Science Can Kill

text: Tabitha Ann
art:   Mike Force

The Fear sets in. You hold on tight, but your car jerks over the pavement just inches beneath your ass. The seat trembles. Too. Fast. Must. Hold. On. A quick glance behind prevents grabbing for the brakes. The pack is on top of you. You’ve got yourself out in front and it’s going to stay that way. Bones rattle. Teeth chatter. The Fear. Should you slow down? Make for the brakes? If you brake, you’ll loose this race. Your tires skim the tires of another car. The brakes. You reach for the brakes and then stop yourself. You didn’t get into this position by being cautious, why start now? Fuck it. You start to turn, and then…

Four spectators loom over you. One is saying your name. Ears. Ringing. Why are you so tired all of the sudden? Today is race day! You’ve been excited about this for a month, no time to nap. If only someone would stop that damn ringing you could finish this race. You get up. Woozy. You HAVE to finish. You have to drag the carcass of your car over the line or else they’ll call you a pussy and you are not a pussy. You are a soapbox racer.

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The San Fernando Valley Illegal Soapbox Federation is an organization of risk takers throwing caution to the wind for the thrill of old time soapbox racing. Four tires and a set of brakes propel these men and women upwards to 60 miles per hour down some of the region’s most treacherous hills. The participants take this race deadly serious. One wrong move and a car could spin out of control, catapulting the driver into spectators, oncoming traffic, or whatever blunt object that is unfortunate enough to be in their way. Observe.

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The setup: Soapboxes gather momentum by gravitational acceleration only. Touch the brakes too much and you shave off all your speed. Touch the brakes too little and you’ll eat shit in a turn. Racers must trust their intuition as to how fast they can traverse turns, dips, and road hazards if they plan on surviving the course. To find out how fast a soapbox can go without eating a shit sandwich, we must examine the forces that hold a soapbox racer steady.

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The force of gravity pushing down on the rider is balanced by the normal force. Normal force is the force that keeps everything on Earth from being crushed to smithereens by gravity. It is measured as the gravitational acceleration multiplied by the total mass of the car and driver. We’ll estimate the combined mass of the car and driver as 140 kilograms (about 300 lbs)

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Centripetal force keeps our rider moving in a circle around the turn. If you’ve ever swung something over your head in a circle, then you’re familiar with centripetal force. It is the force pointing in to the turn our rider is making. To calculate this force, we need to know the mass of the rider, the velocity of the soapbox car, and the radius of the turn.

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A soapbox racer’s best friend is friction. Friction is what keeps a car from spinning out in a turn. If the force of friction is less than the centripetal force, the car will skid and possibly spill the rider onto a well manicured lawn. To calculate the force of friction we need to know the normal force exerted on the rider as well as “u” - the coefficient of static friction. All we need to know about this term is that it is a number between zero and one. The closer to zero it is, the more slippin’ and slidin’ we get.

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The problem: The riders in the video met their match when they hit a patch of cobblestone, thus decreasing the coefficient of static friction. We are now prepared to ask ourselves: at what velocity could a soapbox racer be traveling to survive this unforeseen obstacle?

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Racers traveling any faster than 20 miles per hour were treated to a poop buffet when they hit that patch of cobblestone. As they tried to turn, the wheels of their soapbox couldn’t hold onto the pavement. The end result was one massive pileup, one trip to the hospital, and one valuable lesson learned.

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For more information on soapbox racing and the San Fernando Valley Illegal Soapbox Federation, visit www.sfvisbf.com. You too can build and race your own coffin!   

For more science, visit www.getyourscienceon.com

Special thanks to Lee McGowan, Dirty Dave, Mad Science, and the rest of the SFVISBF crew.

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