[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.