Motorcycle aerodynamics, a slippery and windy, slope?
 

Looking at the all-encompassing fairings on the popular scooters here got me inspired and I went to the drawing board. However, I got to thinking about the differences between a scooter not meant to really go beyond 50mph most of the time, and a motorcycle that's built to handle highway speeds, which I will need for my daily commute. I currently average about 50mpg in my daily commute to and from work in my Corolla, so the bike I get to do this commute will have to be significantly better in gas mileage.

That leads me to the 250cc class*. There's definitely some success stories here with those types of bikes. Of those bikes, the fuel-injected CBR250R is the most attractive.

I'm familiar with fabricating fiberglass stuff, so creating a bigger, more thorough fairing would be on my list, but this article got me thinking, that maybe I shouldn't mess with the aerodynamics. Being aerodynamic in a forward direction probably has no real draw backs for a scooter, but for two wheels at highway speeds, that could possibly spell disaster, if what I'm reading is correct. You'd basically be a sail on wheels.

There's possibly some ingenious ways of getting around this issue, or it's possible that it's not even a real issue. I was looking at all these fully-enclosed, cager-cycles that are in production in Europe, and there's apparently no major issues with crosswinds. If it were as dangerous as the FIM would have us think, there'd already be horror stories about all of these cycles getting in horrible accidents caused by the wind, right?

http://www.youtube.com/watch?v=VbdkZB9-Sd4

If I improved arrow dynamics on a bike, I wouldn't be fully enclosed. I would still need to be able to put my feet on the ground like normal; no fancy gadgets to make it stable while at a stop without my own feet.

What do you think?

* 4/21 - After this thread, a lot more planning, and thought, and also after looking more into Allert Jacobs' awesome scooter, I decided my commuter bike will most likely be a scooter; probably the Yamaha Zuma 125. (Too bad the Honda ANF125i Innova is not available in the US Market) It seems to be best route for fuel-economy and speed (for a daily commuter).

There probably is some truth to the notion that dustbin fairings increase sensitivity to x-winds such that instability could become an issue.

That said, we have the Vetter streamliner, the Ecomobile, and that streamliner on the EM home page, and as far as I know all of them are streetable and are claimed to be stable.

Perhaps the problems with the old dustbin fairings were:
1) they didn't have enough tail so that put the center of pressure ahead of the center of gravity, and that causes instability just like firing an arrow backwards; and
2) they were racing and so the speeds and conditions they were operating in were less forgiving than the street.

So for safety, do you think maybe an all or nothing approach to aerodynamics better? Either turn that bike into a bullet on two wheels, or leave it alone, perhaps?

for safety, I would stay in bed :)

I think there was some issue with early designs that had a high tail, and the whole bike was a bit like a vertical wing with a genuine angle of attack leveraging the whole thing over in a crosswind.

I believe you do want to avoid that scenario, or possibly even figure out how to use crosswind to your advantage (rudder?).

But aerodynamics and small two wheelers are about as efficient as it gets for 1-2 passengers, so absolutely worth sorting out.

edit, interesting link:
http://www.bikeweb.com/node/1805?size=_original
the ecomobile leaned more but got pushed off its path less than a standard bike in a crosswind test, fwiw. They say the bad crosswind press is from pointed front ends with overhang.

Safety is always relative, so until you define it, its hard to say what you should do. As a general comment on design it is posible to design anything poorly. and if you find yourself in the midle of a tornado or a hurican it won't matter much what you are driving, it will be tossed around by the wind and distroyed. You the driver/rider may or may not servive? In 1957 when the FIM banned dust bin fairings most racing was done on normal roads lined with trees and other hard objects. The speed improvements resulting from full dust bin fairings in the absence of other safety provisions found on modern roads and racetracks as well as improvements to tires, suspensions and brakes etc led to many fatalities. The easyest solution was to slow the bikes down hence the baning of the dust bin fairings. although the ban was a solution of sorts it was not the only problem being solved. The almost total asence of safety features found on modern roads and race tracks was the main problem not the dust bin fairing.
My two bits says you should proceed carefully in small steps until you are comfortable with the results. Start at the back of the bike behind the rider.

