“Your 25-pound bicycle is no match for a 10,000 pound truck.” You’ve probably heard that before.
So, if you collide with a 10,000 pound truck, the impact will be much, much worse than if you collide with a passenger car, or another bicyclist, or a pedestrian, because the truck weighs so many times more than your bicycle.
By the same reasoning, you should be much worse off yet if you simply fall and collide with the planet Earth. It is so much heavier than that truck…
Hmm, but we’ve all fallen off our bicycles a few times, mostly without serious injury.
I don’t want to collide with a truck — or a car, or another bicyclist, or the planet Earth. But let’s look at some basic physics.
First of all, the mass of the bicycle isn’t the issue. The rider accounts for most of the mass of the bicycle/rider system.
But also, the severity of a collision doesn’t increase directly with mass. It depends on the severity of the impact.
Consider two bicyclists, a perfect match for each other, riding toward each other at the same speed and colliding exactly head-on. They will both come to a complete stop. This impact would be the same if you put a brick wall, or a sheet of paper, between the two bicyclists.
Now, suppose that the truck is headed toward the bicyclist at the same speed as the other bicyclist. How much more severe is the impact?
Not hundreds of times, but four times. The energy dissipated in a head-on crash is as the square of the speed. The square of 2 is 4. The truck is so much more massive that the bicyclist is pushed back at almost the truck’s speed. It’s almost the same as riding into a brick wall at twice the bicyclist’s speed.
Severity of impact is much more about speed than about mass. The severity of a bicyclist’s collision with a motor vehicle is almost entirely in proportion to the square of closing speed, all other things being equal. The only exception is if you go underneath and get run over. Then, the vehicle’s mass does matter. Getting run over either by a car or a or by a truck is very likely deadly.
My worst ever “hurt” was from a collision with the earth. Afterwards, it did not occur to me to consider its mass.
As I remember my sophomore Physics, weight (correctly: mass) does play a role in collisions. The pertinent formula is M = m * v, where M is momentum, m is mass, and v is velocity. Also pertinent is the law of conservation of momentum. When the bike and the truck collide, the truck transfers some of its momentum to the bike, enough to cause the bike to change direction 180 degrees. Since the bike has a small mass, the momentum transferred to it in the collision can only be achieved through attaining a large velocity (in the opposite direction) very quickly. Flesh, bones, and brains respond very poorly to large changes of acceleration. My father pointed out to me, “A long fall never hurts you. It’s the sudden stop at the end.”
That’s a lousy explanation. I need to get my old physic text out and refresh.
People do get hurt falling off their bikes and colliding with the Earth, but not by only falling off their (stationary) bikes. If they fall off a moving bike, their injuries mostly come from sliding across the ground. If we talk about stationary bikes, to eliminate the injuries from sliding, at the beginning of the fall, the erstwhile rider and the Earth begin moving toward each other, according to the law of universal gravitation. Since the fall is not very large, neither object has time to build up very much velocity, hence there’s not much momentum to be transferred.
“If they fall off a moving bike, their injuries mostly come from sliding across the ground. ”
That will give you road rash but the impact is what causes serious injury. The point I was making is that people speak about the severity of crashes as if it were proportional to the mass of the vehicles, and will be much, much worse if you collide with a heavy vehicle than with a lighter one. It isn’t proportional. it has much more to do with relative speed.
The difference in results from a collision between large and small masses is that the large mass can continue through many more small masses.
In other words, a truck will plow through a larger crowd of cyclists than a Smart Car at a given velocity.
That’s right, in terms of physics. If you lined up forty bicyclists in a double row twenty long, and drove a small car straight at the end of the line at 50 mph, they might bring the car to a stop, with multiple fatalities. But on the other hand, that bears no relation to how crashes happen, or actual crash statistics. Very few car-bicycle collisions involve more than a single bicyclist. I’ve heard of a few car-bicycle collisions involving two, three or four bicyclists riding in a group, but that is rare.
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