A straight sided fairing ?
 

A question for those who know such things. I've been thinking yet again about the ill effects of a tapered fairing... that lovely classic shape which has the unfortunate effect of flicking you around if the wind off a truck or around a building hits unexpectedly.

How about the flat sided fairing, like a pyramid , wider at the front but with the leading edge low tapering up but the maximum width at that point then coming back to a reduced dimension at the extreme back?

This is rather hard to explain but my scanner is dead, a drawing would be a better way of explaining.

This photo of the Dome racing car was the inspiration for the idea. Flat sides ,thus no lift !

No side lift is what I'm after.

http://i1062.photobucket.com/albums/...ed/Dome%20.jpg

The vertical fin area gives a surface for the wind to push from the side. To minimize lift and reduce side wind load we use a curved body. The cross section is like an inverted egg with the larger radius at the top.

I understand the theory Grant but that egg shaped side generates side lift that can be deadly. The flat side would have the same surface area for the win to push against but would not generate side lift if the wind is off to the side.

Side lift? referring to the reason nascar, and land speed record cars use roof rails? adding side skirts might be easier to do, help just as much, and have less chance for unintended effects.

Edit: nevermind, I'm a dumbass stuck in 4wheel mode

What would the side skirts do mc ?

Side force elimination
 

Hi Peter, I'be followed your previous postings about motorcycle body designs and I get where you are coming from
Unfortunately there is no easy answer to your quest (I've been looking for it myself) because of the complex nature of "side lift".

Lift has two components, Newtonian and Bernoullian, and you are searching for a means of eliminating the side force ONLY using Bernoulli's principle. Even if you were able to do this ( as your question implies) it would still leave the Newtonian component, which as it applies to motorcycles is the greater of the two forces.
A useful explanation can be found here http://youtu.be/zUehWUoiPHQ

Your efforts to reduce sidewind sensitivity should be aimed at reducing the "sail area",and lowering the Centre of Pressure (ie height above ground). This will reduce the turning moment around the tyre contact patches and reduce the lean angle, thus aiding stability.
One of the central problems with traditional motorcycle design (ie those that keep the normal hands first riding position) is that a high centre of gravity also implies a high centre of pressure
Attempts to enclose the upper body area of a motorcyclist will inevitably result in a large "sail area", high above the contact patches, and therefore a large overturning moment.
To make any real progress in this area you will need to consider recumbent riding positions or active aerodynamics.

Thank you , a great reply .

It seems to me that flat sides would negate side lift almost entirely, that's the Bernoullian component dealt with or am I missing something ?

As you say the Newtonian component is the problem that remains and feet forward seems the obvious response. I think a Honda CT110 would make an ideal basis for such a bike, for me at least. There is a bloke who posted an excellent thread on his Honda 90 version of feet forward and I've been thinking about it ever since . This basic design should work but if I add a Vetter style rear section I still have the curved life areas .

This bike.

http://i1062.photobucket.com/albums/...20P3100450.jpg

Perhaps another way to solve this would be to get a front trike. Like this: Trike Gas Motor Scooters 50cc 3 Wheels Moped
http://www.scooterdepot.us/media/cat..._d50tka_11.jpg
It's low and the double wheels in the front greatly improve stability. Don't know.

Interesting but not available in Australia in that form or at that price !

"Total Price: $1,758.00 + $399.00 Shipped!"

Normally, when I have seen people say "$x shipped," they refer to the cost, including shipping. $399 for shipping?

It kind of seems that if you want an Elio, you need to purchase one of these and build a body. Lower the seat like this guy: 214 mpg with DIY aerodynamic fairings on a Honda 125cc motorbike

The idea behind the egg shape is that the shape will generate down force instead of lift since the air moving over the smaller radius will have to move faster. Stall strips on the nose help too. Don't take my word for it, ask the Vetter challenge people.

