I was intrigued to see this data from my ride today. Average wattage was 208, which is about what I hope to put out in my next IM bike leg. (Adjusted wattage of 230, according to some algorithm that surely accounts for the warm temperature.)
That wattage produced an average speed of 17.6 over a moderately hilly (3250 feet) 50 miles. I anticipate closer to 20.5 mph in an IM race.
There are two obvious differences. First, I was riding a road bike that is set up to be comfortable. (It is comfortable, too. After two weeks of riding the two Cervelos, getting back on the Gunnar felt like settling into a Lazy Boy and grabbing the remote.) Surely there is some aerodynamic penalty. Second, the bike was loaded more or less how it will be in the 600K 10 days from now -- and not much lighter than it will be in the 1200K in July.
My perhaps naive conclusion is that the weight made a real difference in the amount of work required to maintain basic speed, in light of the constant rolling hills. I further conclude that it is easy to understand why holding 18 mph in a randonneuring event is so doggone hard to do.
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I tried plugging some numbers into analyticcycling.com. It's a bit of a crapshoot because how do you plug in "3250 feet of rolling hills", so I used 1% grade. And of course I have no idea what the inertia of your wheels is.
All else equal, at 208 watts it guesstimates ~8.5 m/s (19 mph) with an 8KG bike, and 8.4 m/s (18.8 mph) with a 10KG bike.
But then adjust the aero position; assuming aero on the 8KG bike it guesstimates ~8.8 m/s (19.7 mph), and assuming upright position, about 7.8 m/s (17.5 mph) on the 10KG bike.
Those numbers are pretty close to what you're observing in your IM and rando rides. So it looks like the bulk of the benefit comes from the rider position on the bike rather than bike weight. Interesting that a 4.5 pound difference in bike weight, even on a 50 mile long 1% grade, makes such a minor difference.
The conventional wisdom on Slowtwitch is that aerodynamics affects speed vastly more than does weight. The difference -- according to some pretty smart people -- is such that one should basically ignore weight entirely when there's an aerodynamic benefit to be gained.
A data point on this. After IM Mont Tremblant last year, there was a Slowtwitch thread in which those who had raced were asked to post three data points: (1) Their weight; (2) Their average watts; (3) Their bike splits. My data were ~165 pounds, 205 watts, and 5:11.
Based on the others who responded, it was evident that my bike position -- which is extremely aggressive -- was pretty key to the result. Most others who were about my weight, and who rode about my split, took ~220 watts to do it. Alternatively, those who were my weight, and rode my wattage, were a good 10-15 minutes slower than I was.
My tri bike is far from the lightest out there. To the contrary, its deep tube shapes, highly integrated front end, rear wheel cover, and the like made it something of a brick compared to others.
In conclusion, I think Sam's right that there's very little benefit to be gained, speed-wise, in shaving grams. Especially in a randonneuring event, where time is almost literally irrelevant.
One other thing: Normalized power is unaffected by temperature. It is affected only by the variance in your wattage from moment to moment. The idea is that it's harder for your body to do a series of sprints and recoveries than an easy effort, so the measurement (developed by Andrew Coggan of TrainingPeaks fame) assigns a much greater weight to time spent at higher power than at average-to-normal power. If you rode 3 hours at exactly 208 watts for each second, your Average and Normalized Power each would be 208 watts. The greater the variance -- which closely tracks the hilliness of a ride -- the greater will be the difference between the two numbers.
This leaves the question of where the tipping point lies. If a nonstop steep downhill prioritizes aerodynamics exclusively, and a nonstop steep uphill prioritizes weight exclusively, somewhere between the two is a point at which weight overtakes aerodynamics.
A not very meaningful data point: both cyclists I encountered going my way passed me on an uphill. I overtook them on the flat and left them behind on the downhill. It could be they were _really_ not aerodynamic, or it could be they were doing hill sprints and coasting between them, but it seems possible and even likely that their uphill advantage was weight related.
The relevance of time: whenever one wants a point of comparison, time ends up being the default. It is certainly as relevant in a randonneuring event as it is in a marathon or an Ironman. Perhaps more so, because beating the cutoff (the only objective relevance) in those events is easier than is beating the cutoff in a randonneuring event.
The time question is interesting.
In single-day rando events even a moderately unfit cyclist can make the time cutoffs, making time an interesting measure only if one is interested in measuring time. There are quite a few randos who use instead as a point of comparison the quality or quantity of food consumed, or the company kept, or the scenery observed.
In multi-day events the situation changes drastically as other factors, whether physical, mental, or logistical, could easily keep an otherwise fit cyclist from completing within the cutoff.
So for rando specifically, time is not particularly relevant for a single-day event, but is very much so for a multi-day event.
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