Tuesday, January 10, 2012

Bubba Stewart, idiot savant of physics

Red Bull just released a promotional video of Bubba Stewart. It's the sort of hi-def, ultra-slo-mo stuff you'd expect. Bubba narrates it, providing a pretty bland and dull commentary. What interests me about it is his discussion of his greatest contribution to motocross, the 'Bubba Scrub'.

The scrub move gave Bubba a huge advantage coming up as a motocross racer. In the long travel suspension/SX era, speed over jumps is an important factor in race success, especially in SX races. Course designers make the challenge harder, by contouring landing areas or locating corners in places that make it more complicated than just hitting the takeoff ramp at a higher speed, flying further and higher, and landing with a higher retained speed.

I remember watching Bubba as a young 125 racer, and he was so much faster through the air than other riders that it seemed as if a different set of laws of physics applied to him. Time after time, he'd pass people in mid-air, on a visibly lower and faster trajectory.

While no one really seemed to know just how Bubba's trick on the takeoff ramps worked, all his rivals quickly realized that it did work, and before long it was part of every motocrosser or supercrosser's arsenal. On a big outdoor track, the scrub allows riders to hit takeoff ramps at higher speed without launching themselves into space; they get back to the ground faster, where their rear wheel can again begin transmitting power. On a supercross track, where landing areas are typically tighter, they can hit the jump harder without over-jumping.

The Red Bull video doesn't really have great shots of the key moment in the scrub -- which is when the rider essentially does a whip on the takeoff ramp, virtually crashing his bike into the ramp at the moment when it goes ballistic. But there are any number of great videos illustrating the technique, such as this one...

Bubba's own narration makes it clear that he has no idea how it works. He claims that it works by reducing aerodynamic drag in mid-air. That's why he's a motorcycle racer and I'm a frustrated genius. (Trust me, Bubba, you'd rather be you. Geniuses don't have groupies who'll boink them in their motorhomes. In fact, we don't even rate motorhomes. Put that way, I'd rather be you, too.)

But, the Scrub's got nothing to do with aerodynamic drag, for several reasons not the least of which is that with average SX lap speeds of about 30 mph, drag's minimal and that there's no reason why drag would be lower with the bike horizontal compared to vertical. If you watch the video clip above, you'll see that in fact during the mid-air recovery phase, his bike's turned broadside to the direction of travel, for maximum aero drag.

So why does the scrub actually work? It's all about getting a lower trajectory off any given launch ramp.

As noted, all else being equal, lower trajectory is faster for two reasons: It will get you back on the ground, with your rear wheel driving forward, ASAP. And/or, it will allow you to hit a jump faster without over jumping the landing area.

Pre-Bubba, motocross racers pretty much always ran up the launch ramp with their bikes straight up and down (or as straight as they could get 'em.) That meant that their bikes' center of mass left the jump on a trajectory parallel (and a couple of feet above) the angle of the launch ramp. (I'm simplifying ever so slightly here, and describing a sharp-edged ramp. It's more complex but the same principles apply over a jump with a rounded profile. Also, amateur physicists please note that I'm using the term 'bike's center of mass' but really mean, 'combined bike and rider center of mass.') (God, can I get one more parenthetical comment into this paragraph?)

Bubba himself accidentally gets part of the scrub explanation right in the Red Bull video when he says that you have to crash the bike as you leave the jump, and have faith that you can gather it back up and land on the wheels. That's pretty much what happens.

When a rider does the scrub, he's basically pushing his bike's center of mass down towards the track at the moment that he breaks contact with the ground and takes to the air. The path that the center of mass is taking through space at that moment defines the bike's ballistic trajectory in the air.

It's critical to understand that if the rider grossly mis-timed his scrub, and scrubbed well before the takeoff lip, he'd simply crash his bike into the ground. I've seen videos in which riders actually drag their cases off the jump. IE, at the moment they go ballistic, their bike's center of mass is at least a foot lower than it would be in the normal riding position. Considering that that one-foot drop happens on the launch ramp -- and as long as it's in progress at the moment the bike goes ballistic -- it effectively reduces the ramp angle by several degrees.

Anyone who remembers the days when Bubba first rode the 125 class (often posting times as fast or faster than the best 250 riders!) will recall seeing him pass many riders in mid-air; going a gear faster while magically traveling on a much lower trajectory.

Given Bubba's tendency to over-ride his bike, I imagine that this technique was simply discovered when he realized that he was on a takeoff ramp and committed to the jump, while traveling at a speed much too high for the situation. Either he'd carried too much speed out of the last corner and was still turning the bike on takeoff ramp, or he realized he was about to over jump a landing area followed by a corner and attempted to start turning the following corner before even leaving the jump. Regardless, he basically low-sided at high speed over the lip of the jump. Once in mid-air, his instincts took over; he'd already landed hundreds of show-offy whips, and this was no different. The Bubba Scrub was born.

Now, I presume that Bubba knows that, sometimes, you can break the law and just get away with it. I guess that's what he was hoping would happen last year, when he was arrested after impersonating a police officer and attempting to pull over... a real cop.

But the laws of physics are not like the law. You not only never get away with breaking them, you just can't break them, period. The flight trajectories of things like motorcycles -- with no appreciable lift or motive power in the air, it's a projectile -- are clearly defined by simple equations that can be found in any high-school physics text. The rider can do things that will adjust the bike's pitch, roll, and yaw in mid-air, but there is nothing he can do to prevent the motorcycle's center of mass from following a ballistic trajectory that is defined by simple physics.

Bubba can't break those laws any more than I can, but he did (probably accidentally) find a way to redefine his launch trajectory. Until he came along, motocross racers had always launched off a jump on a trajectory defined by the jump itself.

Realizing that he'd found a way to change that trajectory was Bubba's genius. I guess it doesn't matter whether he knows why it works or not.

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