Motorcycle Aerobody Research

[3-Wheeler]

Originally Posted by sendler

.... Adding a long tail makes the bike want to go straight through the wind like an arrow. So far, so good. .....

Before adding the tail to my Honda Insight, I would have read your statement and agreed with you, as I had nothing more to add.

But, after adding the tail, and driving the car in some nasty 50 mph sidewinds, I have some comments to make.

1) In heavy sidewinds, the car still pushes the front. This is how the car was when I first purchased it, and it still remains this way after adding a 3.5 foot long tail.

2) The car has been driven on icy-slick roads in the winter, and yet still needs turning into the wind to compensate for high side pressure.

3) I would have thought that adding the tail would allow for more "neutral" steering, but this has not proven to be the case

4) The cross-sectional area of the tail, is I admit, much smaller than a tail on a tall motorcycle would be, so your point may be well taken for a more slab-sided air foil shape for a motorcycle.

5) Would be interesting to hear from any of the older members who have ridden in the Craig Vetter High Mileage Contest from the late 1980's.

I've got several articles scanned in from those days, and they are located here...

None of the articles make much mention about handling in high sidewinds however.

https://picasaweb.google.com/threewh...leAeroArticle3

Just adding to the discussion....

Thanks, Jim.

[godscountry]

Tony Foale Designs, article on motorcycle aerodynamics.

This tells it like it is,again the long wheelbase helps,pushing the back out,turning the front into the wind for better control,where as the short bike gets blown off course and you have to gain control.

[jjackstone]

Originally Posted by sendler

So we can at least look at these HPV's to see what they are doing wrong.

The HPV's are optimized for speed above all. If it can be ridden with "relative" safety it probably will be. However, all bikes are checked for particular safety requirements in case of accidents.

I wouldn't try to see what they are doing wrong, but see what they are doing right and modify that to work with everyday vehicles. The two technologies I like best are velomobiles(generally three wheelers) and the Segway gyroscopic balancing technology. I believe this or an extremely similar technology is being use in the Lit Motors cycle.

JJ

[godscountry]

heres some more on side winds,interesting Stability Or Control

[sendler]

Originally Posted by godscountry

heres some more on side winds,interesting Stability Or Control

That is an excellent experiment with the string. Maybe the best position for the side CoP will be on the vertical line which runs through the center of the head set to best activte the countersteering of the trail. But still as low as possible below the roll CG.
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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.

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[redyaris]

The first question I have is where do the reaction forces take place when a side wind acts on a motorcycle? Newtons 3rd law.

[sendler]

Originally Posted by redyaris

The first question I have is where do the reaction forces take place when a side wind acts on a motorcycle?

Good point. The strings experiment is interesting but less relevant than I first thought. It is only a side wind when the bike is parked. Any time the bike is moving forward faster than the side wind's velocity, the apparent wind will be a vector off of the front. Making the streamlined bike a symmetrical wing with an angle of attack.
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regardless of where the static CoP from the side is, The streamline shape with an angle of attack will have the greatest lift (side) force somewhere toward the front near the point of maximum width.
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[visionary]

Thanks to 3-wheeler and godscountry for two useful sources.

As Scott says the string tests were quite insightful, but ultimately difficult to quantify and that makes it impossible to draw scientific conclusions – but thought provoking nonetheless.

The Aero articles, centring on 80’s Vetter mileage contests, and particularly the wind tunnel testing of different types of motorcycle were most useful. This is some of the most accurate information I have found available – published windtunnel numbers from a reliable source.

In this article https://picasaweb.google.com/threewh...leAeroArticle5 page 2
The figures for the rifle faired Vetter Challenge bike were a shock –Cd 0.3627.
This is way higher than I would have imagined for a streamliner. I don’t imagine that my design would be as good yet I was hoping for a lower figure – some recalibration of my targets may be in order.

I cannot remember where, but I recall a Cd 0.19 for the Peraves Ecomobile and I based my expectations on this figure. Now, in light of this article I question if the eco could make that number. I wonder if we could compile a table from different sources of the top aerobody designs.

