Motorcycle aerodynamics, a slippery and windy, slope?

[Grant-53]

A rudder would tend to steer the bike into the wind but would also increase drag laterally. A 5:1 length to width body with a tail 25% Kamm has good possiblities. See photos of Trek bicycle aero tubing.

[sendler]

Originally Posted by Grant-53

A rudder would tend to steer the bike into the wind

Probably not like you would think. Especially if it is sticking way up high as on the Ecotracer. That rudder would have to be used the opposite way to roll the top of the bike into the wind.

[Grant-53]

I am looking at a rudder that is top to bottom. The other possibility is to use separate rudders top and bottom to affect lean. There has been lots of discussion about steering conditions but I don't think we have gotten a good handle on the physics of cornering and the additional effects of control surfaces. There are some fairly complex mathematical models for two wheel vehicles available. I will continue with experiments on bicycles first where risks of injury are lower if the unexpected happens.

[sendler]

Originally Posted by Grant-53

I am looking at a rudder that is top to bottom. The other possibility is to use separate rudders top and bottom to affect lean. There has been lots of discussion about steering conditions but I don't think we have gotten a good handle on the physics of cornering and the additional effects of control surfaces. There are some fairly complex mathematical models for two wheel vehicles available. I will continue with experiments on bicycles first where risks of injury are lower if the unexpected happens.

Rudders on the tail of a motorcycle are quite questionable for any number of reasons on anything shy of a land speed machine which is more like a missile at that point. How will you generate any control force in normal riding when the air speed from the front is only slightly higher than the potential air speed gusts from the side that you are trying to combat?

[jkv357]

I would expect that anything sticking up above the rear wheel's contact patch would have significant leverage, and would easily force the cycle to lean drastically with any type of side load - the taller it is, and the farther behind the contact patch, the more effect it would have.

On the highway, passing a large truck, I would think the air deflected off of the front of the truck would give you a significant sideways thrust and cause a dangerous handling condition with any type of rudder system.

In Land Speed Racing, rudders work because they add straight-line stability, and the amounts of side loads are insignificant compared to the forward velocity (as sendler noted above).

[sendler]

Actually, on a motorcycle at 70 mph, controlling the roll, not the yaw, with horizontal and opposite moving elevons might make more sense than a rudder. If any of it makes sense.

[sendler]

I am thinking about adding a land speed front fender to my CBR250R in lieu of a full dust bin nose and found these two companies.
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http://www.airtech-streamlining.com/.../landspeed.htm
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Fender, Front, Land Speed, Drag Race, Suzuki Hayabusa - Tiger Racing
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[Grant-53]

The LSR1F would be my choice so I could fine tune the proportion of area fore and aft of the steering axis. Check out the GSXR side car system.
The faired motorcycle is very much like a low flying aircraft but without wings. Lift or down force is generated by the shape, use of flaps, and angle of attack of the body. Airlerons control banking or roll and the rudder controls turn or yaw. If I understand it correctly, the countersteer is used momentarily to get the bike to lean or roll into the turn. To counter a crosswind may require roll as well. On cars, most of the stability issues are fixed by generating more downforce but there is a trade off when drag is increased. On bicycles, very little countersteer is needed. The question comes down to how much stability versus agility is desired. Canards and flaps mounted horizontally are greatly effected by the lean angle. Movable vertical control surfaces may give better control of roll and yaw for canyon carving.

[Cd]

I skimmed over the many pages here, and just wanted to ask if using window screen on the sides of a fairing / tail would allow sidewinds to have less effect on stability while still providing a surface for air to slip over from straight onward.

[Grant-53]

I have wondered the same thing about screens over grill work. From what I have seen the higher the wind speed "overloads" the screen so sudden side gusts would not pass through. Don't be afraid to experiment. I have seen adjustable side deflectors added on a BMW touring fairing to direct flow around the rider or onto the legs in hot weather.