Blast From the Past: The Malicious Variable

In: BLAST FROM THE PAST, In Print By: Dirt Rag Contributor On: September 29, 2015

With the aid of a 4 by 8 foot sheet of plywood, a mattress and an unsuspecting volunteer named "Stinky," John “Bones” Branum conducts an entertaining experiment on the physics of the endo.


Editor’s note: This article by John “Bones” Branum first appeared in Dirt Rag Issue #60, published in August 1997.

V-brakes will stop you! As a tall rider, this impressive promise scares me. A high center of gravity is a playground for stiff stoppers. A while back, I was standing on a trail. While looking for my bike and shaking the rocks out of my helmet, I began wondering, “How come, now and then, even with my weight back, I still manage to fly over the front end if I have to brake hard on a steep descent? Even when I feel like I’m far enough back?”

The answer is simple: The instant I hit the brakes, evil gremlins hop out from behind the trees and lift up on my seat! It’s common knowledge that when momentum is halted, the energy looks for another way to expend itself, hence, the “endo.” This suggests that simply by stopping too quickly our weight is whipped over the bars. While physically feasible, this mishap is unlikely by that definition. We’re smarter than that. As bipeds, we’re keen to balance by nature. We know exactly how far to lean on a turn; we can maneuver a skateboard; if we leap from the tip of one rock in a stream to another, we instinctively know at what angle to land at a given speed and distance in order to balance ourselves. There must be more to this sling-shot effect than stiff braking. So I put my shoe back on, wiped the dirt off my teeth and peddled forth, puzzled and curious.

“Get Your Butt Back” Tip #27

Out of all the “rider tips” articles I’ve read, I’ve never heard a more frequent or passionate suggestion than to scoot back on steep descents. This makes sense and most of us comply, but why so much emphasis? This advice doesn’t only pertain to beginner tips either, it resounds through all levels. Could it be that advanced riders remain bewildered by their inability to predict the “endo” phenomenon? I filled my head with angles, numbers and lines for days with shady conclusions. It would have to be an experiment.

With a promise of seven dollars, my neighbor, Stinky, would be the guinea pig he so curiously resembles. For an extra buck he’d bring “The Boys.” Physically, better left to one’s imagination, they are his silent and strangely obedient accomplices, reminiscent of Larry, Darrel and Darrel. I had my bike on a 4 by 8 foot sheet of plywood lying in the driveway, end to end with a mattress. I convinced Stinky to climb on.

With two holes drilled through the wood on either side of the front tire, I strapped down the wheel with a rope. On “three” The Boys began lifting the back end of the board. As the board angled higher, Stinky’s center of gravity began to pass forward over the front horizontal axis of balance. He let out a yelp as the rear tire lifted from the board. I held the rear down, instructing The Boys to stop, drop it back a few degrees and hold it steady. I stepped back and in circus ringleader form, I commanded, “Stinky, grab and hold the FRONT BRAKE!” and with my pocket knife, I sliced the rope. Stinky went flying over the bars with a jerk, landing on the mattress, the bike crashing to the pavement. The confusion that ensued between his whimpering and The Boys laughing could not overshadow the discovery at hand. With the front wheel locked to the board, Stinky’s center of gravity lay behind the “balance axis.” He could safely be tipped forward until his center of gravity passed over the front hub.


As the brake was locked and the wheel restraint released, the axis of balance instantly shifted from the hub to the tire-to-board contact point which, at this downhill angle, fell just behind his center of gravity.

This event proves that the front balance axis is not fixed, neither to the hub nor to the tire, it is instead a malicious variable fluctuating between both! Stinky squinted, saying, “Sure, that’s very impressive, should your wheel accidentally become strapped to the trail with a rope, but what about the balance axis while rolling?” Uh … Well I couldn’t imagine a simple experiment to measure this condition and I could no longer afford Stinky’s services anyway. But, my hypothesis, to the best of my imagination with a little physics thrown in, is that while rolling, brake-free, the balance axis will fall no lower than the hub; in fact if anything, it will be higher. How high I can’t say. But perhaps the most interesting conclusion here is that the balance axis is not a material thing. It’s not metal, rubber or road. It is a varying point on an invisible segment between (and perhaps beyond) the hub and the tire-to-ground contact point. As brakes are applied gradually while rolling, this point creeps downward from the hub, therefore surprising ones instinctive assumptions of balance.

Another interesting point is that Stinky’s often cloudy instincts told him as he began to go over, to grab the brake harder, when letting go would have safely rolled him down to the mattress. Despite the complexity of this theory, its application is comparatively simple. On those short, steep drops where navigation is strict and relative speed is increased exponentially, stuff that seat in your gut and stretch your arms out more than you think you need to. As Sid Weighbach said, get “flossed” if you must. It should feel a little awkward but not as awkward as a nose dive. “Feather” the Lever of Doom! Braking must be monitored and used sparingly, if at all. In fact, barreling through brake-free with plans to lock ’em up at the next flat spot might prove safer than trying to slow midway, and is certainly preferred over dragging those back knobs down a gently groomed slope.

So the next time the trail seems to be getting steeper when it’s not, blame the “malicious variable” and let off the front brake. If that doesn’t work, get out your gremlin gun.

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