Motorcycle aerodynamics, a slippery and windy, slope?

[Frank Lee]

Is the Lit anything beyond vaporware yet?

[Ken Fry]

Originally Posted by Frank Lee

Is the Lit anything beyond vaporware yet?

I'm really not sure. They have some gyro-stabilized stuff they can kick around the shop floor. I'd like to see a remote-controlled version driving around, at least. They talk about 1300 lb feet of righting moment... but that means a pretty beefy structure to transfer that to the chassis when you want to flick it into a turn.

The guys are definitely creative and bright, but they seem to be going in several directions. Overall, my hunch is that they will pull it off, but I don't have any real rational reason for that.

[Frank Lee]

At least one of them came from Aptera.... A.D.D. all over again???

[Ken Fry]

Originally Posted by Frank Lee

At least one of them came from Aptera.... A.D.D. all over again???

Ewww, I hadn't hear that.

[redyaris]

Originally Posted by Ken Fry

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.

I do not dispute the effects that both you and sendle discribe.

So we have all accepted that the wind force from the left is causing the bike to roll to the right, about a roll axis; which is at the ground level along a line that goes between the contact patches of the front and rear wheels.

The issue can be reduced to the sum of the moments in the wind direction [to the right or clockwise] and the sum of the moments to the left [oposite to the wind direction and to the left, or counterclock wise].

The moments to the right [clockwise] are due to the wind, gravity and a small amount of prosesion caused by the rear wheel that cuases a yawing moment about the vertical axis to the right.

The moments to the left [counterclockwise ]are due to trail and procesion as the front wheel is rotated about the steering axis.

It is my contention that the sum of the moments in the clockwise direction
[due to wind and gravity]; without rider steering input, will always be greater than the sum of the moments in the counterclockwise direction [due to trail and prosesion].

Therefore bikes are not self corecting

[sendler]

Originally Posted by redyaris

I do not dispute the effects that both you and sendle discribe.

So we have all accepted that the wind force from the left is causing the bike to roll to the right,

You lost my acceptance right off the bat. Maybe you mis spoke. I have observed that the wind FROM the left causes the top of the bike to roll TO the left. The top of the bike rolls into the wind. Or more apparently, due to the distribution of mass on the roll axis and the distribution of side area presented to the side wind which is much greater below the roll axis than above when the rider is in a tuck, and the fact that the new counter steering angle generated by the trail, is with the wind, the bottom of the bike rolls out from under the rider WITH the direction of the wind. This is in the roll axis. There has been no yaw in the picture yet. And there will be almost no yaw in the discussion until you put a big tail on the motorcycle. And maybe even still not with the tail unless you are forcing the rear wheel to slip sideways. But that is a whole different discussion.

[Ken Fry]

Originally Posted by redyaris

I do not dispute the effects that both you and sendle discribe.

I should hope not... we are gods. And Sendler is, in addition, a poet.

So we have all accepted that the wind force from the left is causing the bike to roll to the right, about a roll axis; which is at the ground level along a line that goes between the contact patches of the front and rear wheels.

We don't agree on this point. If the wind is from the left, the wind force (applied through the bike's mechanism) is causing the bike to roll counterclockwise, in other words to the left. You may mean that the wind force should roll the bike to the right, or that it should tend to roll the bike to the right (clockwise when viewed from behind), but that is not what happens.

The moments to the right [clockwise] are due to the wind, gravity and a small amount of prosesion caused by the rear wheel that cuases a yawing moment about the vertical axis to the right.

The moment to the right is due only to the wind, and is reduced by a low rider position.

The moments to the left [counterclockwise ]are due to trail and procesion as the front wheel is rotated about the steering axis.

The sequence is this: the steering head gets pushed to the right because there is little to resist that action. The bike cannot roll easily to the right because of the gyro effect of both wheels. The steering head motion causes the bike to steer to the right, causing the bike to fall to the left. The steering head motion, having caused the bike to steer to the right, also causes gyro procession at the front wheel, the effect of which is to amplify the left-leaning moment. So gravity is causing the bike to fall to the left, and procession is causing it to fall to the left.

Bikes are very sensitive to steering, with just a few ounces of handle bar force causing hundred of pounds of gyro-stabilized machine to bank. When I road raced, I would set up for a turn by sitting way off to one side hanging a knee out, and could otherwise move around on the bike with impunity, because they are so stable in roll. Wasn't until I moved the handle bars almost imperceptibly that the bike actually banked quickly.