[visionary]

I wanted to post a link to a page that I feel contains some of the best information on the stability issue. Its from Kraig Schultz’s Deltabike site and it draws on some opinions of highly influential designers.
Delta Bike USA - Forge Design Competition A - Cross Wind Stability

Even in this page there are some discrepancies between views
“but leans automatically into the wind, the more forward the LCP (longitudinal center of pressure) the more leaning against the wind. “ is a quote from Arnold Wagner (the Ecomobile designer)

and this appears at odds with the views of Royce Creasey who wrote
“ This 'rollout' force should be minimized and this has been done by keeping the nose area fairly low”

Many other people have made the assertion that a rearward biased centre of pressure is most desirable, but I feel this is influenced by misapplication of principles that apply specifically to bodies in free flight. Arnold’s design seems to contradict these ideas, has a high side area and a rearward CofG. The ecomobile shell has no obvious transition points to initiate detachment of cross flows, yet it looks like the most stable vehicle in this class.

All of which seems to muddy the waters.

[sendler]

This is a good article but there are a couple places where he miss-spoke.
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Tony Foale Designs, article on motorcycle aerodynamics.
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From the article:
Him..."The component of the wind side force, acting at the front of the machine is passed to the road surface through the tyre via the steering axis, but the steering axis is in front of the tyre contact patch by the amount of the trail, and hence the sideways force on the bike will tend to steer the wheel with the wind. Exactly the situation that we might wish to avoid"
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Me...Bikes don't make lateral acceleration without first having to lean. He is correct in saying that the trail will take any side force that acts on the bike and translate it through the steering geometry to turn the front wheel WITH the wind. But this exactly what we DO want. The wheels are way below the CG of the roll axis and much lighter than the core of the bike/ rider, so the nose of the bike moves with the wind very little while the wheels counter steer freely out from under the bike by a great amount. Which makes the bike lean and corner into the wind.
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His 3 summations at the end are all not quite right.
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Him..."1. Smaller wheel;----- The precessional forces will be reduced in line with the reduction in weight close to the wheel/tyre circumference. This will reduce the coupling between yaw and roll movements."
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Me...He is saying a lighter wheel is more stable but I disagree. It is true that a lighter/ smaller tire and wheel will make less gyroscopic force but if you want more stability, you want more wheel. Yaw forces from a side wind acting on a vehicle with front and rear tires planted to the ground will be miniscule compared to the roll forces and the lateral forces. Unless there is a huge overhang to shift the Center of Pressure, the yaw force will be applied somewhere between the two contact points and have very little leverage. The gyroscopic interaction of yaw, roll and steering in the front wheel with trail is ideal. Side winds will act well above the contact patches and try to roll the top of the bike with the wind. The precession of the movement of the top of the wheel to the right is to steer the front of the wheel to the right. Good. Exactly the counter steering input we want in order to lean and corner to the left, into the side wind. There will now be a slight yaw of the front of the bike to the right, not so much from the side wind, but as the rear wheel tracks to get back in line. Any slight yaw of the front of the bike to the right (whether from steering or the force of the side wind acting at the aero point of max differential, which will always be forward of the CG of a fast moving bike, even with a tail), will precess to roll the top of the wheel into the wind. Very slight but in the right direction again. And the counter steering input that the trail imparts away from the wind to the right, precesses the same way as the yaw, by forcing the top of the front wheel to lean into the wind. Good again.
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A bigger wheel/ tire is more stable. We already knew this.
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He then states that less trail is better in side winds:
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Him..."2. Less trail;----- The component of the wind side force, acting at the front of the machine is passed to the road surface through the tyre via the steering axis, but the steering axis is in front of the tyre contact patch by the amount of the trail, and hence the sideways force on the bike will tend to steer the wheel with the wind. Exactly the situation that we might wish to avoid."
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Me...Automatic counter steering from increased trail is exactly the situation we might wish to implement. More trail equals more stability. At the expense of requiring heavier steering efforts.
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Him..."3. Zero offset;----- With normal steering geometry with about 1-2" of offset, the bulk of the wheel side area is forward of the steering axis, this combined with the effect of today's large tyres and discs means that a considerable steering force can be generated by the action of a side wind on the wheel. But with the zero offset geometry used on the hub-centre arrangement, this wind force is balanced about the steering axis and no turning effect is produced."
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Me...Not quite right again although much less of a factor than having too little trail. Looking at just the wheel assembly, a slight bias of the CoP toward the front will tend to generate a beneficial counter steering force.
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It is a good article and he clearly addresses the aero issues of side winds and at what point they act on a fast moving vehicle which is nowhere near the static side CoP, but (like many of my own hurried posts) some of his statements could benefit from some proof reading and constructive discussion.