I was reading about Alan's results and they are great, he reported no trouble with side lift either .... perhaps they were no 18 wheelers on his course ?

http://i1187.photobucket.com/albums/...r/SAM_0104.jpg
See the 1960's Indy cars prior to wings.
http://8w.forix.com/indy/mg-indy63.jpg
While the boxy Chaparral 2J used vacuum power to generate down forcedata:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD/2wCEAAkGBxQSEhUUEhQVFBUXFx8UFxcUFRwYGBoXGRoXGBcYFh gdHSggGB0lHBcYITEiJykrLi4uGB80ODMsNygtLisBCgoKDg0O GhAQGy0kICQvLSwtLC4sLCwsLywsLCwsLCwsLCwsLywsLCwsLC wsLCwtLCwsLCwvLCwsLCwsLCwsLP/AABEIAJQBVQMBIgACEQEDEQH/xAAcAAEAAQUBAQAAAAAAAAAAAAAABgEDBAUHAgj/xABPEAACAAQDBAUFCwgIBQUAAAABAgADBBESITEFBkFRBxMiYX EUMoGRoRYjQlJTYpKTwdHSFRdUVWNylLEkMzSCotPh8ENEpLLC ZHN0g4T/xAAaAQEAAwEBAQAAAAAAAAAAAAAAAQIDBAUG/8QALhEAAgECBQIEBgMBAQAAAAAAAAECAxEEEiExURMiFEFhgQV xkbHR8CNCofHB/9oADAMBAAIRAxEAPwDuMIQgBCEIAQhCAEIQgBCPLuACSbAZkmM U7Uk/KJ64AzIRhnakn5RPXFPyrJ+VT6UAZsIxBtOT8onrjLgBCEIAQh CAEIQgBCEIAQhCAEIQgBCEIAQhCAEIQgBCNTvBKdwkuVPaQ74g pCkgkKTY5WFrX1F87d2lkMxxgVpa7gAOGBBBZioIztYgi2oQ3y BsBMIRCVr54KYNo07KQpONBjKtiYGwzN+yAMshrfM5Dzavqx/TJWNgWB6g4cKAYza175N69IAl0IjdJXsJ3bqpXVmzBCcLm8tLA KVGpOL1DnG3O1pANutl3184acM4AzYRhJtaQSQJsu4vcYhcYRi b1A3PdD8rSMvfpeenbHf9xgDNhGM20JQAJmIAdO2M7kgWzzzB9 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Trash all that after the indy car jpeg! Yikes

We have 18 wheelers everywhere! The California coast riders with Craig Vetter in Monterey get strong winds off the ocean. I get the thunderstorm winds especially crossing the river here. My bicycle fairings have done well because I attach to the head tube rather than the handlebars as do most fairings available. Since a motorcycle has 4 to 4.5 inches of ground clearance, floating plastic skirts could be made. The Chaparral 2J used them to get a partial vacuum under the car for down force. A gas fan would use fuel too.

Thanks Grant . I'll have to investigate skirts .

A brainstorm (or brain fart) might be to use a pyramid shape as the fuselage of the F-117 aircraft. Being wide at the base and pivoting at the top like a bell and clapper, the motorcycle could tilt inside while the shell is suctioned to the ground with a couple of gas leaf blowers. Good for smooth roads only.

:D Now that is getting way too complicated and I hate leaf blowers !

But the F117 is along the lines of my original brain bubble . Those flat sides and rapid intersections don't look aerodynamic but it works.

The issue also arises of engine (and rider) cooling and how much power would be needed to run an electric fan. The tapered side would give some Newtonian down force. The flat sides might be easier to fabricate but I wouldn't count on being invisible to police radar.

Much of this has been discussed in other threads going back a few years.
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http://ecomodder.com/forum/440784-post225.html
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My new tail is a complete non issue with regard to cross winds. Even the full streamliners are minimally disturbed by cross winds as long as there is an open vent for the pressure differential at the point of max thickness/ lift.
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https://scontent-ord1-1.xx.fbcdn.net...b2&oe=5652D512
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I've been thinking along the lines of sliding "doors" that could be slid back inside the rear to cover those openings in the interests of reduced drag. That would seem to require the flat sides.

Actually the doors could be flat or concentric. The larger cross section would be in front so the door would slide on the outside as in the minivan application.

A straight sided fairing leaves a huge wake since it is not streamlined. But at least the rider can gain some weather protection and some useful locking storage.

I think I need to bandsaw out a shape, take a photo and post it to give you an idea of what I have in mind.....

Here are few photos of a rough model I made today. The sides are flat and the maximum cross section is around mid way but there is no foil. The rear could be narrowed more or even tapered a little without generating lift.

It is quite similar to the Vetter fairing and would have the same frontal area . Could i have some comments on this idea ?

http://i1062.photobucket.com/albums/...P7230533_1.jpg

http://i1062.photobucket.com/albums/...d/P7230537.jpg

http://i1062.photobucket.com/albums/...d/P7230538.jpg

Honestly, it would be much more aerodynamic if you tapered the sides. You punch a hole through the air and if your vehicle taper, you leave a smaller hole, creating less drag. The back of my Civic tapers a few inches behind the rear wheels. This is pretty standard. The Gen1 Insight tapers more than most cars and it is very aerodynamic.

