the answer is to change the gearing rather than to improve the motor?

How steep is the hill, what are other people riding up the hill, how long have you been working on getting this bike up this hill, and are there lesser hills you can use to build up your motor before attacking this hill?

You got a nice bike, but it is a high end race or race-type bike. The gearing is designed to be ridden by a strong rider. Converting it to much lower gearing will be kind of costly. You might just want to put on a 12-28 freewheel - thus getting a modestly lower gear - put it couple months of training and see how it goes.

I love tinkering with bikes too, just trying to come at this from a different angle.

Back in the day, races up the Pyrenees and Alps low was with a 42: 26. or maybe a 28t

older than that and the gearing was higher..

I am 58 years old and been riding for quite a while. I have mainly been riding fat tires. My daughter and son-in-law have gotten into Road biking and have asked me to do a couple rides with them. There is a 10 mile stretch of 6% grade on the shorter of the two rides. That sucker scares me.

I hope to improve the motor, but I need to be in that kinda shape by Sept. 8th. I tackled 3 miles of 4% last weekend. I am feeling my age.

My step-dad has been very happy with his 50/34 x 11-34 10-speed setup. Though, I might suggest an 11-32 for slightly tighter geaing on the higher end.

Back in the day, races up the Pyrenees and Alps low was with a 42: 26. or maybe a 28t

older than that and the gearing was higher..

Good climbers tend to weigh about 2 pounds per inch of height so they don't need as much power to make it up hills. A little structured training can give you a 60% power boost from an untrained state. That makes 42x28, 39x26, 34x23, or 30x21 a great gear for climbing some place like the Rocky Mountains or the French Alps for a decent cyclist.

OTOH, with a bit of middle aged spread and not much training you could have half that power to weight ratio which would take a 42x56, 39x52, 34x46, 30x42, or 24x34 to go up such hills at the same non-fatiguing cadence. Only the last option exists (in the form of a road triple plus a mountain bike cassette).

In the other direction the best professionals can have double the power to weight ratio of decent amateurs and need even less. Lance got rid of his sixteen cog so he could have 21-22-23 large cogs for the perfect high speed spin in the l'Alpe d'Huez time trial.

2 pounds per inch of height leaves me 6'3" and 150. I haven't been there since my Junior year in High School. I am now 3lbs per inch.

I think that I am going to try a SRAM 1070 30-12 cassette. A slight gain, but the rest i have decided I need to man up and get on my own. Strava has been a good challenge. There are several riders in my age group that are posting Top 10 times. I am going to try and join them.

If you shop around, and are not hung up on getting Dura Ace, a compact crankset would help at a reasonable price. JiveTurkey's advice above is the way to go IMHO.

+1 Play around with the gearing calculator on the Sheldon Brown site, before you spend money.

I have migrated most of my bikes now to compact cranks.

I'd go ahead and migrate to a compact. My favorite is the Sugino 48/34: http://harriscyclery.net/product/sugino-xd500-d-compact-double-crankset-110mm-48-34-teeth-3416.htm

Some speed demons don't like that the large ring is only 48, but I find it suits me perfectly.

And if you feel so inclined, go ahead and change out your rear cassette to something with a 28.

If you shop around, and are not hung up on getting Dura Ace, a compact crankset would help at a reasonable price. JiveTurkey's advice above is the way to go IMHO.

+1 Play around with the gearing calculator on the Sheldon Brown site, before you spend money.

I have migrated most of my bikes now to compact cranks.

A compact crankset gets you one gear lower with the same cassette. Ex - 34x23 is like 39x26.

There's a good chance that's not going to be enough, and although the default road triple 30T granny ring is only one more gear lower (30x23 is like 34x26 or 39x30) you can run down to a 24 ring with the possibility of having racing gearing for flat rides and a small enough low gear for the hills without changing cassettes.