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Bill  On The Road

 by: Bill Oetinger  1/1/2023

To Hammer or to Slide?

There has been an interesting discussion on our club chat list lately. I didn’t pay attention to it when it started but I think it was about the efficacy of padded or somehow armored cycling shorts that might do for your pelvis and butt what helmets do for your head in the event of a crash. But it soon moved on, mostly focused on this question: in a bike crash, which is better…to slide at a higher speed or to hit the ground more-or-less straight down at lower speed?

At first the observations were anecdotal and subjective—rehashes of past crashes—but then someone mentioned physics and things got a bit abstruse. Who knew we had so many physicists in the club? Or at least folks with some college-level physics in their resumés. It started innocently enough…

F“Let's say F is the amount of Force produced by an impact and applied to a Bone; P is the amount of force absorbed by (protective) Padding; and B is the threshold force required to break a Bone.  If F-P>B, then the bone will break. If F-P

That got things rolling and flushed the physicists and engineers out of their thickets…

“When falling, a rider's motion can be resolved into two vectors, a horizontal and vertical vector. In a crash, the vertical vector is the same regardless of how fast the rider is moving in the horizontal direction.

“High speed crashes can sometimes seem minor because a person's horizontal kinetic energy dissipates gradually as the rider slides over the tarmac. A rider skids, slides and rolls to a stop over a second or two. But the vertical component of kinetic energy remains exactly the same. 

“A rider's kinetic energy is 1/2mv^2. So, if a rider is moving at 20 mph, she has four times the kinetic energy in the horizontal direction than if she crashes at 10 mph.  But her vertical kinetic energy remains exactly the same. Vertically, she starts at 0 mph at the beginning of the fall. Her kinetic energy vertically when she hits the tarmac is mass x force of gravity x height above the ground. Things become complicated very quickly because crashes are not perfectly inelastic, the slide can be converted into rolling motion, etc.”

Note: just because these folks can cobble together fancy formulas doesn’t automatically mean their conclusions are correct or even that their formulas are correct. Just sayin’…

And having said that, I will add that some of the folks who posted to the list I don’t know very well. Others I know better and a few I have come to respect for their general intelligence and what I might call bike smarts. This post is from one of them…

“Whether you are going fast or slow, the gravity force vector is the same. But if you are traveling faster, the horizontal vector is longer. The total force vector is the hypotenuse of that right-angle triangle. So technically, a high-speed crash subjects your body to more force. However, when you crash going fast, your hip typically doesn't slam into a horizontal brick wall. So I think there is something about the skid angle that is protective. I think your hip decelerates more slowly with skidding, which reduces the maximum instantaneous impact force.”

Then there was this from a guy I know has a PhD in Physics…

“One thing not mentioned yet in this discussion is the concept of Impulse. (Definition of ‘Impulse’ in physics: a force acting briefly on a body and producing a finite change of momentum; a change of momentum so produced, equivalent to the average value of the force multiplied by the time during which it acts.) I teach physics at SRJC and always enjoy discussing Impulse. The equation for Impulse comes from Newton's Second Law and can be written dP = Fdt.

“A change in momentum requires applying force across a change in time. From this simple equation it is clear that if the change in time drops, then the force must increase. If you are in a collision and your momentum drops suddenly to zero then your body will undergo a large force.  The best thing you can do if you anticipate a collision is try to extend your stopping time as much as possible.

“I’ll ask you the same question I ask my students. You're driving down the freeway at 80 mph and suddenly must stop. You have two options: you can turn left into a brick wall or right into a giant mountain of hay. Which way would you go?”

There was a good deal more in the same vein. Those of us who are not physicists or engineers were hanging on at the back of the pace line, trying not to get dropped. But actually, even for the Liberal Arts dimbulbs, it’s not that hard to understand. We may not be able to write the formulas but we’ve had the physics impressed upon our soft-tissue bodies and our breakable bones. Nothing like leaving the ivory towers of academia and visiting the School of Hard Knocks for making this easy to understand.

Above all, the real world is messy, and the crashes we find ourselves enduring are seldom so simple that they can be defined by a simple formula or neatly divided into two types. No doubt the assorted forces and vectors itemized above are all going to be in play, but many other variables will complicate the matter. Just for one example: in a faster crash, the difference between sliding on rain-slick pavement or black ice and sliding—grinding, bouncing—on very rough pavement or gravel or rocks. Even a smooth slide on a slick road can go wrong, whizzing you right over the edge of a cliff or under the wheels of an oncoming car or into a stone wall. The world is not a laboratory, except perhaps for experiments involving chaos theory.

My own experience of crashes—more than I care to recall—tells me the ones where I slam down hard and don’t slide are going to be worse: a brutal hammer blow to the body and probably a trip to ER the next entry in my day runner. My most recent crash in September was one of these. I hit the front corner of the truck, my carbon handlebars shattered, and in some small fraction of a second, I hit the pavement HARD…all the impact concentrated on my upper left side: elbow, ribs, shoulder, collarbone. Would I have been better off sliding? According to a simple formula, perhaps. But in this case, in the messy real world, a slide might have taken me right under the rear wheel of the truck. 

I’ve had three crashes that sent me to ER. All three were the slam-down kind. All three resulted in broken bones. I know of many other crashes of that sort among my pals that ended up with the orthopedic folks involved, including even the simplest goof: toppling over at a stop light after forgetting to unclip. In contrast, the long, sometimes fast slides have resulted in nothing more than rather garish, grisly-looking road rash (and shredded shorts). If the bike was still rideable, I usually hopped back on it and continued after those skids. I might have been uncomfortable for a few days with the abraded epidermis—that gummy, scabby mess—but not nearly as uncomfortable as I would have been, for weeks or months, with a broken collarbone or fractured ribs or busted-up pelvis. No visits to ER. No surgeries. No rehab.

As is noted by our science friends above, it’s the dissipation of kinetic energy that differentiates the two kinds of crashes, trucks and cliffs and other variables aside. Your butt or thigh can act as a drag brake when skidding along the pavement, slowing you down before you hit some immovable object. Making allowances for all the other jokers in the deck that the real world will throw into the equation, I think I’d choose the long slide over the hard hammer blow every time.

Of course the best option is not crashing at all. But this isn’t something we choose to do or not do, except perhaps in the degree to which we choose to push the envelope. There’s probably a formula for that too. Heading into this new year, I wish you all safe cycling. May you please not end up as crash-test dummies in some physics experiment.

Bill can be reached at srccride@sonic.net



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