# Definition of Aero wheels



## lblanch40 (Jul 20, 2011)

I'm pretty new to cycling and keep seeing the term aero used when discussing wheels. What exactly does this mean or refer to? Thanks.


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## PJ352 (Dec 5, 2007)

lblanch40 said:


> I'm pretty new to cycling and keep seeing the term aero used when discussing wheels. What exactly does this mean or refer to? Thanks.


In a nutshell...

_Semi-aerodynamic and aerodynamic wheelsets are characterized by greater rim depth, which is the radial distance between the outermost and the innermost surfaces of the rim; a triangular or pyramidal cross-section; and by fewer numbers of spokes, or no spokes at all—with blades molded of composite material supporting the rim. The spokes are also often flattened in the rotational direction to reduce wind drag. These are called bladed spokes._

Source:
Bicycle wheel - Wikipedia, the free encyclopedia

Once there, look for the headings Type - Road/ racing bicycle wheels (~3/4 way down the page).

Also, if you do an internet search on *aero bike wheels*, you'll get a number of hits. Here's one I found interesting:
Aero Wheel Advantage for Age Group Triathletes


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## Kerry Irons (Feb 25, 2002)

*Marketing*



lblanch40 said:


> I'm pretty new to cycling and keep seeing the term aero used when discussing wheels. What exactly does this mean or refer to? Thanks.


PJ gave you a good technical snapshot. The other side of the coin is "It means what the marketing department says it means."


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## Cni2i (Jun 28, 2010)

Kerry Irons said:


> PJ gave you a good technical snapshot. The other side of the coin is* "It means what the marketing department says it means."*


LOL  So true.


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## Schneiderguy (Jan 9, 2005)

My understanding is that rim depth needs to be a minimum of 38-40 mm to have any meaningful effect. And you need to be going fast to get any real benefit. I have some Rol Vigors with a 32 mm alloy rim (same as American Classic 420) and wide aero blades. In reality not very "aero" but cross winds do effect the wheels due to the wider blades. The wheels are light and very stiff and bullet proof. But the rims are heavy. A heavier wheel with lighter rim would accerate faster. My HED Jet 50s are much more aero and don't weigh much more with a lighter rim I suspect.But if you are not hammering it isn't going to matter and even then rather marginal unless you are a compeitive time trialist.


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## Kerry Irons (Feb 25, 2002)

*Discontinuities*



Schneiderguy said:


> My understanding is that rim depth needs to be a minimum of 38-40 mm to have any meaningful effect. And you need to be going fast to get any real benefit. I have some Rol Vigors with a 32 mm alloy rim (same as American Classic 420) and wide aero blades. In reality not very "aero" but cross winds do effect the wheels due to the wider blades. The wheels are light and very stiff and bullet proof. But the rims are heavy. A heavier wheel with lighter rim would accerate faster. My HED Jet 50s are much more aero and don't weigh much more with a lighter rim I suspect.But if you are not hammering it isn't going to matter and even then rather marginal unless you are a compeitive time trialist.


There are no "magic numbers." A deeper rim typically is faster than a shallower rim, but there are plenty of details that make a difference as well. The wheel doesn't suddenly get faster when you add 2 mm of rim depth. 

A slower rider actually saves more time with aero wheels than a fast rider since they are spending more time out in the wind. Again there is no magic number, no discontinuity in speed and power, when using aero components. A wheelset that will save you 0.4 mph at 25 mph (about the fastest wheels there are) will save you 0.3 mph at 20 mph.


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## skepticman (Dec 25, 2005)

Everyone should keep in mind that with a head wind, the apparent wind speed is your speed combined with the wind speed. If you're riding 15MPH into a 20-30 mph head wind, the wind is effectively hitting your wheels at 35-45 mph.

HED has a calculator to show apparent wind speed with head and cross winds.

HED Aerodynamics


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## Salsa_Lover (Jul 6, 2008)

The thing with aero wheels is: they make you faster only when you are fast enough.

If you are slow they won't have any effect. ( only the aesthetic and wallet thinning one )


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## Kerry Irons (Feb 25, 2002)

*Apparent wind*



skepticman said:


> Everyone should keep in mind that with a head wind, the apparent wind speed is your speed combined with the wind speed. If you're riding 15MPH into a 20-30 mph head wind, the wind is effectively hitting your wheels at 35-45 mph.
> 
> HED has a calculator to show apparent wind speed with head and cross winds.


And perhaps even more important to understand is that on a bicycle you are NOT riding into the wind speed that is reported by the weather folks. That speed is taken 30 ft (10 m) above ground and away from buildings and trees. While there are a lot of variables, it is not a bad estimate to assume that the actual wind you face on the bike is about 1/3 of the reported number. Otherwise a 10 mph (16 km/hr) headwind would slow you from 20 mph to 14 mph (32 to 22 km/hr) and we all know that does not happen.


