Originally Posted by
chas58
very true above 20mph - aero is the majority of the total resistance.
Riding along a flat road, it's also usually true at 15mph. And quite significantly so. Making air drag a minority of total resistance at 15mph pretty much requires getting into a Graeme Obree tuck and/or running absurdly slow tires.
You probably know it goes up exponentially, where friction is linear.
It goes up cubically, not exponentially. Power lost to aero can be modelled with:
.5 * (air density) * CdA * (velocity ^ 3)
If we assume a rider with a reasonably-svelte CdA of .32, then at 15mph (6.7 m/s) and with a typical sea-level air density of 1.225 we have:
Air resistance = .5 * 1.225 * .32 * (6.7 ^ 3) = 59 watts
That's obviously not very many watts, but this same cyclist is probably doing significantly less than 100W in total to maintain 15mph!
At 10mph, aero is trivial and friction is the vast majority of the load.
On flat road, even at 10mph aero tends to be a pretty large chunk of total resistance. Especially since people tend to lean up more when riding slower.
Most people riding at 10mph on the flats just don't care about whether they're having to pedal at 45 watts or 35 watts, and they certainly don't care enough to bother getting aero.
On a road bike, the question never really comes up, because most folks can do >10mph even while bonking.
In my case, I'm in roughly the same aero position on the mountain bike or on my hoods with the gravel bike.
That's unlikely, especially if your MTB uses typical modern wide bars. Even if you're as low on the MTB as on the gravel bike, you're probably less aero.