Triathlon Aerodynamics

So, I have been looking through google endlessly from some real number on aerodynamics. I have seen people saying things like "Well, I am not aeronautical engineer but..." or making statements like drag is reduced by 10% when... (which is completely untrue)over and over. The fact is, I am currently a senior in aeronautical engineer, which means very little money do to strained time, meaning I want to buy the most bang for my buck as far as tri equipment goes, but mostly I understand all of the jargon. Because of this I was wondering if anyone could give me actual numbers on aero equipment. For example, things like the drag coefficients for different road helmets, aero helmets, road/tri bikes. Just a legit web site would do as well. I am not too interested in weights of things due to the lack of a need for acceleration in tris, plus I only weigh 140.

Thanks for any help,
Alex Rowe

 

Hope some of the guys post links. Excellent topic. I know I've come across a few sites that have actual numbers you're looking for. I know Ive seen it. Wind tunnel tests etc......

 

You probably won't find drag coefficients for individual pieces of equipment, except possibly bikes and wheels, and if you did they would be as misleading as all the marketing figures that are readily available. The rider's body and all the other parts in the system affect the airflow over each component, meaning that any drag values would be drastically different when tested separately vs. ridden in competition. Beyond that, the position of the rider also greatly affects how the air moves over something like a helmet or aero bars, so a helmet which tests really well in an equipment comparison may not be the best choice for a given rider's position.

If you really want to see drag coefficients for bikes or wheels, you can usually back them out of the wind tunnel test data by reading the fine print about the test speed and assumed riding speeds that the "wattage savings" or "time savings" are based upon.

Most of the serious aero folks here, on the Slowtwitch forums, or on the Wattage Google group do their own aero testing to tweak their position and equipment choices for the reasons described above. If you train with a powermeter there are some pretty effective ways to test one's aggregate CdA &Crr without paying for a wind tunnel session. Tweak one aspect at a time between and that's how we do our aero comparisons.

Bottomline: I'd still be cautious about making equipment choices based on Cd data that you find, because they may not be applicable to your body shape and position.

Here are some threads you might find helpful / interesting:

http://www.cyclingforums.com/power-training/467741-aero-drag-power.html
http://www.cyclingforums.com/cycling-training/472961-tt-bike-fit-progress.html
http://www.trainwithpower.net/

 

I do understand that the Cd of something like a helmet does change, but most peoples heads are not too differently shaped. Also people always give drag coefficients in CdA. Because something like a helmet design has a fixed A there is nothing you can do about that. Having a power meter would be great, and I could figure out everything I need to know based on that, but just an average power meter cost nearly as much as my bike. I am just trying to figure out for myself something like how much an aero helmet helps compared to a regular or even how much regular helmets vary. I wish I could care more about bike aerodynamics, but I doubt that will happen soon. I would still like to know how different tri bikes and road bikes compare though, just for kicks.
p.s. Thank you for those links. That is getting much warmer to what I am interested in.

 

Equipment will help you to mitigate overall drag BUT your position on the bike will possibly have as great-or-greater of an impact until you actually optimize your frontal displacement ...

Several years ago, I did a comparison based on simply moving my hands from the hoods to the crook in the drops where I reckon the actual change in my body 'height' didn't change by more than 2" since the difference in positions is more for hand relief ... if the difference is actually more-or-less, then it's whatever it is.

At altitude (6000+ feet) on a False Flat where the coasting speed was greater than 12 MPH, the difference in speed was about 2+ MPH for the different hand positions (and, subsequent change in body position).

On a steeper descent where the coasting speed was greater than 30 MPH (actually, closer to 35 MPH), the difference was approximately the same (i.e., 2.n MPH).

At-or-closer to Sea Level, the difference in coasting speed would probably be greater for the two positions as the speed of the rider increased ...
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In other words, you could be wearing all of the latest-and-greatest aero gear + be riding on a TdF level TT bike, but if you were sitting taller than someone else who is more aero on his/her bike, then you might nonetheless need to expend more watts to maintain a given MPH.

I reckon that Lance Armstrong's less-than-ideal TT position (i.e., Armstrong's slightly 'hunched' back appears to raise his frontal displacement by at least 2" more than it would be if he could 'flatten' his back the way other top tier riders do) finally caught up with him at last year's TdF -- undoubtedly, that is one reason why he spent a portion of this past year working toward improving his aero positioning.

Presumably, diminutive Leipheimer (who has a pretty good aero position) can go incrementally faster for a given wattage output than someone whose frontal displacement is greater.

So, as a suggestion, if the stem on your Road bike is currently barely lower than your saddle, then you probably want to set it so that it is at least 4" lower than the top of the saddle to help you acclimate to a more aero position ... then, when that is a comfortable riding position, get a TT bike where your back position is as close to flat as you can manage ...

That's a very circuitous way of vaguely inferring that some aero gear will undoubtedly help an individual to reduce their frontal displacement (you know, a skin suit is better than wearing a blousey T-shirt) & drag, but if they are not riding in an aero position then the benefit may be smaller than for someone else.

Hope that helps a bit.

 

Just to be clear, it's not the shape of the head that I was referring to. Different riders hold their heads at different angles while riding, which means the airstream is hitting the helmet differently and the tail of the helmet is laying differently along the rider's back. Also, the position of the hands relative to the face will direct the air into or around the helmet differently. Basically, the aero characteristics of the helmet in isolation are less important than finding one that fits your riding position well, an unfortunate reality that the textbooks sometimes neglect to mention.

For example, my neck isn't very flexible in my TT position, so I tend to let my head hang down and periodically look out the tops of my eyes to clear the road ahead. In studying the shapes of various helmets available to me I noticed that tail on LG helmets are much lower relative to the brow of the helmet than the Giro helmets that many use. I got an LG helmet since it lays against my back in a low-head position, instead of a Giro which doesn't.