I'd say find out if the examples of faired in bikes I provided are really stable and if so, there is probably no reason not to copy them if something like them is what you want.

Yeah, the more consideration I give it, the more I want to figure out the motorcycle aerodynamics.

The article really just brought to light that the dustbin fairings were banned for safety concerns, but it doesn't reveal the true cause of the danger. It does seem like the increased speed was the only true danger.

It wouldn't surprise me if I could hop on one of these old vintage dustbin racing bikes, and not be in any real danger, simply because I'm already familiar with riding naked vintage bikes and my riding style is pretty cautious; not race-oriented at all.

Since I started saving money for a bike, it's just given me way too much time to think and plan ahead for this kind of stuff. After nearly a year when I have enough cash to buy a bike outright, I'm sure I'll have this all figured out.

Ps. Here's another tidbit on the crosswind thing. It's the same quote, but with a picture for reference:

http://www.bikeweb.com/node/1808

Sounds like the overhang is a big no-no.

http://www.bikeweb.com/files/images/Baumm001.jpg

One other point about motorcycles aerodynamics and cross winds is the mass of the bike. If you have two bikes with the same side aera to work on and one of the bikes is twice as massive, then the more massive bike will be less affected by the cross wind F=ma. That is why; as you reduce the mass of the bike getting the aerodynamics right become ever more critical as well as the location of the center of gravity and the center of pressure... Take a look at the side area of one of the big touring bikes like the
goldwing and then ask the owner what its like in a cross wind? I ride with a friend who has a gold wing and it leans much less than my GS500 or my KLR650 both of which have less mass and less side area.;)

here is another interesting link with some crude testing
Stability Or Control

summary:
for aerodynamically stability, you want the weight up front and the side area in back (like a dart or an arrow). You don't want too much stability though or you can't turn ;)

For crosswind stability in a car/trike you want the center of pressure to coincide with the center of gravity.

On a bike, they concluded experimentally that a side force (center of pressure) in front and perhaps some steering geometry changes made side forces manageable or even self correcting. (though aerodynamic instability introduces new complications at higher speeds, which there may be ways to address, like active aerodynamic stability)

From the string experiments, it looks like the "sin" might have actually been having more side area in the rear than in the front? At least from a crosswind perspective. Of course these are high speed runs where aerodynamic stability gains in importance (you don't want your bike to try to swap ends when you hit a bump).

Also take a look at another current design:
214 mpg with DIY aerodynamic fairings on a Honda 125cc motorbike | Hypermiling, Fuel Economy, and EcoModding News - EcoModder.com
http://ecomodder.com/blog/wp-content...da-v2-rear.jpg
claims: “A 40mph (65 km/h) side wind is no problem,”

no front overhang, but also not a tapered long high tail either.

Have a look at the fairings, streamliners and related info on Craig Vetter, designer and inventor of the Windjammer fairing,Triumph Hurricane Motorcycle and fuel economy motorcycles .

And of course there's Allert Jacobs streamlined 125cc on the ecomodder.com homepage.


You've got some big shoes to fill when you want to walk with giants .
Craig's fairings are legendary, and Allert is world famous in the world of velomobiles and recumbent cycles ;)

Yes, I saw that. It's a beautiful design. One way you could improve on it, is by sitting where your feet normally are, and then you'd be able to cut down the side surface area and lower your center of gravity.

I'm really not sure if lower CG helps or not here. Certainly lower center of pressure helps, and having the center of pressure below the center of gravity means the side winds will have to work harder to tilt the bike.

Sort of like the c-clamp on a yardstick trick.

