Until 15 days ago (when we returned to working from home) I was having a good run of commuting time on the bike. Minimal intrusive weather events and lots of days pushing the pace meant my time to work was consistently shorter than what I was used to. I felt like my fitness was progressing. But commuting "work" and training "work" are usually quite different. In duration and impact.
Still, it is regular riding of a decent length and I've proven I'm pretty good at using commute time as training.
However, I've felt pretty slow on the bike - especially the mountain bike - when I'm not commuting.
Given the slightly muddy conditions of late, most of my MTB time has been on the single speed. I got a single speed years ago in part because there are no derailleur gears or expensive drivetrain bits to wear out. I am on my third single speed now because they do run forever with little maintenance and they're a lot of fun.
Last weekend I had two decent rides on that bike. Both felt like I was struggling for speed.
Enter Strava. I got several PBs on both rides. Mostly climbing ones (where fitness matters most). Including one that was set in a race about 8 years ago. While Strava is GPS reliant and therefore not accurate enough for proper race timing, to-the-second results are fine and I don't care about a second here or there anyway. On a 3 minute section, being one second out matters in a race but not in a "how fast did I go?" query.

The coaching lesson from this is one I've seen many times before. How you feel and how you go are two distinct and separate things. The feel slow, but are fast situation is infinitely preferable to the feel fast, am slow situation at the other end of the spectrum. For me personally, the feel fast, am fast situation leads to tactical errors.
If only there was some good racing to really test my fitness on...

To make a long story short, I have used 180 mm cranks on my road bikes for nearly 30 years. This is becoming virtually impossible - even the new Dura Ace 12-spd lacks the 180 mm option. It's all about shorter cranks these days.
And the 177.5 mm cranks on my commuting bike cracked over a year ago. SRAM couldn't easily replace them (they were just out of warranty cover so they offered to give me a great price on replacements) because they're all about the DUB axles now and DUB doesn't fit BB90.
At this point I thought "what about 170 mm cranks?"
Why 170?
Because that is the length I have migrated to on the MTB (well, 2 of 4) for ground clearance. The remaining two MTB are the DH bike > 165 mm and the Single Speed > 175 mm. I'd like to change the SS to 170 mm as well, but basically can't be bothered.
Enter a set of loaner cranks with all the critical dimensions OK for my BB90 Trek. A 24 mm axle, a compact spider, the 170 mm length and enough offset to clear the chainstay. That they happen to be lightly used Rotor cranks is a bonus because they look good with the bike and the Record parts on the rest of the bike.

To make the swap I had to change the off-side bearing. SRAM GXP uses a 22 mm inner dimension while Shimano (and equivalents) use a 24 mm axle the entire length from left to right. In the end I put the original Trek bearings back in (the bike came with Shimano parts) on both sides. They have a thin nylon sleeve that covers the outside of the bearing and runs inside the bore to sit between the axle and bearing race. I have zero idea how durable this will prove, but maybe it will be quieter than a metal on metal system?

A side note: this bike has had a very noisy BB area from day one. The drive-side bearing doesn't seem to fit tightly enough in the bearing seat and the off-side bearing fits too tightly. So anything that makes it quieter is welcome!

The Rotor cranks might also be good in that they are not a fixed width system like Shimano cranks. Perhaps a bit of inwards pressure on the bearings will keep them quieter? Yes it might come at expense of a couple of Watts of power and early bearing failure, but I'll take it after 2 years of listening to noise from down there.

First up after installation was front shifting. It was perfect. The Rotor cranks located the two chainrings in the same place as the Red cranks before them.
Second was the test ride. I didn't have much light left, so I headed up Stromlo access road for one "effort" that would test out the system. Big ring all the way (which is so possible with compact cranks). No noise. The cranks did feel a bit short. I think I was running smaller than usual gearing to compensate.

Only time will tell if the shorter cranks become OK in a road environment. I'm also concerned about jumping back on either Madone with their 180 mm cranks. Will they feel too long?
Another side note: I haven't changed my seat height. The expected thing would be to lift the seat up by 1 cm to compensate for the 1 cm shorter crank and still achieve maximum leg extension. But that changes the top of the stroke by 2 cm. I've not been convinced for a multiple-crank-length riding cyclist that different seat heights are optimal. Many tell me I'm crazy. I always ran my track seat at the same height as my road seat. The centre of the circle of pedalling remains the same that way (concentric circles). I may also experiment with seat height as I go.

So expect me to report back in a few weeks with how radically shorter cranks are going for me. Dependent on going back to the office too, as working from home provides few demands on my commuting bike.