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Going low and frontal area
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I am trying to understand how going lower impacts frontal aero

When the snow melts I will do this with field testing, but I am trying to understand the math behind frontal area, especially how much would I save for every cm drop, assuming I am starting from a position where my head is way above my back. I understand that there is a low point where you will get diminish returns.

In this article (http://www.endurancecorner.com/Alan_Couzens/bike_fit_4) the author claims “For an average 6ft tall cyclist, moving from a 73 degree seat angle to an 80 degree seat angle will decrease the frontal area by ~.03m^2,”. This is about a 10cm drop according to him. So let's say .003m^2 per cm.

I then put this .03m^2 in a power calculator and it says that if I reduce by .03m^2 I could save 15watts.
15 watts, 10cm, about 1.5 to 2watts per cm drop

Does this make sense ?

If I take the widest point of my shoulders when crouched/compacted is say 30cm * 1cm lower = .003m^2. Does this make sense?
Last edited by: marcag: Jan 20, 12 7:19
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Re: Going low and frontal area [marcag] [ In reply to ]
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marcag wrote:

I am trying to understand how going lower impacts frontal aero

When the snow melts I will do this with field testing, but I am trying to understand the math behind frontal area, especially how much would I save for every cm drop, assuming I am starting from a position where my head is way above my back. I understand that there is a low point where you will get diminish returns.

In this article (http://www.endurancecorner.com/Alan_Couzens/bike_fit_4) the author claims “For an average 6ft tall cyclist, moving from a 73 degree seat angle to an 80 degree seat angle will decrease the frontal area by ~.03m^2,”. This is about a 10cm drop according to him. So let's say .003m^2 per cm.

I then put this .03m^2 in a power calculator and it says that if I reduce by .03m^2 I could save 15watts.
15 watts, 10cm, about 1.5 to 2watts per cm drop

Does this make sense ?

If I take the widest point of my shoulders when crouched/compacted is say 30cm * 1cm lower = .003m^2. Does this make sense?


Along with feild testing you can do some quick dirty work with a camera and a tripod set up about 20 feet in front of your bike on a trainer. Mount the camera lens at your chest level when you are in the aero position. Take the photo against a while wall behind you. Photoshop the image into black and white and "sort" the pixels. You could even baseline by taking a photo of the bike w/o the rider as a tare. It is not too difficult and cheaper than a wind tunnel.

In some cases though a reduced FA does not yeild a significant change in drag; that's where your feild tests will help. A curved back may be faster than a position with a perfectly flat back. The right helment shape is critical, too. Trying to find the tipping point and ~1.5 - 2 watts per position change isn't going to be easy without dozens of trials (and equipment options).

-SD

https://www.kickstarter.com/...bike-for-the-new-era
Last edited by: SuperDave: Jan 20, 12 14:46
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Re: Going low and frontal area [SuperDave] [ In reply to ]
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Thanks !!!

I actually tried the video cam thing. I have a my Computrainer PC in front of me and it happens to have a webcam. Videod a few arm positions and use ImajeJ to calculate various areas although I didn't play with up/down, just my arm position. Is was just for hoots. I know the real adjustments can only be done with field testing.

But do the calculations in my original post make sense ? Only 15 watts for a 10cm drop ?
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Re: Going low and frontal area [marcag] [ In reply to ]
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i don't have an answer for you other than to say, it is going to depend greatly upon your initial speed
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Re: Going low and frontal area [marcag] [ In reply to ]
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marcag wrote:
Thanks !!!

I actually tried the video cam thing. I have a my Computrainer PC in front of me and it happens to have a webcam. Videod a few arm positions and use ImajeJ to calculate various areas although I didn't play with up/down, just my arm position. Is was just for hoots. I know the real adjustments can only be done with field testing.

But do the calculations in my original post make sense ? Only 15 watts for a 10cm drop ?
Hi marcag,

Dave is right, a camera and power meter can help with time and patience.

In general though, you're calcs agree with my experience. Based on 14 runs in the wind tunnel where drop was the only position change, I get (drawing through the data points with a BIG crayon!) about 20 g/cm.

Lots of scatter in the data and lots of caveats in the protocol, so I'd say your 1.5 W/cm are roughly "in agreement" for now.

Cheers,

Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)
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Re: Going low and frontal area [damon_rinard] [ In reply to ]
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Cool !!!! I am glad you confirmed this. I had this wild idea (optimism) it would be significantly more. I have been slowing moving down over the winter on the trainer.

Well I have convinced myself I need a P5 to get 100g of drag (?) less than my P2. Am I way off ?

I plan to spring this on the wife by saying I am doing this to spend less time on the bike and more quality time with her.

Then I'll just spend time on the bike and get more FTP and less drag. Life is so simple.
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Re: Going low and frontal area [marcag] [ In reply to ]
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Hi Marcag, its not just about getting low.Some riders can be more powerful riding high. Its also about reducing your shoulder width. The best way of doing this is to scrunch your head into your shoulder, which will reduce your frontal area. Get it right and hold for a long period of time you could save a good few watts. Try it in front of your video camera, you will see the difference!
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Re: Going low and frontal area [matthewbottrill] [ In reply to ]
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Ya ! I have been trying to figure this out and sure enough the scrunched head yields the narrowest shoulders.