Yeah, on second look, he's already sitting very low. He'd have to extend the frame and sit in front of the engine to get any lower; which might defeat the purpose, since it'd extend the surface area longwise, for just a few inches off the top.

He also mentions 80lbs of aero mods helping with crosswind stability.

Another thing to note is that he has rounded trailing edges. Normally we like a nice sharp transition into the void for reducing drag, but I have heard folks comment on that having a negative effect on crosswind response in cars (increases the effective tail size with a wall of wind?)

My varna has been ridden in high winds during competition at 50 mph for an hour. The wind definitely makes a difference in handling, our rider didn't slow down when crossing dust devil cyclonic storms in his path. A trike might be a lot easier however. Graft on an ATV front end onto a motorcycle and fair it in. The CanAm I think really missed the boat when it comes to efficiency with their trike.

Yeah, the trike would be a simple design fix, but I really like the two-wheel set-up. I still want a bike and the handling that goes with it.

It's been slow at work, and I've just been sketching away at various ideas. It's really kind of silly, but I've got so much time to think about this before I have a bike. In theory I could have a lot of stuff built before I even get the bike.

Surely there are some forum contributors who can give us honest feedback about how the Ecomobile rides in sidewinds.
Since this is the only real fully enclosed motorcycle in widespread use it must be our best point of reference.

One thing is, on Allert Jacobs' blog, he mentions some crosswind trouble when he had only the front dolphin-esque fairing. He was able to eliminate the instability when he went all out, covering the whole bike.

http://www.velomobiel.nl/allert/Recu...n/image028.jpg
"The front wheel fairing has to go to. I was quite confident that it would work and not affect the steering to much.

It turned out to be very scary to ride even with as little as 3 beaufort (about 10mph) side wind I did not dare to go over 35mph. It is a pity I made a mould and all without ever testing with a cardboard model."

Taken from:
http://www.velomobiel.nl/allert/Recu...0motorbike.htm

A lot of aero car designs have fin(s) in the rear for stability. The Tatra and the Panhard are the best examples.

Center of pressure ahead of center of gravity = instability. No surprise there.

My experience with driving in gusty sidewind condition is that when I start leaning forward on the seat the handling gets much better.
I made my doors on the scooter dismountable in case of heavy sidewinds, but I never used that option. In a way you get used to some more bike movement with a fairing. Possible in my case the enlarged front wheel fairing is giving negative effects too. I am preparing a fixed front so maybe when that is ready I can find that out.

Hello all ,I've just joined the forum and am extremely happy to have found this thread !

My current ride a BMW R80 and it is unfortunately uneconomical ,which is a pity because it is otherwise an excellent bike .I don't think trying to make it economical is practical so have decided on another course .

Some people have been converting Indian Enfields to diesel usually using 10 hp Chinese aircooled engines .These bikes are slow , with a top speed of around 50mph unfaired but get remarkable fuel consumption figures ,1.5 to 2.2 litres per 100 km being normal .(156 to 107mpg US ).

However back in the 50s at the time of dustbin fairing in racing the Enfield factory decided to build a moderate fairing for roadbike users .The result was the Dreamliner .

http://pic40.picturetrail.com/VOL282.../397836281.jpg

I have other photos should anyone be interested .

It weighed 45 pound , added approximately 8mph to the bike's top speed ( which incidentally was a 13hp 350cc model )and reduced fuel consumption by 25% .

This fairing looks as if it would be a good basis for development .
I've derived it's drag coefficient at .85 which is a great improvement on the standard bike's 1.2 .
With a diesel engine and it's possible 40% efficiency versus 25% for petrol,some quite good fuel consumptions are theoretically possible .

1.44 litres per 100 km or 163 mpg at 50 mph .
.95 l /100km or 247mpg US at 40mph .
Maybe !

Can anyone suggest improvements to the fairing ?

I'm now going to try to get up 5 posts so I can put up a photo ...please forgive my impatience .

Three .

