If you want to truly do an “apples to apples” comparison for different riders on different bikes, shouldn’t the weight of the bike be considered? I know it’s a moot point if the bike is on an indoor trainer, but if it’s riding outdoors the OVERALL Watts/Kg would make a big difference.
I don’t know what got me thinking about this, but by my calculations two riders of the same weight and 20 min FTP would have a disparity of about 2% for every 5 lbs. (3 kg) of bike weight.
So if my FTP is 230 and I’m riding with a friend who weighs the same and has the same FTP but whose bike is 5lbs lighter, I would need to generate 234.6 Watts to match her speed at 230 Watts.
Of course, if our bikes weigh the same and our FTP is the same, I could lose some weight from my body (instead of my bike) to match her total Watts/Kg. Losing 6Kg would improve my Watts/Kg by 6%. And it’s a heck of a lot less expensive to eat less than to buy titanium and carbon fiber…