Well, I think you're right to take anything you read online with a grain of salt, as your mileage *will* vary, but if you're just looking for entertainment.... here's some more.

If you've already scoured the web then you might have seen these:

Specialized Bicycle Components : S-Works Transition
(click on the More Info tab, then read "Transition Aero Whitepaper")
How Aero Is Aero? - BikeRadar
Rider Aerodynamics
Performance and Wheels Concepts
The Zipp site has lots of drag comparisons for wheels, but I can't seem to get the link right now.

 

Well, I think you guys are kind of missing my point. I realize that the rider has a significant effect on aerodynamics. My handle bars are already very low for a road bike, and I have clip on aero bars. So other than that, it would be hard to definitively say what is the best position is for me to ride in. That means rider position is out because I do the best I can. I am mainly trying to determine what to do about a helmet. My current helmet is a hand me down from my dad with a visor that one of my friends kindly put "Yeah, it does kind of look like you are about to go on a family bike ride". So, I am looking for a new one. In this process I am trying to determine if I should just tough it out and get an aero helmet for races only or if I should just get a nice road helmet, and does a nice road helmet even matter! That is kind of my main dilemma; however, I thought it might be more useful for raw number and I could do the calculations myself. Of course, maybe just flat out asking would work too .

 

Between your first post and this one, I might have been missing what you're looking for. I thought you were trying to definitively compare one aero helmet vs others, which would be highly rider-position dependent. The Bike Radar link I provided will give you a representative CdA difference between a rider with road helmet vs. aero helmet. Going from a .267 CdA to .256 will add about .3 mph at 250w. Depending upon the length of your rides, the time difference there is not too shabby.

Visors can sometimes be removed from road helmets (many times they just unsnap from the shell), which will improve the look if you wanted to continue using it for training. If you're competitive then I would definitely look for an aero helmet for racing, and that would be my recommendation if you can only afford one or the other (road or aero).

 

BTW, on www.trainwithpower.net the applicable links are at the bottom of the page in the Tech section.

 

Ok thank you. The Cds of the two different types of helmets is what I was looking for. But, is there any significant difference in Cds of various road helmets? Those seem to be harder to find than the Cds of aeros. Also, you can not think in a saying there is a .3mph difference at 250 watts. Watts and velocity can unrelated, unless there is no wind, incline, and constant friction. As velocity increases, drag goes up by a power of two. The numbers, however, were what I was looking for, as I have already said I can not get in a wind tunnel, nor do I have a power meter. Unless I can find a road helmet with a low Cd you may just have convinced me to get an aero helmet though.

 

I just now read "How aero is aero" that is exactly what I needed. Thank you.

 

Road helmets are usually tested/marketed on weight, comfort, and cooling rather than aero. Any differences in road helmet aero would probably come at the expense of cooling to the head.

Careful now..... how can velocity and power be unrelated?

....and the propulsive power required is drag (force) times velocity again, so it goes up by a power of three.

The reason I said .3 mph at 250w is because the amount of speed *difference* generated (between the 2 equipment scenarios) is dependent upon the speed being travelled since faster objects benefit more from streamlining than slower objects. You can do the math yourself or use a modelling website like www.analyticcycling.com to see what I'm talking about.

 

When I said it is unrelated I didn't actually mean just that. I meant that you may have to generate 250W to go 10 mph is the hill is steep enough. You can only talk about power to speed increase on flat vacuums. That is what I mean. The reason I do not like saying it goes up by .3 mph is because, exactly like you said, it is a function of speed. So, instead that .3 would vary based on speed. But anyway, I have all of the info I need to make my decision now. I am currently in the process of calculating calorie savings based on equipment in mathcad if you are interested when I am through.

 

Ah... ok. The .3 at 250w was based upon a flat paved road of average smoothness (coefficient of rolling resistance = .006), at sea level (air density 1.226), 73 kg rider + bike weight, CdA = .267 vs. .256. IOW, .3 mph is the actual gain in speed for a typical 250w cyclist on road bike + clip-on aero bars on flat asphalt -- 23.8 --> 24.1 mph.

There are actually some pretty robust modelling tools out there for cycling geeks so we don't have to assume that we're riding in a vacuum, on frictionless surfaces, or similar nonsense. If your values are drastically different than those used above, then you can easily predict your actual speed gain using the tools at www.analyticcycling.com

Also, I trust that you caught that the numbers in that aero article were Cd*A values for the rider with helmet, as opposed to Cd values for the different helmets.

Good luck, and definitely share your research findings when they're finished.

 

I dunno if this is helpul, but....

This test was done by the German Tour Magazine. There's a pdf full version of the article somewhere on the net, but it's in German. It was done on a velodrome, but I'm not sure if it was indoor or outdoor. As you can see, they made note of how the required power to hold 45km/h reduced as the rider got more aero, bit by bit. His aero helmet cut 3w of drag



They first put Uwe Peschel on a 'normal' road bike

Required output (watts) to sustain 45kph.

'Standard' road bike, 32-spoke wheels, hands on hoods: 465 Watts
Same bike, hands down on the drops: 406 watts
Same bike, Easton Aeroforce aero bars: 369 Watts
Same bike Triathlon position (5.5 cm lower bar, saddle forwards): 360 Watts
Same as above, with 2 carbon Tri-spoke wheels: 345 Watts

Cervelo TT bike + tri-spoke wheels: 328 Watts
Cervelo TT bike + tri-spoke front + disk rear wheel : 320 w
Same as above with Giro aero helmet: 317
Same as above with speed suit: 307