Four.

Please provide a referance for the Cd of 1.2 for the standard motorcycle?

Neato, but probably still an awful Cd.

It was the figure given to me by an engineer of my acquaintance but I note your reference on your other thread .I have found it very difficult up to now to get any substantive information on this subject.

''Aerodynamics of road vehicles edited by Hucho the chapter on motorcycles.''
Based on that could you estimate the cd of the old fairing in the photo above ?

and thanks for any help ,seriously !

The overwelming problem is the size of the windscreen and its shape. The lower part of the bike is ok. The side view is not enough even to guess at the Cd. I would guess that the overall bike is very poor aerodynamicaly.

It's certainly not ideal from an aerodynamic perspective ....I agree! I would definitely lower angle and height of the screen .


What number do you apply to a standard bike re ''Aerodynamics of road vehicles'' ?

My problem is that my state registration department will not allow any modifications to the basic motor cycle or seating position .The wonderful ''Allert'' would not be registerable here .I need something that can be bolted to a standard bike .

If you click on Gray you can see what i have done, the other place to look is craigvetter.com. an other referance is an online book called without the hot air or something like that.

Thank you , an excellent resource !David MacKay FRS: Sustainable Energy - without the hot air: Contents

Gray is an interesting project ,how do you find it in cross winds with the extra bodywork on the rear ? I can see something similar being very useful on the Dreamliner modification .

Any difficulties getting on and off ?

I'm reading this thread , there are gems everywhere .This above was particularly helpful ! I have to admit that after I saw Allert for the first time I immediately drew an Enfield sized version .... the rear is designed to slide back as Allert's slides forward .The bike plus rider would weigh around 230 kg / 500 pound .


http://pic40.picturetrail.com/VOL282.../397436022.jpg


I can see from dcb's comment above that the higher centre of pressure and the rider's weight higher too would very likely cause instability .

thx Pete :) Here is a diagram I slapped together looking at a tall streamlined bike as a "wing" of sorts from the top, don't know if it is entirely accurate and haven't figured out if the wheels make a significant gyroscopic stabilizing contribution at higher speeds or not either, so take w/salt.

http://ecomodder.com/forum/attachmen...1&d=1312561083

re: diagram above, I sort of think it is not accurate in this case as you approach 90 degrees. If you had just a sidewind pushing on the area, you would expect some force to be created, dunno.

I've heard about this book on motorcycle dynamics, although I haven't been able to go through it yet myself: Amazon.com: Motorcycle Design and Technology Handbook (Motorbooks Workshop) (9780760319901): Gaetano Cocco: Books

It might be fruitful to look at the reviews and the "other things people bought" too.

dcb, good info but the problem is more the unexpected burst of wind from the side at any angle.I can't work out how a wind from say 15 degrees off the front would effect things but if it was a constant it would probably be OK .


There seems some small advantage in the higher ''fuselage " ,a slower roll period.The shorter the structure ,the faster it will lean but how this relates to the distribution of mass and the balance of the areas ....I have no idea !

A couple of bicycle related links which discuss gyroscopic effects and side loads .Not perfect for this discussion as bikes are pretty porous from side wind perspective .
http://en.wikipedia.org/wiki/Bicycle...xternal_forces
http://www.sheldonbrown.com/brandt/wind.html

and a review of the Hucho book
http://www.recumbents.com/wisil/demma/aero_review.htm

I'll keep reading through this excellent forum and try to learn something !

I really wish I could put a question to Allert's builder ...I'm sure he could put me right in a second !



Edited to add .In the original 1957 article on the Dreamliner there was mention that a prototype fairing that was 'slabsided ' had been rejected as bad in cross winds ,the final Dreamliner was the same as an unfaired bike , the reason given by the designer was the indented sides .The flat sides of the original had been much worse .

Thanks Frank , I looked at the reviews but there's no mention of the silly kind of stuff I have in mind !:)