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## skepticman (Dec 25, 2005)

So a 5-10 mph range measured at 30m would be about 1.67-3.33 mph at the ground and a 20-30 mph wind would be 6.67-10 mph at the ground. The difference between a 5 and 30 headwind when pedaling into it seems far larger to me than that range would indicate.

I'd also like to see sources for the 1/3 claim.


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## Guest (Nov 6, 2011)

skepticman said:


> So a 5-10 mph range measured at 30m would be about 1.67-3.33 mph at the ground and a 20-30 mph wind would be 6.67-10 mph at the ground. The difference between a 5 and 30 headwind when pedaling into it seems far larger to me than that range would indicate.
> 
> I'd also like to see sources for the 1/3 claim.


There have been cases where I've been riding in winds claimed to be 19mph steady w/ gusts of 30+ by weather people -- I was maintaining 32mph for about 20 minutes on a mostly flat road where I usually manage 20mph in non-windy conditions at the same effort. That would suggest _actual_ windspeed of more than 12mph at the ground. Considering rolling resistance, friction in the drivetrain, and other forms of drag will still be greater at 32mph compared to 20, the net headwind speed would have to be under 20mph for me to be able to cruise at 32mph. The return trip (into the headwind) was ~14mph with significantly greater effort. 

Note that the road I was riding on was at the top of a ridge with nothing to shield the wind (ie no nearby trees, buildings etc). 

I think wind on ground vs wind in the air would depend a lot on the geography. Riding in a canyon without many trees with wind blowing parallel to the walls of the canyon would probably still result in pretty fast windspeed near the ground -- maybe even some enhancement due to the "Wind tunnel" effect. Riding in the same canyon with wind perpendicular to the canyon, I'd expect to feel very little effects from the wind as the walls of the canyon would block in the wind.


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## Kerry Irons (Feb 25, 2002)

*Claims*



skepticman said:


> So a 5-10 mph range measured at 30m would be about 1.67-3.33 mph at the ground and a 20-30 mph wind would be 6.67-10 mph at the ground. The difference between a 5 and 30 headwind when pedaling into it seems far larger to me than that range would indicate.
> 
> I'd also like to see sources for the 1/3 claim.


Like I said, there are a lot of variables and I am not aware of any research on this subject. Just passing by some trees or a nearby building can shift your speed a couple of mph if the winds are strong. Also recognize that the wind speed at ground level is, by definition, zero. That said, try this on for size:

If a 10 mph headwind were really 10 mph, you would slow to a bit over 14 mph at the same power you would go 20 mph in calm conditions. Is that your experience? Mine would be more like I would lose 3 mph if I were out in the wide open on flat ground. That 2-3 mph speed loss corresponds to 3 mph of "actual" headwind. If I'm riding with trees and buildings alongside the road, my speed loss is maybe only 1-2 mph.

You can plug your own experience into thisthinking and see what you come up with.


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## Guest (Nov 7, 2011)

Kerry Irons said:


> Like I said, there are a lot of variables and I am not aware of any research on this subject. Just passing by some trees or a nearby building can shift your speed a couple of mph if the winds are strong. Also recognize that the wind speed at ground level is, by definition, zero. That said, try this on for size:
> 
> If a 10 mph headwind were really 10 mph, you would slow to a bit over 14 mph at the same power you would go 20 mph in calm conditions. Is that your experience? Mine would be more like I would lose 3 mph if I were out in the wide open on flat ground. That 2-3 mph speed loss corresponds to 3 mph of "actual" headwind. If I'm riding with trees and buildings alongside the road, my speed loss is maybe only 1-2 mph.


14mph into a headwind requiring _greater_ effort than riding 20mph normally sounds like it backs up my experience with the "19 mph+" winds still being roughly 12mph near the road surface. Note that on other roads, the 19mph nominal winds have had significantly less effect. 

Another thing which makes this difficult to compare is when I was going into the headwinds, I got into an unusally low tucked position -- similar to descending. This means I traded off a bit of power production (giving up a bit of leverage to be leaned way over with back flat) in exchange for being more aerodynamic, but because of the unusual headwinds, the power savings from being extra aerodynamics more than made up for the loss in efficiency/power production at the cranks. 

This is why I belive there's a limit to these online calculators being able to predict "you will go x speed with y power with z headwind". The calculators assume a constant drag cross-section which isn't really the case. I'm betting most cyclists going into an actual 10mph headwind, at similar effort as going 20mph will actually still go faster than 14mph by getting into a more aerodynamic position than normal.


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