A little nudge from the wind is all it takes to make this work. If you sit tall in the saddle or straight arm the bars, you can defeat this automatic self-correction. Some bikes have negative cross wind stability. But Sendler's bike has friendly crosswind stability when he keeps his body low. Most of the bikes I've ridden (which have tended to be cafe racers) have the same characteristic.

[Frank Lee]

In aviation, an auto-control action (designed in stability, not to be confused with autopilot) is said to be self-correcting if said action has any countering effect on the external force being applied to the aircraft. It doesn't mean the "self correcting" action achieves perfect stasis without any control inputs (although with proper trim it's pretty darn close); all it means is that the tendency is for stabilization and NOT for ever more extreme deviations or oscillations from the intended path.

Just like bikes, which are self correcting (unless you get a tank slapper).

P.S. If bikes were not self correcting, a crosswind from the left would make the bike lean to the right and want to go in the ditch, badly.

[sendler]

I can see where some of the confusion is coming in. I thanked Ken for the link he posted out of a great appreciation for his (and everyone's) generosity, before reading it. The article does tend to get things turned around a bit at times. The initial " Push left, lean left, go left" is correct as we all know. Since I generally lay on the tank, I quite often ride with my left hand on the top of the left fork to eliminate the parachute on that side. So I am guiding the bike with only my right hand. This makes the effects of counter steering very obvious as I change lines with slight little pushes or pulls on the right grip. A fun exercise I would recommend to anyone. Most of the rest of the article I'm sorry to say seems a bit backwards. Which is not too surprising. Motorcycle dynamics is very complicated and even as yet, there is no consensus on a "unified theory". The main page at Wiki even seems to contradict itself from one paragraph to the next regarding gyroscopic precession.
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But, my bike does self correct it's lean into a side wind. I don't mean to say that it is perfectly balanced to the point of riding a straight line forever. But it's initial reaction is the correct direction and seemingly instantaneous and without sticktion.

[sendler]

Originally Posted by Ken Fry

The Ecomobile was available with a rudder.

Back to the topic of the windy slope. The concept of an airplane like rudder on the back of a full aero motorcycle makes an interesting thought experiment for my stance on the futility of an extended fin behind the rear wheel for the purpose of straight line tracking in side winds. A big fin in the back makes the bike into an arrow when it is going straight down the road except for the fact that arrows don't have two tires stuck to the road. The rear tire is fixed and can only roll straight ahead so even with the tail fin, your bike is not really like an arrow. The tail fin can only push the rear of the bike with the wind but it is forcing the rear tire to go sideways in order for anything to happen. Now look at the rudder sticking up high above the cg and far behind it like the low fin is. Which way would you actuate the rudder if you were flying the bike and got hit by a side wind? Would you actuate the rudder to amplify the force of the wind at the rear as you are trying to do with the tail fin? Again, this is not an airplane. The rear wheel is stuck to the ground and the most effective way to generate the lateral force needed to push back against the side wind is to LEAN the bike so it is camber thrusting and steering into the wind. You would actually use the high mounted rudder in the opposite direction that the tail fin provides by trying to rudder the top of the bike, even though it is behind the rear wheel, to roll INTO the wind. Think about it. Maybe counter intuitive. The rear of the bike to roll INTO the wind. Do tail fins help anything or are they just adding pressure for the side wind to blow you off of the road? I also wonder what any yawing action that did make it sideways past the tracking rear tire would do to the trail in the front geometry. If you could have the cg at the steering axis it would tend to stay neutral like the arrow. But the cg will probably be closer to midway between the wheels with a feet forward design. The yaw will pivot on the cg, forcing the steering axis laterally into the wind, causing the trail to counter steer and precess and lean the wrong way, with the wind.
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Just thinking out loud in the spirit of advancing the design concepts of a streamlined motorcycle which I believe in 100% and will one day own. I think Allert has the better handling design concept by possibly foregoing perfection in frontal drag to get safer rideability (the weather is terrible where I live but I still ride right through it every day) although his design is very high in the back to gain luggage space.
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I would like to see Craig and Allen truncate their tail fins into a Kamm to see if it is better in side winds as it may still be just as aero. And much easier to back out of the garage.