Motorcycle aerodynamics, a slippery and windy, slope?

[sendler]

Thinking through the issue of side winds and motorcycle aerodynamics some more:
I question the theory of adding a long tail behind the rear wheel in order to improve stability. I know it is very aerodynamic and reduces wind drag from the front but will it make the bike track better in side winds? Adding a long tail makes the bike want to go straight through the wind like an arrow. So far, so good.
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Bikes don't turn from steering, they turn from leaning.
When a side wind hits the tail, it steers the front of the bike into the side wind. So far, maybe not so good. Bikes need to lean to turn, not steer. Does yawing the bike into the wind with the fin (rather than leaning) load the trail of the front steering geometry to cause a counter steering event that actually leans the bike the wrong way. Probably.
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Is the wrong way lean more than the right way yaw? Don't know.
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Another draw back to the rear fin is the sheer increase in area for the side wind to push on. Does it increase the ability of the wind to skid the whole bike to the side all at once? Definitely. The wet roads that I ride on half the time would be revealing.
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Does any increased fin area that is sticking up higher than the cg of the roll axis, ie a rudder, blow the bike over to lean the wrong way. Definitely.
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For motorcycles, keep your bodywork as small as possible while preserving low drag from the front. And the side. And keep the center of pressure from side winds as low beneath the cg as possible to help the bike lean into the wind the right way. And keep your cg high.

[FXSTi]

Sendler,

Read the link in post #107.

Kirk

[sendler]

Originally Posted by FXSTi

Sendler,

Read the link in post #107.

Kirk

I have seen the No BS bike report before. Very cool demonstration and a fairly serious commitment to do that to a bike to prove his point. Why do you recommend I read about Code's No BS bike?

[euromodder]

Originally Posted by Ken Fry

Like the Monotracer, mine was touchy about the transition from outriggers-down to outriggers-retracted. OK for enthusiasts, but not so good for a mass market.

The Lit could solve the low speed problems that come from having to first steer like a car when the outriggers are down, and then steer it like a motorcycle when the outriggers are up.

What's the issue ?

The mass market for motorcyclists seems to cope fairly well - motorcycles do the same, steering like a car at low speed.
It's all a matter of speed.
And it's done without thinking about it.

[sendler]

Originally Posted by redyaris

Sendler
I do not dispute any of the effects that you referance and discribe.
What I am disputing is that they are enough; on there own, to get the bike to roll into the wind.
It is my position that without rider input the bike will not roll into the wind.

I will actually have to go ahead and try it no handed to let you know for sure that I am not subconsciously guiding the bike as it leans into the wind.

[Ken Fry]

Originally Posted by euromodder

1. What's the issue ?

2. And it's done without thinking about it.

1. The issue can be seen by watching someone start out on an Ecomobile or Monotracer. Unlike a motorcycle, which the rider naturally makes vertical with respect to gravity at a stop, enclosed motorcycles (so far) list at a stop to be perpendicular to the road, which is typically crowned. So the driver has to first get the wheels directly under the CG, which requires a swerve. If the bike is leaning to the right, the driver must steer to the right. If there is a car immediately to the right, this is a problem.

In a normal motorcycle, this stuff is so easy that it requires no thought and almost no effort: even if you are leaning slightly at a stop, a tiny push to vertical is simple.

Designing a linkage that makes the enclosed motorcycle vertical with respect to gravity (by individual control of the outriggers, manually) is possible but not stunningly simple. The fact that the $80,000 Ecomobile has not sorted this out is an indication of the complexity. Adding to the problem is the fact that enclosed motorcycles tend to be heavy: the Monotracer is something like 1000 lb, if I recall.

I used to ride my brother's Harley 74 when I was 12. I was fine, on tippy toes, as long as I kept it vertical. Let it lean at all, and I quickly got to the point where I could not hold it up.

In my enclosed two wheel prototype, I had independent manual control, via foot pedals, but getting sufficient control authority (high leverage) in combination with large enough travel to quickly deploy the outriggers (low leverage) is not trivial.

