Understanding gearing and drive train.

[diegogarcia]

However, I seem to be tiring quicker on the bike with 53/39 than I do on the bike with 52/36.

DG, why to you attribute the tiredness to gearing? I can see it if you tire more specifically on climbs where you mash a 39/25 (3.27 m development) and wish for a 36/27 (2.88 m development). But riding in the big ring on the flat, the 1 tooth difference up front is immaterial. The cassettes are the same for the first 8 gears, with 23-25 tooth cogs at the low end for one, and 24-27 for the other.
How different are your 2 frames? if the 53/39 is overbuilt and stiff, that could play a role.
How different is the usage? if you use one for hard intervals and the other for smell the roses rides, well...

I dunno, but I have been rather ill this year with shingles, chicken pox and I am battling depression which is affecting me on the bike as it is very debilitating on physically as well as mentally. I also spend 98% of my working week stood up now and my power and form has dropped due to this so feel that changing things may help. According to the gear chart, running 50/34 with a 12/25 will be better for me than my current set up, but to be honest, I am so out of form, I cannot push anything like 53/12 or 11 as not got the strength. Presently I am running a junior cassette with 16-27 which helps a great deal and actually, works really well with the close ratios on from 16 though to 21. I am trying my best to get through it however. I have thought about walking away from bike riding, but it is the one thing that is helping me stay on top of my mental health though it is all getting a little confusing.

[artray]

It seems like you have health issues . I would just work on staying healthy and feeling good again. Keep riding your bike, but ride for enjoyment and to stay healthy . Your form will come back but if you are not getting the results and gains as quick as you like it can affect your mental well being. It's always important to take the pressure off yourself when you are struggling or you will keep going backwards. Things will get better. Good luck

[basilic]

Sorry to hear about your health problems. As Artray says, keep riding, for fun only.
Re gearing, nobody pushes a 53/12, that's for passing hapless tourist cars on mountain descents. I have found the 50/34 with a 12-x cassette to be awkward, because you go from 34/13 (5.5 m development) to 50/17 (6.2 m). Not only is that a wide gap, but as you shift up front you need to add 4 cogs in the back. And you do that a lot, about 6 m is a useful range. I now run 48/36 with a 11-28 so I can stay in the 36 on flattish terrain (up to 36/12), and when I shift to the big ring I only add 1-2 clicks on the right shifter. Call me a wimp.