BMX Science: The Physics of a Stall-to-Tailwhip
Take a glance at the BMX riders spinning and whipping through Austin during the X Games and you’ll see tricks aplenty. And if you look at the right time you may even see Canada's Drew Bezanson (among others) rip off a signature move, a stall to tailwhip.
And, as you might expect, the successful completion of the trick doesn’t rely solely on nerve and nifty riding—although it takes plenty of that too—but rests solidly on a foundation of sound science. Matthew Christensen, Red Bull High Performance’s air awareness coach, head coach of the U.S. Freestyle Ski Team and member of three Olympic teams, explains to Edge just how Bezanson can successfully leave the ramp, hit the wall horizontally and whip the bike while making it look oh so effortless.
From the very start, Christensen says, Bezanson’s upper body follows a set trajectory based on his forward speed and the curvature of the ramp. “If you watch the video, you can see that his center of mass, his upper body, tracks pretty consistently along the same trajectory from ramp to wall,” he says.
As Bezanson heads up the ramp, which he traverses slightly off vertical to start his motion heading in the proper direction, he leans forward and twists slightly to the left with his head and shoulders. This motion, while not ejecting his center of mass from its trajectory, serves to “unload the rear wheel” and lift it from contacting the ramp. “This gives him the proper amount of angular momentum to spin a quarter turn to hit the top feature,” Christensen says. “He uses his lower body to ever so slightly accelerate the spin just before coming in contact with that top feature.”
But Bezanson can’t just slam the wall. No, that would kill all his momentum and render his tail-whip undoable. Instead he uses his legs to absorb the energy of the impact.
“By retracting his forward momentum against the feature he is able to stay in contact with it longer,” Christensen says. By pushing his legs into the pedals, he transfers energy to the rear wheel, allowing him to pop the front wheel off the wall before the transition back downward. Getting the front wheel off first allows him to line himself up for a smooth landing in line with the ramp, even with a tail-whip included on the way back down.
Keep in mind, though, that while in the air, Bezanson applies the brake to stop the rear wheel at all times.
“Spinning wheels act as a gyroscope and can effect in-air rotation,” Christensen says. “Additionally, the sudden stop of the rear wheel from spin enables that torque to be put into the bike to give some additional torque on take off versus having to put that into the system through body movements.”
Effortless? Only if Bezanson follows the science.
Tim Newcomb covers stadiums, design and gear for Sports Illustrated. Follow him on Twitter at @tdnewcomb.