I see now that you mean to taper the sides so they are narrower in the back. By "straight sided" I had assumed that you meant parallel. The Vetter tails are tapered at 16 degrees and flat sided just like your truncated tail that you are showing.

I guess I should have labelled the model .... it is tapered the 16 degrees as per the Vetter form but without the curve. The sides are flat .... but not parallel.

Thus some increase in drag is likely but far less side lift.
The side lift is something that worries me a great deal, the winds were i ride are extremely variable and if i ride on main roads or freeways the are a large number of very large trucks of the huge moving van variety.

Side lift could easily toss me into the path of another vehicle.

As little side lift as possible is preferable, even at the cost of drag..

All the Vetter tails have straight sides.

That is not problematic?

A correction, because of the required geometry with a flat sided taper the angle isn't 16 degrees , probably between 5 and 7 but as it starts further foreward the max width and the end width are the same as the Vetter..... no curve .

comment
 

*you're streamlining to reduce pressure drag.
*you must reduce or eliminate separation to do this.
*historically,gentle,progessive plan taper is the only way to accomplish this.
*and this requires a curvilinear pathway.
*a straight curve has too much pressure gain to respect a turbulent boundary layer's needs to mitigate counterflow,eddy initiation,then full-blown turbulence.
*to complicate things,the body upstream of the taper is mutilated due to the cavity for the rider,so the onset flow to the tail is adulterated already.
*boat-tailing requires clean,turbulence-free flow to perform properly.
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*Since your fairing is a crude,upright wing section it cannot tolerate a Kamm truncation.Chopping it off creates an exponential drag increase.
*for drag reduction the tail needs to be curved to a point,even if it's not to an ideal length.
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*if you're concerned about crosswind and gust dynamics,it's my opinion that you need to ride with extreme vigilance,and use the body cavity to lean out,using body English to address your immediate riding environment.
here is the minimum drag section (center)
http://i1271.photobucket.com/albums/...rohead2/-2.jpg
Here you can see how drag is compromised as you leave the ideal contour
http://i1271.photobucket.com/albums/...head2/8-24.jpg
Here is the numerical relationship with which you can predict your drag penalty for truncating the tail
http://i1271.photobucket.com/albums/...2/8-16-132.jpg
Here are some examples of proper streamlining
http://i1271.photobucket.com/albums/...2/HPIM2007.jpg
http://i1271.photobucket.com/albums/...2/HPIM1997.jpg
http://i1271.photobucket.com/albums/...2/HPIM2002.jpg
http://i1271.photobucket.com/albums/...2/HPIM1979.jpg
http://i1271.photobucket.com/albums/...2/HPIM1954.jpg
http://i1271.photobucket.com/albums/...2/HPIM1920.jpg

Thank you but I am well aware of what ideal streamlining looks like , the problem is that it isn't practicable to use on a busy road . I'll end up dead .Lovely pictures though.

''*a straight curve has too much pressure gain to respect a turbulent boundary layer's needs to mitigate counterflow,eddy initiation,then full-blown turbulence.''

Could you explain that in other words ? I'm finding your meaning a little hard to derive .
I've modified my rough model , improving the nose area shape and smoothing it more back to the rider's position, the rear remains flat but tapered.

My understanding of this is that it is the angle at which the body leaves the air that is most important. I appreciate that my design will generate some flow separation forward .... but then that is the whole point if the side lift is to be avoided and that seems the primary gain, it could keep me alive.

Basically what he is saying is that your design won't be very low in drag.

I guess I'll have to build it and see .

Tuft test, if you can.

But you will gain some nice storage capacity and weather protection so even if it is only a slight improvement in drag, it is still nice to have a fairing and trucated tail. My tail and a new chain have given me an extra 10% in fuel economy to now average 102 mpgUS and now I can go shopping for more items or lock up my stuff when I am gone camping.
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https://scontent-lga1-1.xx.fbcdn.net...b2&oe=5652D512
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I hope for a fair bit more than 10% !

I guess I'll see.

Sendler, I'm a little disappointed your well made tail is only improving your mpg by 10%. From my studying on the subject I would have guessed around 20% improvement.

The utility aspect is surely a nice gain though. I need to build a tailbox. Though I have been hesitant since I carry my mountain bike on my rear rack once a week. I'd have to make the tailbox quickly removable.