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 Comparing drag of different bikes.
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Victor Ragusila
recumbent enthusiast

Canada
352 Posts

Posted - 07/30/2012 :  13:52:44  Show Profile  Reply with Quote
Hey All.

I think there is a lot of wrong info around about how to compare drag of different bikes. Most people seem to use CdA numbers. Others just look at the frontal area. A discussion was started on the topic about Obree's bike, so i would like to move it here and try to give a better explanation about how i think bikes should be compared.


Drag is measured in newtons. It is a force, lets call it D. The issues come up when the idea is to compare different designs, sometimes of very different sizes. Lets say you design a solar car. You want to know how it compares with a smaller solar car. Of course the big car has more drag, but it has a lot more power generated by the solar cells. So you compare the drag over the solar panel area (D/A_solar). The lower the number, the better your car.

For a Zeppelin, the classic measurement was the drag divided by the volume, because the volume was the desirable quantity. The bigger the volume, the more cargo the Zeppelin could carry.

For cars, the desirable quantity is interior space. Thus, the frontal area is taken as a measurement of comfort and interior space, and CdA came into use. It compares cars independent of frontal area, meaning, for a similarly desirable car, which is better aerodynamically?

For our bikes, the desirable quantity is pilot power. Unfortunately, it is very difficult to link this to a specific geometric measurement. Some riders (Sam?) can output a lot of power and be quite short, with short thighs. Others (dutch?) can output a lot power but are taller.

The question is, how do we compare bikes? that is, if the Varna is big enough to fit Sebastiaan, would it be better than Velox? This is hard to know, because Sebastiaan doesnt have the proportions Sam has, so it is very difficult to scale the bikes. A varna that fits him would look quite different than Sam's Varna.

Frontal area doesnt appear anywhere in the analysis. Should we increase or decrease frontal area? It doesnt matter. The overall drag force and pilot power is the only thing that matters. A measurement of drag/power is pretty much the speed achieved.

What does this mean? It means that comparing the aerodynamics of different bikes without looking at the riders they were built for is meaningless. Whether a Velox or a Varna is better aerodynamically than a EIVIE doesnt matter unless we know the power of the riders that can fit in it.

Where does this leave a rider trying to build a bike and to learn what is the best design? Well, try to understand how your body measurements compare with other riders. Then, if the proportions are close to other riders, one can try to compare the drag (N) over the height of the rider. Of course, if the bikes can fit the same size of riders as yourself, then go ahead and just compare the drag in (N).

I think it is a lot more useful for everyone to understand the basics of aerodynamics and how they affect the bikes, instead of concentrating on CdA , frontal area and how it is related to their bike.

cheers

Victor

Speedbiker
human power expert

USA
2465 Posts

Posted - 07/30/2012 :  15:25:14  Show Profile  Reply with Quote
Nice, Victor. But, as you have said before, compromises in egronomics figure into the equasion. Things like short(or very short) cranks, K-drive, linear drive, body position(prone?) can all affect shell design and therefore, drag. But who can say which trade-offs are worthwhile. For years Sam's more upright seating and canopy seemed a good compromise between aero and power. Then Damjan builds a bike that seems to have many risky compromises, yet it clearly works. The Delft team builds a big bike for a vig motor and it works very well. Now Mr Obree has a design that has innovation and it's compromises. With no easily available windtunnel and power lab available, about all we can do is go to BM and see who guessed the best.
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Adam C
recumbent enthusiast

USA
227 Posts

Posted - 07/31/2012 :  11:53:02  Show Profile  Visit Adam C's Homepage  Reply with Quote
Good post Victor. I brought up this topic somewhat tangently in a thread I started about recumbent vs semi-recumbent riding position. I studied the design of the Cheetah fairing for some time. I dug up the detailed article that was originally printed in Mechanical Engineering magazine. There was a section of that article that peaked my interest.

quote:

The controversial approach the team took in developing the shape of Cheetah's fairing was to reduce drag and increase its aerodynamic design...

"Though we didn't have the time or money to test fairing shapes in a wind tunnel, we did work with Michael Selig, now an assistant professor of aeronautics and astronautics at the University of Illinois (Urbana-Champaign). Selig has become known for his work in low-speed aerodynamics", Frantz said. Selig had been obtaining good results developing airfoils for California radio-controlled glider enthusiasts preparing for world competition, which is why Frantz asked him to get involved with the Cheetah project...

Frantz said, "Under Selig's influence, we decided to optimize the fairing aerodynamics rather than minimize its frontal area"...


Franz seems to imply that optimizing fairing aerodynamics is not necessarily tied to minimizing frontal area. At least that what I took from this quote.

I then posted some quotes from Rob English commenting on his Mango HPV. A few posters responded by saying that the semi-recumbent position is inferior to the recumbent position and that minimizing frontal area is a crucial factor in going fast.

Edited by - Adam C on 07/31/2012 11:54:18
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Larry Lem
human power expert

South Sandwich Islands
2300 Posts

Posted - 07/31/2012 :  12:36:00  Show Profile  Reply with Quote
"I then posted some quotes from Rob English commenting on his Mango HPV. A few posters responded by saying that the semi-recumbent position is inferior to the recumbent position and that minimizing frontal area is a crucial factor in going fast."

Can you dig that old thread up? Then we can see who the posters were, see their entire statements, then see if they made any valid points (before we simply judge these comments as stupid or incomplete).