Enclosed motorcycles (assuming they are like the Monotracer (low and streamlined) feel much different than a motorcycle. Because the only thing that can cause the bike to lean to the right (to turn to the right) is counter steering (steering to the left), you find that you are more conscious of counter steering. When the outriggers are down however, you steer it like a car: turn right to go right.

The course for learning to drive the Monotracer is something like a week, for good reasons.

2. You're right that many motorcyclists do not think about (nor need to think about) countersteering, etc. When they panic, however, something goes haywire, and far too many start to "think" that to avoid hitting the truck they are drifting into, they must turn the handlebars away from the truck. That banks the bike right into the front of the truck. It is the classic accident that the "push right go right" campaign of the Motorcycle Safety Council was designed to prevent.

But still there are many, even some racers, who just don't understand how to steer a motorcycle -- this is the reason for Keith Code's no BS bike.

[Ken Fry]

Originally Posted by redyaris

Motorcycles are not self correcting

Incorrect.

Motorcycles are designed to be self-correcting. Touring bikes are highly stable, and race bikes are less so -- that is designed-in, not coincidence.

This article describes motorcycle stability. Whether the bike leans to the right because of some road-induced perturbation, or is pushed by a crosswind makes little difference. In either case the bike's trail causes the required counter-steering to occur.

The bike, of course, has no way to know how much correction is required. Rider position can have a large effect as can the distribution of area in the side view. Some bikes correct for crosswinds better than others.

[Ken Fry]

Originally Posted by redyaris

How does the bike know how much to counter steer?
How does the bike know how hard the wind is pushing it?
At what point does the bike stop countersteering?

I think Sendler's answers here are already quite good. This might be simpler... maybe:

The bike, of course "knows" nothing, but the whole system functions as a feedback loop.

The bike knowing how much to countersteer and knowing how hard the wind is pushing are interrelated: the sideward component of the crosswind causes a countersteering force: the more the wind force, the greater the countersteering effect. (The bike is designed to do what a rider would otherwise have to do.)

The bike stops countersteering when equilibrium is established. A cross wind from the left causes countersteering to bank the bike to the left. The bank to the left, however causes countersteering (from gravitational effects)* to tend to lift the bike back up to vertical. Equilibrium is established when the two tendencies are balanced.

Obviously, for this to work, the bike has to be well-designed (from this perspective), and the rider has to avoid tensing up and applying a white knuckle grip. There have been bikes that will oscillate (all the way up to tank slappers) at the slightest provocation, so motorcycling handling dynamics are not as simple as they might appear.

* You can see this effect if you straddle a bike and tilt it to the right. The handlebars steer to the right without your input.

[sendler]

Thanks for the link Ken. I have wanted to buy a video camera for the bike anyway so when the weather clears I should be able to post a video showing my bike self correct in a side wind blast with my hands off of the bars. But not too far off the bars in case a tank slapper starts to set in. The steering is fairly quick on the CBR250R. A pity to me that apparently more people haven't experienced this as it is one of those mystical feelings that certain activities can give you such as walking up when on an escalator that is already going up. On the bike it is very enjoyable and addictive. It is one of the things about riding a motorcycle that makes me want to get out and ride with no particular destination. Feeling the wind flowing around you, the minute temperature changes, the smells, and being one with it.

[3-Wheeler]

Originally Posted by sendler

....On the bike it is very enjoyable and addictive. It is one of the things about riding a motorcycle that makes me want to get out and ride with no particular destination. Feeling the wind flowing around you, the minute temperature changes, the smells, and being one with it.....

Wow!

I thought I was the only person that felt this way!

Most of the people at work drive cars, and for the most part, so do I now that I have a car that gets more MPG's than the bike can possibly muster, so it's hard for most people to relate.

But still, your description above is exactly what I feel on a beautiful summer day, loafing along an old country road, with the sunlight skipping across the blades of shimmering green grass, butterflies fluttering about, and the wonderful smell and color of flowers spread along the sides of the road, all with a slight breeze wisping under the helmet across my face! Ahhh!

That's what a nice motorcycle is ride is all about.

Jim.