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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?

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    Interested readers might want to listen to some of the Marginal Gains podcasts hosted by Josh Poertner of Silca and his colleagues. silca.cc is the company link, otherwise the podcast can be found on iTunes or similar sites. I recall hearing that going from worst in class to best in class, wheels, frame set, rider position, and clothing would all produce about equal average savings. I’m refraining from posting this as an answer until I can be bothered to track the correct podcast and verify what I remember. – Weiwen Ng Jan 20 at 0:31
  • It's all over the map. – Daniel R Hicks Jan 20 at 2:37
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    @DanielRHicks That's not true. For example, I doubt you'll find any instance where upgrading the wheel skewers reduces as much drag as the frame. You will also not find much disagreement that the rider introduces most of the drag. Just because something can't be precisely quantified without detailed testing of a specific bike in controlled conditions does not mean widely applicable, accurate estimations are impossible. – Phil Frost Jan 20 at 2:58
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    @WeiwenNg Yeah, that does sound like something Josh said but I don't remember exactly which podcast it was in either. However, I do recall that Chris Yu said that in the first half of this podcast: cyclingtips.com/2019/07/… . You can ignore the last half. – R. Chung Jan 20 at 3:41
  • I think there's too much variability for a good answer here. "aero helmet" might help a lot of people, unless you have Fignon hair or wear the helmet wrong hurting your aerodynamics. – Criggie Jan 20 at 21:42
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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.

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    Some things are relatively independent (like, helmet and clothing) while others, as you said, interact (like wheels and frame, or wheels and tires, or shoe covers and frame). There are rough general rules of thumb but they're only general so if you really need to know you have to test. That can happen in the wind tunnel but also in field tests (if you're very very careful and conscientious in your testing protocols). – R. Chung Jan 20 at 3:46
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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.

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