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-   -   Motorcycle aerodynamics, a slippery and windy, slope? (

Kincurd 04-08-2011 12:39 AM

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?

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).

Frank Lee 04-08-2011 12:52 AM

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.

Kincurd 04-08-2011 01:04 AM

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?

dcb 04-08-2011 01:23 AM

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:
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.

redyaris 04-08-2011 01:33 AM

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.

Frank Lee 04-08-2011 01:44 AM

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.

Kincurd 04-08-2011 02:09 AM

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:

Sounds like the overhang is a big no-no.

redyaris 04-08-2011 02:58 AM

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.;)

dcb 04-08-2011 09:40 AM

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

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)

"This experiment is so easy to do that I hope you repeat it. I am tempted to leave out the results but, for the curious, here is what we found.


With the string tied at the head tube, Max pulled sideways (gently at first!) and I found that it was very easy to make a slight steering correction to return the bike to roll-and-yaw equilibrium and to keep the path essentially straight. With a little practice, I was steering and rolling the bike slightly and could resist as much side force as he could pull. Sharply varying side forces (gusty winds) were tried next with the same ease of control.

Next, we moved the string back to the seat post simulating a CP aft of the CG. We kept the height above ground the same. Here the control required was much more difficult. With practice, I could steer and roll the bike to counter this side force but there always were several big swerves and the heading always changed. A varying "gusty" side force was very difficult to
control -- most of the effort went into roll stability (keeping balanced) and the heading went all over the road!

Finally, we moved the string back to the head tube and reversed the front forks to increase the trail. Now the side force also produced a large steering torque. This torque steered the bike "down-wind" which resulted very quickly in a roll angle "up-wind", just what is required to "lean into the wind". With a loose grip on the handlebars, the bars wiggled around as the string was jerked but the bike kept going nearly straight.

The interesting conclusion is that the "aerodynamically unstable" location of the CP forward of the CG is the easiest to control and appears preferable over an "aerodynamically stable" configuration! Control appears more important than stability for this situation. The experiment we tried did not go to very high speeds so I am not suggesting that this result is valid at higher speeds. My experience with large, frame-mounted front fairings has generally been good at speed (on long hills) in moderately gusty winds.

dcb 04-08-2011 10:48 AM


Originally Posted by Kincurd (Post 230403)

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).

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