Physics behind a motocross whip

Question

I've been struggling for quite some time to find an answer as to why motocross motorcycles mysteriously return or swing back to the original position after a "whip".

Please refer to the following links if you don't know what a whip is: Whip1 whip2. Please note that there are different types of whips, but they all share a swing, some more angled than others, but they all turn and then return to a straight landing position. Also, huge whips like the ones in the video are more stylish and can cause some confusion, the effect is actually clearer in more amateur whips where the bike just goes sideways and then returs with no fancy movements at all.

It is important to know that to do a whip, the rider must lean the bike just before takeoff, kind of carving before the lip of the jump.

My first thought was that gyroscopic effects might be responsible, but no, I ride myself and I can turn the handle in mid-air with no noticeable effect (some effect might exist, but it's just not enough). This can also be debunked since some riders do same sided whips by turning the handle in different directions.

My second thought was that the carve needed to lean the bike generated transversal forces on the suspension that when in the air, would turn the bike, although this doesn't explain how it swings back.

My current idea is that we always jump with a certain amount of throttle (force on the rear tire) and that when the bike is leaned, this force is not aligned with the center of gravity, it creates a torque. This torque will turn the bike sideways in the air and (this is the part that I'm guessing) maybe the Dzhanibekov effect or some sort of sinusoidal spin returns it?

I wanted to create a formula for whip magnitud but I'm struggling with the combination of the initial torque, projectile motion and this other mystery effect that makes the bike swing back.

Any help would be greatly appreciated!

Answer 1

About the bike turning back to straight landing position:

One question is: how much air resistence is there on the bike when bike is sideways? If the center of aerodynamic drag is to the rear of the center of mass then the air resistence will tend to push the bike back to straight.

To find the center of mass of bike plus rider you would have to manufacture a way of suspending the bike on a cable. If bike doesn't pitch (up or down) then you know the center of mass is somewhere along that vertical line.

To assess the center of aerodynamic drag you would need a strong wind, and then see whether that wind tends to turn the suspended bike.

If the air resistance explanation falls short then I'm stumped. In the extreme whips in the video's it looks as if the bike is whipped all the way to close to going backwards. It looks as if they could choose to go on and do a 360.

Actually, is doing a 360 possible? If so, does footage of it exist? Comparing the two moves would be interesting, I think.

Is there also footage of whips shot with a camera at some distance, and perpendicular to the direction of motion of the bike?

The reason for asking that: Rhett Allain (youtube channel: dotphysics) has put out many video's in which he uses software that analyses motion. Example: Video Analysis Physics: World Record Pole Vault. If the camera itself is non-moving Rhett uses the motion with respect to the background to get a set of data points, and then he can do motion analysis.

It may be that Rhett will find the motorcycle whip an interesting case to make a video about. (That does require a video that lends itself to getting data points from it.)