Larry Lem
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Adam C
recumbent enthusiast

USA
227 Posts

Posted - 07/31/2012 :  12:48:44  Show Profile  Visit Adam C's Homepage  Reply with Quote
quote:
Originally posted by Larry Lem

"I then posted some quotes from Rob English commenting on his Mango HPV. A few posters responded by saying that the semi-recumbent position is inferior to the recumbent position and that minimizing frontal area is a crucial factor in going fast."

Can you dig that old thread up? Then we can see who the posters were, see their entire statements, then see if they made any valid points (before we simply judge these comments as stupid or incomplete).

Larry Lem



Here you go...

http://www.recumbents.com/forums/topic.asp?TOPIC_ID=4331

Even Jon Garbarino chimed in as well.

Edited by - Adam C on 07/31/2012 12:49:31
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Grant-53
New Member

USA
51 Posts

Posted - 08/06/2012 :  16:52:03  Show Profile  Reply with Quote
It looks a little confusing at first but there is a reason why CdA is used. The Cd is a measure of how efficient a body is in terms of aero drag. The max cross sectional area is the size of the hole the body pushes through the air. The third factor is the velocity that must be specified to determine power requirements. Atmospheric conditions have some effect. Weight affects rolling resistance.
A rider's power to weight ratio is a consideration and VOx is often measured in relation to output. Body type and proportions affect optimum rider position, rpm, and crank length. To determine an optimum rider position one would need to compare a number of riders of varying body types.
To compare design features then the critical measurements are Cd and weight. These reflect the fundamental efficiency of the design.
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Steven Challenge
recumbent enthusiast

Netherlands
142 Posts

Posted - 08/07/2012 :  07:00:11  Show Profile  Reply with Quote
With my limited knowledge (mechanical engineering and watching discovery channel) I'd like to give my point of view.
If I would have to design and build a recumbent for a specific person, I'd find out first what this person can do. What are his/her dimensions, strenght, muscle tone, weight, fitness, weaknesses. With this information you could create the perfect ergonomic position for the rider. Keep in mind the use of the bike (urban, long distance, hills, wind conditions) as far as that goes. Test this position in a wind tunnel and ajust if wanted.
With this body position, I would fit in the main components, wheels, steering, crank, gears. Test this configuration in a wind tunnel and ajust.
Next, fill in the blancs with the frame. Test this again in a wind tunnel and ajust. Now back to the rider. Train him/her to use the bike to the fullest. When to apply force and when to freewheel. Optimising the riding position and muscle tone.
From what's written here, there are about two dozen variables that makes a perfect bike. And there are probably a couple dozen more variables that I haven't mentioned that would infuence the overall drag.
To compare the drag of recumbents could be done, but you would be missing the point. There is more to a fast bike than just drag. You could compare recumbents by configuration, wheels, weight, gears etcetera, but you'd be missing the point again. I think, the only way to compare bikes is to ride it yourself and experience for yourself. Get on the bike and ride it. Scientifically, in a wind tunnel or just by feel.
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Grant-53
New Member

USA
51 Posts

Posted - 08/07/2012 :  12:07:14  Show Profile  Reply with Quote
There is much information in the book "High-Tech Cycling" by Edmund Burke even if it primarily concerned aimed for upright racing. As Lennard Zinn points out, a bike must be comfortable to allow best output from the rider. This is one reason The Bicycleman, Pete Stull, encourages people to test ride several different recumbents before making a selection. Many of us have to substitute coast down testing for wind tunnel time and get a chase rider to video tuft tests. There is some benefit to the practice of design evaluation before starting a design project if only to avoid reinventing something that has already been tested. There are considerations that involve lift and crosswind resistance. Ignoring minor variables may make a significant difference at high levels of competion.
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Speedbiker
human power expert

USA
2465 Posts

Posted - 08/07/2012 :  14:21:22  Show Profile  Reply with Quote
Zzzzzzzzzzzz........
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sean costin
human power supergeek

Lesotho
1978 Posts

Posted - 08/08/2012 :  20:06:59  Show Profile  Reply with Quote
I thought there was an AP on my I phone for calculating drag and doing CFD on objects you take pictures of.

It seems that sometimes all this analysis mostly tells us what we already know. Sure I understand the tweaking of elements, but if you are 95% there already, then how hard is it to do some testing.
Probably easier than doing the CFD.

Grouchy old man.

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Speedbiker
human power expert

USA
2465 Posts

Posted - 08/09/2012 :  07:32:44  Show Profile  Reply with Quote
The problem with quoting old data is that much of it is incorrect. It is an assumption, not based on comparitive testing. I know of no hpv designer who can prove at what point being in a less supine pedal position allows you to produce enough more power to offset the aerodynamic drag of a larger body. And saying you are going to optimize aerodtnamics to compensate for the larger body is ridiculous because the guy with the smaller body is also optimized. The only team I see testing at this level is Delft. No surprise they won BM last year. They chose a larger vehicle for a bigger motor, not a less supine rider position. You can easily assume they ran CFD tests on many variations and decided using the biggest engine in the smallest envelope was best. Our team also did CFD tests and it was easy to see that a less supine rider was a very poor idea. We chose the rider size based on our rider, Sean Costin, world recumbent sprint records holder. But, even if you have CFD, it is only giving you a better guess at where to start. The approach that is likely to advance the sport the most is one like the Delft Team. That being the ability to do true, extensive testing, while building new models yearly. Only then can you see what works, and why. Then attempt to build something new and better. But, if you want to design and build an hpv based on old, likely incorrect data, have at it. If you are up to date on present hvp design, I don't believe there is anything to be learned from old data.
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