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Obviously numbers will vary based on the specific bike, but what are some approximations of the drag contribution of individual components in a typical (non-aero) road bike?
I have read in many places that the rider accounts for most of the drag. So obviously, riding in a more aerodynamic position is free speed. But after that, what are the most significant upgrades to the bike itself, and how large might those gains be?
I believe this is a hard question to answer because there are many interactions between different parts of a complex shape like a bicycle and rider.
At a guess, I'd rank components by frontal area and position toward the front on the bike. For instance the rear wheel has less effect that the front wheel because it is moving through disturbed turbulent air.
It's difficult to answer this question because one part might be much more aerodynamic than an equivalent, but when installed on a bike, the advantage largely disappears. I can imagine in some edge cases a freestanding part is more aero, but when installed on a bike, the overall system would be less aero.
GCN recently did some wind-tunnel tests and according to them (somewhat to my own surprise) one of the biggest improvements you can make is to your helmet.
Some people use the "hairsine ratio" as a metric for grams saved per dollar spent; you can do something similar for watts of aerodynamic drag saved dollar spent, and this is what the aero assistant at Aeroweenie attempts to do (it uses seconds over 40 km, not watts). It is general--it's not comparing specific brands of stem or whatever, and it doesn't let you configure every part of the bike, but it picks all the low-hanging fruit (note: this hasn't been updated in a while, and it disagrees about the helmet); this also adds low rolling-resistance tires to the list of improvements. There are lots of other interesting resources at that site.
This is an evolving answer and it will be periodically updated. I haven't found information in precisely the format requested, but here goes. Also, I will expand this answer to include aerodynamic drag from helmets and clothes - they aren't on the bicycle, but their contribution to overall aerodynamic drag is meaningful.
Specialized produced a vid based on wind tunnel testing where they started with a rider on a Specialized Tarmac, in somewhat loose kit (I've heard this called club fit in the US context), a standard road helmet (Specialized Prevail), and standard wheels.
The video was dated 2014. I'm not able to date the tester's bike exactly, but in 2014, the Tarmac was one of Specialized's road racing bikes. It was not a round tubed bike, and it may have had some aerodynamic consideration in its design, but it was not marketed as an aero road bike. In any case, the video appears to sequentially add tight, race-fit kit, then an aero road helmet (the first generation Specialized Evade, as contrasted with a time trial helmet), then Roval CL60 carbon wheels (60mm deep, likely contrasted with ~25-30mm aluminum clincher wheels), then changed the frame to the first-generation Specialized Venge frame (their aero road frame). My recollection is that the Prevail helmet was designed with some aerodynamic considerations, so it may have been (designed to be) slightly faster than the average road helmet of its day. Current higher-end road helmets may have received similar consideration.
The test shown below, as well as the second Specialized video linked below, was done with the wind at 0 degrees yaw. This is likely to under-state the magnitude of the gains from aerodynamic wheels and from an aero road frame. As the wind changes yaw, the amount of drag saved is likely to favor the aerodynamic wheels even more than standard depth wheels. One manufacturer that presents drag data for its aerodynamic wheels versus a Mavic Open Pro (a box section rim about 20mm deep, most likely with 28 or 32 spokes) is Flo Cycling. Their graph corresponds to the trend I described, and it is consistent with other manufacturer-reported data I've seen in public. I would expect a similar trend to hold true for bike frames, and the presenters in the video below alluded to this.
Gains are presented in the format of seconds saved over a 40 kilometer solo effort (26 miles). Remember that these changes were made sequentially.
Another 2014 video tested a 1980s steel road frame versus a then-current Venge with a different rider. That test kept the wheels and all the fit coordinates and rider position the same between bikes, but the bikes were equipped with different components. That test found that the Venge saved 50 seconds over 40km versus the steel bike.
Henec, going to race-fit kit and an aero road helmet are big improvements that should be comparable in magnitude to, and possibly greater than, aerodynamic wheels or an aerodynamic road bike. If you are willing, changing to a skinsuit will save even more time. Objectively, those changes are cheaper than changing wheels and bike. It is worth considering that not all skinsuits have pockets, and they definitely do not appear to be fashionable among amateur riders, and it is worth remembering that aerodynamic road helmets may be more poorly ventilated than traditional road helmets.
