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Old 06-26-2013, 07:52 PM   #1 (permalink)
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Motorcycle Aerobody Research

What’s it all about?

Motorcycles with extensive or “full” bodywork are rare beasts indeed. Informed opinion, let alone genuine academic research into their aerodynamic performance is hard to come by. I’ve started this thread in order to open the debate and deepen the pool of knowledge, as well as supporting my own project, which could benefit from better sources of information.

If you have knowledge of good sources please post links. Personally I have found very little real scientific knowledge to be available. I do however come across quite a lot of misinformation and misapplication of aero principles from other disciplines.

My “pet” hate is the oft cited “centre of pressure must be behind the centre of gravity” which applies to bodies in free flight and is not directly applicable to road vehicles. Yet this phrase is routinely applied by “experts” in motorcycle dynamics. Even if true, it must be seen through the complex relationship between front and rear tyre slip angles, which I have never seen mentioned.

There seems to be a lot of conflicting theories about sidewind stability and how a low drag motorcycle would be affected. Is it more, or less prone to disturbance than a normal rider seated on a naked bike, and if so – why? My personal opinion is that the critical factor is body height, since a lower body also has a smaller moment around the contact patches.

I have been looking at some youtube videos of small-scale wind tunnels, with a view to building a simple version solely to investigate these issues. Making provision for a balance instrument that measures overturning forces at various angles of lean and yaw. I have never seen a motorcycle in a wind tunnel at anything other than upright and “zero yaw”.

Since the accuracy of numbers is not critical, we only need to understand the relationships, would a small-scale tunnel purpose-made for this task bring any benefit? For those with windtunnel knowledge, what would you imagine a suitable design to be – just to get a feel for the principles.

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Old 06-26-2013, 08:00 PM   #2 (permalink)
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I'd look for whatever is available about the Ecomobile; they've been the living proof for decades.
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Old 06-26-2013, 08:16 PM   #3 (permalink)
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Hi Frank - yeah I've seen a lot of stuff on that, but it does conflict!

Peraves make bold claims about sidewind stability and use the tests of the ecomobile being blown from one side in video ( I think you posted about that onanother thread, do you have a link for the video, I've only seen the stills) and photo images - yet there is significant deflection from the path

On the yahoo usergroups feet first forum, which is used by some Ecomobile owners, there is a different attitude, which leads me to think that the Peraves statements may be "sales talk"

Even so, its not the deep technical stuff I'm really hoping will surface in this thread.
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Old 06-26-2013, 09:31 PM   #4 (permalink)
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My 2 cents.

Can i just say that enclosing the wheels will have a dramatic effect on aerodynamics, and make your goal easily achievable. You should look into the Human Powered Vehicle speed records for bicycles. Those are supremely aerodynamic vehicles capable of 80mph using less than half a horsepower! The basic outer shell design could be easily adapted to the scale of your vehicle to allow the same roominess, while still maintaining nearly the same drag coefficient as an HPV. Which is i think below 0.1cd.

On the HPV land speed record vehicles, i've read that nearly half the aerodynamic drag comes from the tiny slits where the wheels are exposed, as it does from the rest of the vehicle body. And these slits are for really skinny bike tires capable of only turning the front wheel an inch to either side. This goes to show how much aero drag an open wheeled vehicle can have in proportion to the aerodynamic body it's attached to.

Your goal is easily achievable, it just depends how much you want to sacrifice to styling.
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Old 06-26-2013, 11:32 PM   #5 (permalink)
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An airfoil or tear drop is the most efficient shape. Concessions must be made to make it practical.
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One simple design trait I see in modern race motorcycles is that the bodywork between the wheels is designed to be just shy of dragging the ground under full compression. This smooths the flow, which wouldn't be an issue with a full streamliner, but also makes the Center of Pressure from the side as low as possible. Having the side CoP lower than the CG of the role axis aids in getting the best self correction in side winds by pushing the bottom of the bike more than the top to lean it into the wind. Trail also takes the side load and through gyroscopic precession leans the bike into the wind as the trail also countersteers the front wheel away from the wind to increase the lean further. You see this in dramatic fashion in the 50mph side wind test of the Peraves. It was blown off line 3 feet but leaned into the wind and automatically recovered in one oscillation.
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A "symmetrical wing with an angle of attack" is created by a bike moving forward at a significantly higher speed than the side wind. Venting the resulting pressure differential also helps reduce the (side) lift that is acting on the bike as Craig found out when he added doors to his streamliner and then had to take the back off when he found it was worse in side winds.
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Nose overhang is bad
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The human powered vehicle has way too much nose overhang which makes them very difficult to ride in side winds of more than a few mph.
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Old 06-27-2013, 02:21 AM   #6 (permalink)
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In fact the the world record holder said just that,about the incredible loss through the wheel openings.Had aerodynamics not been banned,[moto gp racing] by now we would of through real world testing learned what works and what doesn't.There is not a lot of hard data on vehicle motorcycle aerodynamics[mainly because they are void of any aerodynamics],I like, most of the people on this forum pass on little bits I read here and there.I do remember seeing a race version of the Ecomobile,single seat,short wheel base with a tail rudder.Like a arrow with feathers,the longer wheelbase and body workm does help with directional stability.
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Old 06-27-2013, 05:02 AM   #7 (permalink)
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Quote:
Originally Posted by sheepdog 44 View Post

Your goal is easily achievable, it just depends how much you want to sacrifice to styling.

you said it yourself!
You are absolutely correct about the drag reduction measures, but my thinking (in this thread) is about aerodynamic stability. My observation is that there are areas where minimization of drag is critical, and they are mostly competitive events. Here though I hope to focus on issues of stability, particularly from sidewinds and other road traffic. As Scott says, these HPVs look quite difficult to control.

If I were a budding degree student, I would write my thesis on motorcycle aerodynamics. Its such a neglected subject area that anyone with access to reasonable equipment could easily become a world leader.

Maybe now is the time to redress the balance and push back against those FIM restrictions. I have always felt that motorsport governing bodies are misguided, but now it seems they have woken-up to how their rules shape future technology. Formula 1 rulemakers seem to understand how a framework can stimulate innovation, I wish the same was true of the motorcycle world.

I certainly feel that the future of motorcycle design is brighter than it has been, as the world wakes up to the concepts of efficiency and practicality.
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Old 06-27-2013, 05:53 AM   #8 (permalink)
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Regarding the arrow with feathers concept and having tail fins sticking up as a rudder:
This is copied from the other motorcycle aerodynamics thread.
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http://ecomodder.com/forum/showthrea...-16754-12.html
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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.
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Old 06-27-2013, 09:20 AM   #9 (permalink)
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Try checking out Team Delft.

Human Power Team Delft & Amsterdam

Although they are working with streamlined bicycles, they have accumulated a fair amount of data about streamlining in the past few years which may apply to motorcycles as well. AFAIK each iteration has been tested in a wind tunnel. They are also the only other team to reach the 80 mph speed at Battle Mountain. Hopefully a lot of their info is public and available since they are a school.

Quote:
As Scott says, these HPVs look quite difficult to control.
This is correct. The vehicles at Battle Mountain generally race in near no-wind conditions and still can crash due to stability issues. Some of that has to do with rider input but any more than a few mph wind can easily blow these vehicles across the road.
JJ
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Old 06-27-2013, 10:06 AM   #10 (permalink)
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Quote:
Originally Posted by jjackstone View Post
any more than a few mph wind can easily blow these vehicles across the road.
JJ
So we can at least look at these HPV's to see what they are doing wrong.

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