I just need to use a "pocket door", open it to allow 30cm through and find the optimal way to squeeze through. Sure enough it's with the head scrunched into shoulders and the shoulders rolled slightly forward. I can reduce only reduce my shoulder width 2 or 3 cm. But I suspect this will yield a small frontal saving. But every watt counts.

I will focus on finding a compact position I can hold for long rides.

I have been trying to understand what are the theoretical gains that could be made and there seems like upping your FTP is the biggest bang for the buck.
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Re: Going low and frontal area [marcag] [ In reply to ]
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Have you had a bike fit? Scrunching your shoulders and holding the correct way could save you a massive amount over a long time trial.
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Re: Going low and frontal area [matthewbottrill] [ In reply to ]
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I had two bike fits. The first I was very uncomfortable. The second guy (very reputable) put me very far back, very high in front but comfortable. Neither fitter spent much time on shoulder width.

I had a terrible cda, doing about 41km/h on 305 watts in a 17km TT. So I have been trying to improve the position while doing my winter workouts.

I have been moving forward, downward in front preserving all the same angles, just rotating around the BB. I try to see how it impacts my ability to generate power. There are some key workouts I can use to compare difficulty in generating power in a new position. I do a long ride every week (on the trainer) to make sure I stay comfortable.

But the original intent of the thread was to understand the theory behind frontal aera and improving cda so I can make better decisions as I experiment. I don't think I can go much lower, I need to now go narrower, maintain comfort and keep on upping my ftp.

You guys have been a great help
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Re: Going low and frontal area [marcag] [ In reply to ]
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Looking at the article and your posts am I interpreting this correctly that very generally speaking a 300 square cm reduction in frontal area translates to a 15 watts improvement? In the article it translates this to 7 minutes in an IM bike leg at 200W.


Basically this is 1/2 of a piece of 8.5x11 paper in reduction in frontal area.

If this is true it is not clear to me if we are talking at what speed. I assume it is the typical 30mph that is most common. I do realize a reduction in size of one shape versus another in the wind is going to cause different results. Therefore a 300cm^2 reduction in an aero helmet (aero shape) would most likely get you less of an improvement than a 300cm^2 reduction is say your chest (flat and non-aero).






Last edited by: bartturner: Jan 21, 12 8:25
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Re: Going low and frontal area [marcag] [ In reply to ]
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So let's go with the current massively-approximate assumption that a 1 cm drop saves you 20 grams.

Then let's say I drop my bars 4 cm, which is quite a lot (assuming I'm not starting out sitting straight up) to save 80 grams.

That's still less than I'd save by upgrading my P2C to a P4. And it could possibly be that much more again by upgrading from the P4 to the P5 (we'll find out soon!).

If I'm getting all this correct, than I call BS on all the people who chime in to say "Body position matters is 20 times more important than the bike anyway"... or similar such sentiments. Assuming you start with semi-reasonable position, and you own a bike like the P2 (which is one of the best of the non-"super bikes"), then the remaining improvements from fit are on the same order as buying a new bike.

Cervelo fanboi's unite!
Last edited by: matto: Jan 23, 12 18:30
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Re: Going low and frontal area [matto] [ In reply to ]
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matto wrote:
So let's go with the current massively-approximate assumption that a 1 cm drop saves you 20 grams.

Then let's say I drop my bars 4 cm, which is quite a lot (assuming I'm not starting out sitting straight up) to save 80 grams.

That's still less than I'd save by upgrading my P2C to a P4. And it could possibly be that much more again by upgrading from the P4 to the P5 (we'll find out soon!).

If I'm getting all this correct, than I call BS on all the people who chime in to say "Body position matters is 20 times more important than the bike anyway"... or similar such sentiments. Assuming you start with semi-reasonable position, and you own a bike like the P2 (which is one of the best of the non-"super bikes"), then the remaining improvements from fit are on the same order as buying a new bike.

Cervelo fanboi's unite!

This is exactly why I wanted to understand the true savings and am really glad a guy like Damon confirmed it.

Base on the hoopla some make about going low, I was expecting much bigger savings. But hey, every gram is worth while but I will not compromise ability to produce power to get it.

Also remember. You can spend $10k on that 100g of drag, put a pile of junk on the back and negate it all. Done that. Or you can use the wrong bottle and lose 1/2 the benefit. Done that. Or sit up a few times too many. There goes your $10k of drag savings.

Those 10g here and there add up to a lot.

As far as going down 4cm, it's actually not that hard if you are starting from a bad position. I have rotated myself around my BB, going forward and down, taking baby steps and confirming I am not losing power on the way. I am 7cm lower than when I started and have not closed my hip angle at all. But I am getting to the end of that optimization.

I am doing this slowly, while doing my workouts, because as many tweaks as I can find, I believe the real solution is upping my fitness/FTP and making minimal if any compromises on power for position.
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