How do you (personally) measure an improvement in fitness on a road bike?

[L CC]

Not sure about that. I row pretty regularly, and ride pretty regularly, and I'm not sure there's much correlation between 'fitness' on the two. The rower is much more forgiving of excess lard, for a start.

I'm not training for races, so short distance speed doesn't really matter. It's all about recovery time and intimidation for me.

(I.e. I know I'm getting fitter if I'm not intimidated by #### ride, and I've recovered enough from last weekend's n00km ride to be in the gym at 7am Monday)

[Gus]

Normally I wouldn't care.
But if I have to test, I'll ride my prefered 32 km roundtrip. Note the time and when I have to test myself again, I might ride it 3 times in a week, and take the time that had the most similar weather.

[CrazyEnglishTriathlete]

If I really want to know how well I am going then I sit on a Concept II rowing machine - that boils everything down to cardiovascular fitness x leg strength x core strength.

I think there's sufficient crossover* between bike fitness and boat fitness that this would work pretty well. (*it's not 100%, but higher than any other sport - cue 30-page thread digression ... )
BUT you'd have to restict your rowing. Do it too often and you'll develop rowing-fitness that doesn't map to bike-fitness - thus skewing the results!
Although you'd need to have done SOME rowing for good results, or your technique will be so erratic that you'd also skew the results.

I think.

I'm a month into my cross-training on the rowing machine and getting close to my PBs, however, my road cycling is still in its usual parlous state for December, so it would suggest that there isn't a direct cross-over between performances on both.   However, two points in rowing's advantage.

1)  It is working my lower back and back-leg connectivity.  After a couple of years with limited gym work I was aware that my back wasn't as strong as it used to be when I did hit the gym each winter.  So it might not benefit me now, but I suspect that I will feel the benefit when my back doesn't hurt on la Touissire after the Madeleine, Croix de Fer, and Mollard…

2) I do more consistent cardiovascular work on the rowing machine than I can on the turbo - maybe its the constant reminder of 500m split speed glaring back at me in big letters

[Ningishzidda]

Use the road bike to build the muscles, cardio and pulmonary.

Use a simple test indoors in a controlled condition to measure.

The indoor controlled test is the simple ‘Step test’ on and off a box.

If it’s the same box and the same steps per minute for the same duration, at the same room temperature and you are wearing the same clothes, fitness improvement is noticed when the End of Test heart rate reduces.

The fantastic attribute of the step test is you can calculate Watts from first principles using your body mass, the height of lift and frequency of lift.
Then there is a formula using Watts and HR to calculate VO2 uptake.

You can, if so inclined, wear a big belt and as you lose body fat, add on weight disks to compensate, to keep your mass constant.

[Ningishzidda]

There is no need for a cyclist to shell out on a rowing machine to build spinal muscle.

Sit on the floor and with a half dozen old inner tubes round the soles of your feet and gripped tightly in both hands, do the rowing motion.

[velosam]

Personally, this: "Not feeling like death at the end of a ride?"

I use this, but currently the only way not to feel like death, is to drop my average speed which is now around 12.5mph on my commute.

Its takes longer but I keep telling myself 'its building base strenght'