Head-on this morning on MVT

[Mariner]

I was headed south on the MVT just past the Gravelly Point parking area. As I rounded a slight bend, which is a blind curve, there were three cyclists headed NB in close single file, well to their right. Unknown to me, someone else was behind me on my left attempting to overtake. There were shouts, a crash of bikes, and I turned around to see bikes and bodies flying.

At least one rider from the three NB riders was on the ground. I didn’t see blood but I’m sure there was some from hitting the asphalt. The person who caused the collision was profusely apologetic while I screamed at him.

Dude, it is a blind curve. Next to the Parkway. Next to a roadway with no separating barrier. On a trail with nowhere to go. On a blind curve. Your behavior caused some injuries, I don’t know how bad, but could have sent all five of us to the hospital. And all because you wanted to pass.

Location:
https://www.google.com/maps/@38.863862,-77.0421732,20z

and here:
https://www.google.com/maps/@38.864186,-77.0421626,3a,49.4y,204.53h,84.42t/data=!3m6!1e1!3m4!1sqcDkuFOHbenZ7s6BzxVWwQ!2e0!7i13312!8i6656

I have video but it doesn’t show the crash. It probably did record my screaming though.

Topic

[sjclaeys]

@dkel 142061 wrote:

Point taken, but closing speed doesn’t determine the force of the impact. Two objects colliding head-on, each at 25 mph is not a 50 mph impact; it would be the same thing as a moving object colliding with a stationary (immovable) object at 25 mph—i.e. a 25 mph impact.

Sorry. Couldn’t resist. :p

I do not think that you are correct. The force of impact is determined by the combined closing speeds of the objects. Two objects moving toward each other at 25 mph generate the same force as if one was stationary and the other was moving at 50 mph. This is the best I’ve found so far on the subject: http://www.experts.com/Articles/The-Physics-Of-Collision-By-Gary-R-Kilpatrick-And-Associates-PA

Plus,

[ATTACH=CONFIG]12014[/ATTACH]

[bobco85]

@dkel 142061 wrote:

Point taken, but closing speed doesn’t determine the force of the impact. Two objects colliding head-on, each at 25 mph is not a 50 mph impact; it would be the same thing as a moving object colliding with a stationary (immovable) object at 25 mph—i.e. a 25 mph impact.

Sorry. Couldn’t resist. :p

Huh? This does not compute.

Corrected physics: 2 objects colliding head-on, each at 25 mph, is a 50 mph impact; it would be the same thing as a moving object colliding with a stationary (immovable) object at 50 mph-i.e. a 50 mph impact.

You are correct, however, that closing speed doesn’t determine the force of the impact outright, but it does play a part.

[rcannon100]

@sjclaeys 142066 wrote:

I do not think that you are correct. The force of impact is determined by the combined closing speeds of the objects. Two objects moving toward each other at 25 mph generate the same force as if one was stationary and the other was moving at 50 mph. This is the best I’ve found so far on the subject: http://www.experts.com/Articles/The-Physics-Of-Collision-By-Gary-R-Kilpatrick-And-Associates-PA

Plus,

[ATTACH=CONFIG]12014[/ATTACH]

This was the subject of a Mythbusters in which Jamie had to concede that he was wrong – force was not additive. That said, I have never understood the math.

[TwoWheelsDC]

@sjclaeys 142066 wrote:

I do not think that you are correct. The force of impact is determined by the combined closing speeds of the objects. Two objects moving toward each other at 25 mph generate the same force as if one was stationary and the other was moving at 50 mph. This is the best I’ve found so far on the subject: http://www.experts.com/Articles/The-Physics-Of-Collision-By-Gary-R-Kilpatrick-And-Associates-PA[/quote]

Dkel is basically correct. It’s a little more complicated than “they’re the same,” but the practical result is the same.

[Anonymous]

Oh, for heaven’s sake. Cars are specifically designed to absorb a lot of force by crumpling into a smushed mess, unlike that wall they were crashing into. OF COURSE crashing Car A into Car B is not going to damage Car A as much as crashing Car A into a wall (at the same relative speed differential) because Car B is absorbing much of the force of the impact as it crumples. When you crash Car A into the wall, the amount of force absorbed by the wall is bordering on negligible; Car A takes full brunt. When you crash two cars into each other, assuming similar crash crumple design, each car absorbs about half.

It’s not just whether or not the thing they’re crashing into can *move*, it’s also how much force it absorbs by deforming and breaking.

Regardless, cyclists are not immovable objects, so comparing an oncoming cyclist to an immovable wall is not particularly valid. Of course relative speed matters.

[DrP]

Since we seem to be talking about crashes and forces in two threads, here is what I mentioned in http://bikearlingtonforum.com/showthread.php?10456-e-bikes-legal-in-DC&p=142075#post142075:

“The rider with more momentum (mass x velocity, in a straight line situation) will impart a larger impulse on the other rider, which would be the force felt (F = {difference in momentum}/{time length of collision}). The details will depend on the angle of impact and how elastic or inelastic the collision is.”

Both feel a force and you can recast it in multiple reference frames. Pick your favorite reference frame for your computation, but in the end the forces and energy are the same.

The car crashes you show are all very inelastic due to the crumple zones they have, as Amalitza points out. If you want simple, roll marbles at each other.

It has been too long that I have thought about such equations and late on a Friday.

[dkel]

@Amalitza 142073 wrote:

Regardless, cyclists are not immovable objects, so comparing an oncoming cyclist to an immovable wall is not particularly valid.

