Motorcycle aerodynamics, a slippery and windy, slope?

[Hugh Jim Bissel]

Originally Posted by renault_megane_dci

Do we need this much gear reduction ?
The 250 crowd is switching from 1/3 final to close to 1/2 already without particular issues in 1st gear.
Unless you were to change to a diesel engine (and then the motorbike gearbox sounds like a good idea) I can not figure out the need for two gearbox in series on a motorbike engine ...

As I recall (been a few years since I've ridden it) my ninja 250 with (as far as I know) stock gearing was spinning well over 10mph/1000rpm, to the point of being somewhere around 8-8.5k at 70mph. Even without any sort of fairing I would have needed another gear or two to get me to the point of cruising at close to full throttle. That could have been done just by final drive gearing; I recall starting off from 2nd a few times, no problem on the flat, but add a hundred pounds of full fairing streamlining and a steep hill, and getting going without stalling might take some work. And once it's been streamlined I'd think the engine would need to be spinning half as fast (or slower) as stock in top gear to cruise at close to full throttle. If you just do that with final drive, it would be similar to starting in 3rd or 4th on a stock bike!

Whenever I finally get a round toit (all the ones I've found so far are square, I may need to cut some corners ) and build a streamlined bike (or car for that matter) I'd like to have it geared where on the highway top gear at full throttle still needs a tail wind to hold a constant speed. Then if there is a tail wind I'm at maximum efficiency, or if no wind or a head wind I can pulse and "glide" between the top few gears, or just downshift as needed to hold my speed. This will take a lot of gears! (Hmm, I wonder if I can shoehorn a big rigs 18 speed under the hood of my vibe! )

[sendler]

Originally Posted by Hugh Jim Bissel

Whenever I finally get a round toit

Having two trans in series on any diy modified vehicle is a complete pipe dream. Changing the final drive on a motorcycle is a perfectly acceptable solution for reducing the cruise rpm.

[renault_megane_dci]

I might be wrong but your target throttle position on cruise is more 75% rather than full throttle because there might be richer setup for cooling / reliabillity at full throttle.

Anyway, I think I understand your point of view but I feel if you want to keep revs down with at least some efficiency, you need to alter engine internals.
Best exemple is the EL 250 (custom declination of EX) is around 45 mpg stock when a GN is 70 and up.
Their engine is not efficient FE wise, it is tuned for power.

[Grant-53]

To take up from previous discussions, the handling of a vehicle whether car, motorcycle, or bicycle is a function of weight distribution, steering geometry, aerodynamic forces. The blending and balancing of the forces is what we, as designers, strive to achieve for a given application. A low nose and a high tail are general terms that should be quantified and considered with respect to weight distribution. A rider's center of mass is usually just above the belt but back angle on a bike can moves the CG forward and aft. A scooter's CG is typically lower and further aft than a sport bike. Trail and head angle are set to give the desired responses. Adding a full fairing affects the setup with fin area applying forces at the center of pressure, the point where the net affect is applied.

[sendler]

It's about time for this thread to pick up again. Much has been discussed in various other places since the last posts here. Some of my conceptions:
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Motorcycles automatically lean and corner into a side wind simply due to the trail in the steering geometry as shown by the dramatic correction of the ecomobile video. As long as the handle bars are not burdened with a terrified rider's rigid arms.
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Keeping the CG on the roll axis high accentuates this to a good effect.
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Keeping the side CoP low helps push the bottom of the bike out from under the top to lean into the wind to a good effect.
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Streamlining a motorcycle turns it into a vertical airfoil which generates horizontal lift when sidewinds combine with the headwind of forward motion to create an angle of attack. This increases the side lift beyond what a normal, dirty motorcycle would see.
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A streamliner only sees a crosswind when it is getting blown off the kick stand. At highway speed, even a 30 mph side wind gust will combine with the head wind of the forward motion to form a vector at a slight angle to the nose. This angle of attack causes the vertical airfoil shape to create lift (side).
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The greatest lift of an airfoil is located just behind the max thickness. An interesting experiment showed the best place for the max side lift to occur to activate the self correcting aspect of the trail of a two wheeled vehicle is actually at the head tube. Not well behind the center of gravity as would be best for an arrow. Because bicycles have two tires anchored to the ground with front steering trail and turn by leaning. A bicycle was ridden while a helper traveled along side tugging on a string that was alternately attached to the bike at various places. When attached to the seat post or the luggage rack above the rear wheel, as in a long, high tail, it was easy to pull the rider off balance and out of line. The higher, the worse. When attached to the head tube, the bike self corrected from the effect of it's trail to lean against the string as hard as it was being pulled. Unfortunately, the max lift ends up being well behind the head tube in practice. So Craigs streamliners use a vent across the lap of the rider to equalize the pressure differential. Another option for a fully enclosed machine would be a spoil strip running up the nose.
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An easy mistake to make would be generalizing two wheeled vehicles, multi wheeled vehicles, and aircraft. Motorcycles turn by leaning after COUNTER steering. Cars turn by steering. A vertical stabilizer on the tail of a motorcycle that still has traction from both tires will be counter productive to cross wind stability.
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Camber thrust - Wikipedia, the free encyclopedia
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Cornering force - Wikipedia, the free encyclopedia
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Precession - Wikipedia, the free encyclopedia

[sendler]

Excellent info from Aerohead shows that Kamming the tail starts a big loss at anything more than 10% of the airfoil length. He has shown that even a very short fineness airfoil is much better than a long airfoil Kammed at 40%. And a round nosed cone with straight sides is fairly poor also compared to even a much shorter airfoil. We need to make room for the license plate. I would shape the side view to match the aero template starting just in front of the front wheel, truncating at the back. And make the top view as a non truncated short airfoil along the bottom of the bike, tapering up to a 9 inch wide Kamm at the top of the tail for added storage, and a place to mount the license plate and rear flush mount led turn signals.

[Grant-53]

As to multiple transmissions both Kreidler 50cc bikes and the Austin Healey Sprite have used two gear boxes. This would only be needed if the power to weight ratio is very low and the torque curve peaky.

I have a new camera so I can start posting better photos and drawings. The desired handling of a two wheel vehicle can be 'tuned' by adjusting the aero loads both fore and aft as well as top to bottom. When I describe a high tail I mean CoP above the CG but not necessarily a fin above the rider's shoulder. The question becomes how stable vs how responsive do we want. The next consideration is the resultant of cross wind and forward net speed as Cd varies with the effective yaw angle and lean angle. Right now we are looking at the AeroTech Bonneville fairing which is designed for high speed (150 mph) and low cross wind conditions (5 mph). In contrast is the Vetter style which are designed for more crosswind (50 mph) and highway speeds (85 mph).

I will try to make some drawings of the shell I am looking into using and perhaps interpret the above description as well. Then we can have a clearer concept of the vector forces acting on a bike and rider.

[sendler]

I don't understand why you would want CoP higher than CG on the roll axis. This is completely contrary to my way of thinking. The higher the CoP, the more the bike will blow over and lean with the cross wind. So now the wind is not only blowing the bike to the side but making it steer off line as well.

[sendler]

Here is a good example of what you don't want. The Honda factory streamliner from the Vetter fuel economy challenge of 1982. It was flipped by the wind.
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[jkv357]

Originally Posted by sendler

Here is a good example of what you don't want. The Honda factory streamliner from the Vetter fuel economy challenge of 1982. It was flipped by the wind.
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What do you see as the biggest problem with their design? (other than it's ugly as....)