Motorcycle aerodynamics, a slippery and windy, slope?

[redpoint5]

Originally Posted by Frank Lee

Yeah, I know. I wonder if that sort of efficiency is scaleable and if not, why not?

Efficiency almost always correlates to size. Big businesses are more efficient than small ones, big cargo ships are more efficient than small ones, and big engines... well you get the idea.

Large cylinders are efficient because they have a large combustion volume relative to the surface area of the cylinder walls. Surface area is a source of drag, while volume is a source of power. There are likely other reasons big engines are more efficient, but I'm not really an engine efficiency expert.

[sendler]

[QUOTE=redyaris;257334]

Originally Posted by PeterS

Thanks for the reply ,can I assume that the area on the rear turns you up into the wind in a side gust ? Is this comfortable ?Too quick or is it predictable ?[/QUOTE
The cross wind does not turn the bike, what happens is that the bike starts to roll in the direction of the wind [force] what happens at the same is the "counter steering" of the front wheel which causes the bike to roll in the opposite direction, resulting in the bike leaning into the wind as it goes down the road. The bike is comfortable. I have only had one incident that caused some anxiety, but the fact that it resulted in no great change in trajectory gave me confidence that only a hurricane would be strong enough to cause a dangerous change in trajectory. The bike almost corrects itself, I suspect that most problems that new riders have is panic as the bike rolls.

Has anyone figured out what forces cause some motorcycles to lean into a side wind and some streamliners to get blown over? My CBR250R leans into the wind very well when ridden in a tuck. I had thought that it might be because of the cg on the roll axis being above the center of pressure when view from the side. But the "Gray" is said to behave the same way even though it has crazy amounts of side surface area, way up high.

[redyaris]

[QUOTE=sendler;279793]

Originally Posted by redyaris

Has anyone figured out what forces cause some motorcycles to lean into a side wind and some streamliners to get blown over? My CBR250R leans into the wind very well when ridden in a tuck. I had thought that it might be because of the cg on the roll axis being above the center of pressure when view from the side. But the "Gray" is said to behave the same way even though it has crazy amounts of side surface area, way up high.

The forces acting on the bike/streamliner are the same. Wind pressure. What the rider does to counteract the the rolling of the vehicle, due to the wind pressure, is the same. Counter steering, to get the vehicle to role in the opposite direction to the wind force. At the angle of lean into the wind that the two forces are equil the bike will proceed down the road.
The reason that some vehicles get blown over are; the wind velocity [force], the mass and its location, on the vehicle, the traction available to steer the bike, the lift force and its location on the vehicle...
As the mass of the vehicle goes down the rate of roll goes up. as the lift force on the front wheel goes up the available traction goes down.
in the end when the force of the wind is greater than what the rider can counteract the vehicle will get blown over.
A Honda Gold Wing has more side area than my motorcycle yet it rolls slower and to a lesser degree in side winds, this is due in large part to its mass...

[sendler]

What you say you have noticed, and I agree, some bikes exhibit an ability to self correct for side winds blasts. My Honda CBR250R when ridden in a tuck somehow automatically leans into the direction of the side wind blast, the perfect amount, without any input from the rider. Or rather, the wheels alone seem to get blown out from under the bike by the wind, causing a lateral correcting force from the new lean angle, while the nose of the CBR stays right on line.

[redyaris]

There is no Self correcting on the part of the motorcycle. The motorcycle, on its own, would simply veer off in the direction the wind [or other forces...] was pushing it and fall over on its side.

[sendler]

Originally Posted by redyaris

There is no Self correcting on the part of the motorcycle. The motorcycle, on its own, would simply veer off in the direction the wind [or other forces...] was pushing it and fall over on its side.

There is on my bike. I thought you said yours did this as well but I must have misunderstood you. I ride in stormy conditions quite often and the CBR250R is quite impressive in it's ability to keep the nose of the bike right on line as the wheels float back and forth under it, correcting for truck buffeting and sidewind blasts with no rider input. A very relaxing, addicting experience. Honda did a great job with the ergos and aeros of this bike.

[redyaris]

sendler
For your own safety, read this artical;

Countersteering - Wikipedia, the free encyclopediaen.wikipedia.org/wiki/Countersteering

[sendler]

Originally Posted by redyaris

sendler
For your own safety, read this artical;

Countersteering - Wikipedia, the free encyclopediaen.wikipedia.org/wiki/Countersteering

Uhh? Safety? Thanks for your concern. Actually, I did read that article and many others all day long one day when I got on a kick to try to explain my observations on the topic of sidewind performance and the advantages of riding in a tuck. Many people think the CBR250R (and any other less than cruiser weight bike) is too light to safely ride on the highway. Imagine what they would experience on a front and rear faired 250.

[Ken Fry]

Originally Posted by sendler

I ride in stormy conditions quite often and the CBR250R is quite impressive in it's ability to keep the nose of the bike right on line as the wheels float back and forth under it, correcting for truck buffeting and sidewind blasts with no rider input.

Good description.

Yes, most bikes are self-correcting. A gust causes the steering axis to translate away from the wind side. (The gyro effect of the rear wheel tends to restrain the bike's rolling more than it prevents the frame from yawing, because of the leverages involved. The frame yaws, and the steering head translates.) The contact patch, due to trail, is behind the extension of the steering axis. Therefore, in a crosswind from the left, the steering axis moves slightly to the right. The contact patch is then slightly to the left of the steering axis, meaning that the bike is steering itself to the right. This banks the bike to the left, into the crosswind.

You can take your hands off the bars entirely, and this feature works just fine (provided you don't have a bike prone to tank slappers)

If you really freeze your muscles and clamp onto the handlebars with a death grip, you can defeat this built-in trait. Taking the weight off the grips by tucking and putting your chest on the tank makes the effect more pronounced.

On some bikes, cranking in a lot of steering damper will interfere with this self- correction, and the bike will tend to wander.

[sendler]

Great! An explanation. I knew this effect was so pronounced that someone would know about it. So it is really just as simple as trail. I guess this is why we need the side wind pushing on the bike frame and not the front wheel its self? So full streamliners that are having trouble with sidewinds can increase trail to get more self correction at the expense of having slower and heavier steering transitions in chicanes? I have seen photos of land speed bikes with the front, top, quarter of the front wheel fully faired and the back half fully open. Does this help to force the counter steering movement in a side wind or is it just for the aerodynamics?
.

Originally Posted by Ken Fry

Good description.

Yes, most bikes are self-correcting. A gust causes the steering axis to translate away from the wind side. (The gyro effect of the rear wheel tends to restrain the bike's rolling more than it prevents the frame from yawing, because of the leverages involved. The frame yaws, and the steering head translates.) The contact patch, due to trail, is behind the extension of the steering axis. Therefore, in a crosswind from the left, the steering axis moves slightly to the right. The contact patch is then slightly to the left of the steering axis, meaning that the bike is steering itself to the right. This banks the bike to the left, into the crosswind.

You can take your hands off the bars entirely, and this feature works just fine (provided you don't have a bike prone to tank slappers)

If you really freeze your muscles and clamp onto the handlebars with a death grip, you can defeat this built-in trait. Taking the weight off the grips by tucking and putting your chest on the tank makes the effect more pronounced.

On some bikes, cranking in a lot of steering damper will interfere with this self- correction, and the bike will tend to wander.