Road Bike Aerodynamics

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climbandpunishment
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Joined: Fri Mar 20, 2015 4:40 am

by climbandpunishment

Hi all! A friend and I are designing an aerodynamic road bike, and in the thread describing the project over in the Introduction/Gallery forum someone asked a particularly good question: if I want an aerodynamic bike, how can I, as a consumer, choose one without a supercomputer or wind tunnel?

As an aerodynamicist by trade, I thought I'd try to put together a set of guidelines that would make it easier to eyeball-judge a given bike and its design methods, and to evaluate how much truth there is to a given company's claims. I obviously can't write a set of rules that will always be unimpeachable for every frame, but I figured if I can help people identify the best few frames that likely really do live up to marketing claims, it'll benefit the industry through competition and therefore me when I look to buy a new bike.

I need help to make sure the guide is useful for its purpose, and therefore:
1) accessible to as many people as possible but also informative
2) not chock-full of errors, both linguistically and technically

Below is a link to the guide in its current (rough) form. I would very much appreciate your guys' feedback.
https://www.dropbox.com/s/30qwz14oi2jrnzr/Easy%20Guide%20to%20Bike%20Aerodynamics%201.1.pdf?dl=0

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kgt
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by kgt

I was the one to ask. Many thanks!

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H0RSE
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by H0RSE

What you have there are a few elementary and unfinshed principles of fluid dynamics. Unless you're trying to educate someone who is uniniated in the dynamics of fluid flow you're barely helping them understand what the corporations are putting out in their products. Basically you have to take the product that companies produce at face value. Even if you understand fluid dynamics producing the data of flows around the product is yet another step thereafter after which point you determine if the company is lying or not. Short answer is face value.

seaneT1
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Location: Thessaloniki, Greece

by seaneT1

it seems helpfull for someone who enters the world of aerodynamics in bikes for the first time...it will help him know some basic things like what to avoid and what to look for in general.Great job, keep :up:

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Kayrehn
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by Kayrehn

Great writeup especially for non-engineers like myself! With Trek and Specialized now hiding their front brakes, do you figure the watts savings significant, since conventional front brakes have arms that sticks out of the frame area?

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climbandpunishment
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Joined: Fri Mar 20, 2015 4:40 am

by climbandpunishment

Thank guys! Glad it's helpful, and thanks kgt for asking the question.

@Kayrehn: That's a good point; I'd left out specific component breakdown numbers from this guide because I wanted anything in the guide to be based on my own testing, and the testing I've done so far for integration was a bit preliminary. I didn't find huge benefits compared to other changes, so didn't test further. Hiding front brakes may get you a few watts if done very, very well, compared to a normal brake. It's really up to you whether you feel that's significant enough to be worthwhile or not. Note that all that assumes that you don't sacrifice aerodynamics elsewhere in order to hide those brakes, which is a possibility on the new integrated bikes you'd mentioned.

Maybe I can add some more concrete component drag breakdowns to the guide, prefaced by saying that they're based on very preliminary testing and will differ significantly between bikes. Cervelo's numbers seem to mirror them within a decent margin, so they're most likely reasonable.

@H0RSE: Is there a way that you'd suggest I make the guide more in-depth? Keep in mind I'd like to maintain its accessibility.

sawyer
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Location: Natovi Landing

by sawyer

Keep it short and accessible, otherwise it won't be useful

For example ...

Wheels
- Deeper rims
- Fewer and thinner spokes
- Wider U shaped rims
- Front wheel is more important than rear
- Smooth tyre to rim transition with no "lightbulb" shaping of tyres
----------------------------------------
Stiff, Light, Aero - Pick Three!! :thumbup:

davidalone
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Joined: Fri Feb 10, 2012 4:27 pm

by davidalone

As a fellow engineer, I have to say, great work!

something you may want to add in: Theres alot of marketing spiel about watts saved at x km/h, time saved etc, when real engineers talk in terms of CdA. maybe provide a lowdown so your layman can cut through all this and understand HOW relevant it is WHEN it is relevant ( i.e. at what yaw?)
maybe give some rough guides, like 0.1 CdA= x watts saved etc.

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H0RSE
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Location: Mosad

by H0RSE

Image






davidalone wrote:As a fellow engineer, I have to say, great work!

something you may want to add in: Theres alot of marketing spiel about watts saved at x km/h, time saved etc, when real engineers talk in terms of CdA. maybe provide a lowdown so your layman can cut through all this and understand HOW relevant it is WHEN it is relevant ( i.e. at what yaw?)
maybe give some rough guides, like 0.1 CdA= x watts saved etc.

Nicos
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by Nicos

Awesome I was really looking forward to this guide, saving it for later

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H0RSE
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by H0RSE

You won't lean any aerodynamics here. It's a load of waffle.

climbandpunishment
Posts: 62
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by climbandpunishment

Thanks david and sawyer, great points and extremely helpful! I'll work on shortening stuff up wherever possible. I tend to talk in watts just because most bike people seem to think that way, but it's a huge oversight not to at least qualify it with a "watts @ x mph/kph" or better yet add some conversion guidelines in. I'll try and work on adding bits about when and how specific things are relevant too; yaw and crosswinds are definitely lacking in this version. Thanks for pointing those things out, and I'll work on fixing them as soon as I'm able. I really appreciate your guys' help!

Also, I know the wheel list you gave was just an example, but speaking of that, I will probably hold off on discussing wheels too much because I don't feel qualified doing so; testing wheels to the accuracy where I'd be comfortable making any sort of definitive statement is beyond my resources at the moment.

wingguy
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Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

climbandpunishment wrote:As an aerodynamicist by trade, I thought I'd try to put together a set of guidelines that would make it easier to eyeball-judge a given bike and its design methods, and to evaluate how much truth there is to a given company's claims.


Ok... can you give me a few examples of specifically designed and marketed aero bikes that are not very good based on your eyeball estimate?

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Kayrehn
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Joined: Thu Mar 24, 2011 6:06 pm

by Kayrehn

wingguy wrote:
Ok... can you give me a few examples of specifically designed and marketed aero bikes that are not very good based on your eyeball estimate?


I have a feeling he deliberately avoids mentioning any particular frames at all to illustrate his points, perhaps to protect himself from being taken to legal action (improbable but not impossible). For example when he mentions surface treatment and tripwire features certain frames pops up in my mind immediately, which I have no doubts he'll be aware of.

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DanW
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by DanW

Keep it short and accessible, otherwise it won't be useful


Agreed.

Engineers are usually good at writing at length. Some call this waffle but I am an Engineer so I wouldn't dream of using this word :D

I agree with Sawyer that the guide needs a bulletpointed list of "rules of thumb", probably in the summary.

It is an interesting guide but I think it does not need to be written from the perceptive of disproving manufacturer claims- bulletpointed, general rules of thumb would also give you more freedom to not have to dance around the points you are trying to make and could be much clearer to the reader. For example, rather than say "be skeptical of manufacturers claiming big aero gains from integrated components", you could just leave it as "intergrated components typically account for an insignificant amount of the bikes total aerodynamic performance" or similar.

General rules of thumb from your own testing can not be offensive to manufacturers and would let you get the information across much better

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