Physics is not a crashing bore
Almost all of my dirt-riding experience consists of times that I accidentally left a paved surface, and most ended badly. So, regular Backmarker
readers may roll their eyes at the thought of me writing about Supercross. Still, the big motorcycle news last week was Bubba Stewart
finally returning to form.
Stewart’s win in Atlanta
(just) prevented him going a full calendar year without a win. It’s hard to believe that a guy who’s third in all-time SX wins is at risk of being thought of as a disappointment, but it is what it is.
Being a road-racing guy, I wasn’t paying attention as Stewart came up through the amateur ranks. And, I still lived in Europe when he arrived on the 125 pro scene. So I read about him, and heard from about him from friends, who generally felt that they’d seen some kind of motocross messiah.
I was, like, “Sure.”
Then, I finally caught coverage of some random outdoor national in which Stewart, on his 125, lapped faster than almost anyone in the 250 class. What blew my mind was the way he was so much faster through the air than other riders. Time after time, he passed people in mid-air, on a visibly lower and faster trajectory.
Now, every competitive motocross rider does the ‘Bubba Scrub’ but 10 years ago it was as transformative in motocross as dragging a knee was, after Kenny Roberts popularized that technique in road racing. (Come to think of it, I bet Roberts is pissed that putting your knee on the deck is not called the ‘Roberts Drag’.)
Most people intuitively got why shifting weight to the inside and dragging a knee worked. What interested me as an armchair expert was, while the effects
of the Bubba Scrub were obvious - a lower trajectory that reduced flight time at any given takeoff speed that was a huge advantage on big outdoor tracks, and a shorter flight distance at any given speed that was just as important in Supercross
- no one seemed to know why
As an aside, it’s interesting to note that while KR was the first high-profile rider to systematically and intentionally drag his knee, he wasn’t the first to do it at all. Jarno Saarinen came into Grands Prix after ice-racing in Finland. Back then, ice racers achieved lean angles that GP riders could only dream about, and Saarinen got used to feeling his lower leg skimming the ice. He occasionally dragged his knee while road racing, and had he not been killed before his time, he may well have been credited with demonstrating the technique’s advantages.
It’s safe to say that many motocross riders accidentally scrubbed before Bubba “invented” the technique. I found this cool Eurosport coverage of a race in which Tallon Voland (rider #3, at around the 11:00-minute mark) does a perfect Bubba Scrub
. The thing is, the race was in 1997; Bubba was 12 years old.
Given Bubba's tendency to over-ride his bike
, I imagine that he discovered his signature move 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 and attempted to start turning the following corner before even leaving the jump. Regardless, he basically crashed 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.
By 2005, Stewart had stepped up to the premier class, and ‘how-to-do-the-Bubba-Scrub’ stories were populating the web. Even Transworld Motocross, however, still characterized it as “the art of scrubbing speed off jumps like James Stewart” as if Bubba’s advantage came from his ability to stay on the gas all the way up the the jump and slow down at the last second, when it was obvious that the real advantage to the technique was carrying more speed in the air.
Since the cat was out of the bag, Bubba went on to tell Transworld how he did it, but it was pretty clear that while he had some idea of how
he was pulling the move, he didn’t know why it worked
. And, he still doesn’t (unless he’s just read this). The reason I say that is that years later, Red Bull released this video featuring Bubba in hi-def, ultra-slo-mo.
In Red Bull’s video, Bubba says that the scrub 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 find umbrella girls waiting for them back at their motorhomes. In fact, we don't even rate motorhomes. Come to think of it, I'd rather be you, too.)
After all this time, I’ve still read very little in the way of a coherent explanation of why the scrub works. I recently Googled a bunch of phrases like “How does the Bubba Scrub work?” and “Bubba Scrub physics”, and even came across a thread about it on the Physics Forums website, but in the absence of a better explanation, I think it’s time that an asphalt scholar took a shot. Hence, this column...
I suck at motocross, but I was pretty good in physics. That’s why I know the Scrub's got nothing to do with aerodynamic drag. With average SX lap speeds of about 30 mph, drag is minimal. Anyway, there's no reason why drag would be lower with the bike horizontal compared to vertical. In fact, Red Bull’s video clearly shows that in the mid-air recovery phase, Bubba’s bike is broadside to the direction of travel, maximizing aero drag.
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 when he was arrested
after impersonating a police officer and attempting to pull over... a real cop.
This nice graphic (thanks, Wikipedia Commons) illustrates the way that at all launch angles less than 45 degrees, a lower trajectory results in a shorter flight. That means that at any given speed, a lower trajectory allows a rider to land and get on the power sooner. It’s equally true that a lower trajectory allows a rider to hit jumps faster without overjumping his landing. The much crappier drawing below is my own illustration of the way a scrub lowers the trajectory vis-a-vis the launch ramp.
But the laws of physics
are not like Florida Statutes. You 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.
A gifted rider like Bubba 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 combined motorcycle-and-rider’s center-of-mass from following a ballistic trajectory that is defined by Newtonian physics.
So why does the scrub actually
work? It's all about getting a lower trajectory off any given launch ramp. Since all MX and SX launch ramps are shallower than 45 degrees, a 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. That meant that the bike-and-rider combined center-of-mass left the jump on a trajectory parallel (and a couple of feet above) the angle of the launch ramp. So pre-Bubba, the angle of the launch ramp, and the angle of take-off, were the same.
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 “crashing” 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 it breaks contact with the track defines the bike's ballistic trajectory in the air.
I've seen 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.
Until Bubba came along, motocross racers had always launched off a jump on a trajectory defined by the jump itself. He can't break the laws of physics any more than I can, but he did find a way to redefine his launch trajectory. That was Bubba's genius.