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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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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.

  • Changing to race fit kit (from club fit kit): 45 seconds saved
  • Adding aero road helmet (from standard road helmet): 42 seconds saved
  • Changing to 60mm wheels (from ~30mm wheels): 34 seconds saved
  • Changing to aero road frame (from road frame): 59 seconds saved

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.

  • I'm so glad skinsuits "do not appear to be fashionable among amateur riders". No-one wants to see a slightly tubby mammil in a skinsuit! – Andy P Mar 2 at 16:36
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    I wish Chris Yu had described the aero drag improvments in terms of CdA rather than in "seconds saved." In the GCN video cited above by Adam Rice, the improvements are described as "watts saved." In that GCN video they give enough information so one can calculate the implied CdA (and they appear high). In your answer, you might think about adding some description of how to convert between CdA changes, watts saved, and seconds saved. – R. Chung Mar 2 at 17:10
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    @R.Chung I will try to work this in. It's a little annoying that this SE site doesn't support Mathjax, unlike the statistics or math SE sites. – Weiwen Ng Mar 2 at 17:28
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    @WeiwenNg Here's a handy conversion rule of thumb that works okay at sea level, "normal" air density, and "time-trialing" speed (~43 km/h): a change in .01 in CdA is roughly equivalent to 10 watts, or about 1 sec/km, or a change in Crr of about .001, or a change in slope of about 0.1%. – R. Chung Mar 2 at 17:53
  • @AndyP your point is well-taken, but consider this: we are already in a fringe sport to start with, and the people thinking about optimizing aerodynamics are at the fringe of the fringe. so, if someone has a bit of extra weight and wants to lean in, maybe I don't care so much. that's providing the skinsuit fits, however; I believe that for clothing, wrinkles are one big cause of drag (citation forthcoming), so you have to take caution there. – Weiwen Ng Mar 2 at 17:59

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