But if two cyclists of approximately the same mass collide head on, the effect—from the standpoint of force—is very similar to a cyclist hitting hitting a wall; in terms of actual damage, the dynamics of moving human bodies and bikes is a lot more complicated than hitting a wall, so that aspect of my analogy is not accurate.

The speed of a head-on collision is not additive, but a faster collision of any kind has the potential to cause greater damage than a slower collision.

I shouldn’t have said anything about physics in the first place.

[mstone]

@dkel 142078 wrote:

But if two cyclists of approximately the same mass collide head on, the effect—from the standpoint of force—is very similar to a cyclist hitting hitting a wall; in terms of actual damage, the dynamics of moving human bodies and bikes is a lot more complicated than hitting a wall, so that aspect of my analogy is not accurate.

The speed of a head-on collision is not additive, but a faster collision of any kind has the potential to cause greater damage than a slower collision.

I shouldn’t have said anything about physics in the first place.

If you’re going slower you 1) have more reaction time and 2) are likely to be less seriously injured if you end up in a single-bike crash as you’re trying to maneuver. As to whether your speed matters in a head on collision with another cyclist, I’d submit that most of the examples, equations, and hypotheticals mentioned thus far are irrelevant. Cyclists are not marbles, they are not points, they are not spheres of constant density, they are not enclosed in a car or other frame, etc. In a bike crash there are a series of elastic and inelastic collisions, including the bikes hitting each other, parts of bikes hitting human flesh, humans separating from the bikes and continuing their existing velocity, bikes separating and forming projectiles, etc. Especially the “humans separating from the bikes” part is extremely relevant and completely unrelated to any model involving two cars colliding; the faster you’re going, the more energy you (personally, not your car or airbag) need to absorb when decelerating to zero.

[Steve O]

@ShawnoftheDread 142063 wrote:

Anyone slower than oneself is an idiot, anyone faster is a maniac.

Actually, the slower ones are morons.

[dkel]

@mstone 142079 wrote:

Cyclists are not marbles, they are not points, they are not spheres of constant density

Speak for yourself! :rolleyes:

[Steve O]

Ruth had a bike

She hit a tree

The bike went on….

Ruthlessly!

[scoot]

dkel is correct regarding the simplified physics problem, but the damage will be worse when one hits a wall, for the reasons mentioned by mstone.

If you hit a rigid immovable wall at 25MPH, your change of momentum will be your mass * 25MPH. If you collide head-on with another cyclist of equal weight, you would each lose (mass * 25MPH) worth of momentum. If the masses are not equal, both cyclists will lose the same amount of momentum so the heavier cyclist will not lose as much velocity.

But in practice, there will be higher forces involved in a wall collision than a cyclist-cyclist head-on. The change in momentum = impulse = force * collision duration, and the collision duration will be lower for a rigid wall than it would be for two bicycles and human bodies. Because the bike-on-bike crash spreads out the impulse over a series of lower-force collisions.

[notmarian]

@BTC_DC 142045 wrote:

Actually sounds like a relatively long commute if Gravelly Point is just your midpoint going to College Park. ~30 miles each way?

Less than that on distance–~22 miles (Gravelly is actually between mile 7 and 8 for me), so not really a super lengthy commute, but yeah, maybe longer than average around this area (I work with folks riding in from Germantown and Annapolis, which seems like a long haul with some big hills). But, I can travel much faster on open, largely residential roads in the District and in Maryland than I can on the trails (at least going in–coming home is different), so it’s usually ~40m to Gravelly via W&OD, FMR, MVT, then another 45 or so once I go over the 14th St. Bridge, if I hit the lights just right. I’ve thought about bypassing MVT taking Eads or the secret road up through Crystal City, but that doesn’t get me around this particular grief point, and I’m not a huge fan of that trail connection.

I was wondering when this thread was going to end up a physics argument!

[mstone]

@scoot 142085 wrote:

But in practice, there will be higher forces involved in a wall collision than a cyclist-cyclist head-on. The change in momentum = impulse = force * collision duration, and the collision duration will be lower for a rigid wall than it would be for two bicycles and human bodies. Because the bike-on-bike crash spreads out the impulse over a series of lower-force collisions.

Maybe. The cyclists are (usually) not tremendously sticky. So the question is what you hit after hitting the other cyclist–maybe a wall, maybe a post, maybe something else. I’d probably prefer to hit a wall rather than a post (force spread over a larger area). If the bike-on-bike collision results in the two cyclists banging into each other and rolling it is probably better than the wall. If the collision results in the bikes knocking into each other and one of the cyclists flies freely off the bike and onto a spike, it’s probably worse than the wall. The physics of the problem is ultimately less interesting than the topography.

TL;DR: there’s nothing really good to collide with other than a pile of pillows

[Anonymous]

This is an approximately true statement.
@dkel 142078 wrote:

But if two cyclists of approximately the same mass collide head on, the effect—from the standpoint of force—is very similar to a cyclist hitting hitting a wall;

This is where you are completely wrong.
@dkel 142078 wrote:

The speed of a head-on collision is not additive, but a faster collision of any kind has the potential to cause greater damage than a slower collision.

There’s no such thing as an absolute zero speed or an objective velocity. Velocity only has meaning relative to something else and it totally doesn’t matter how you define your zero point. If dkel and I are moving towards each other at a relative velocity of 30mph, it does not matter one iota whether you have
– defined your motionless fixed point as me, meaning we view dkel as moving at 30 mph towards me, or
– define dkel as the motionless fixed point meaning that I’m moving 30 mph towards him, or
-define the surface of the earth as the fixed point and we’re each moving 15 mph (or dkel 10 and me 20, or whatever) , or
– define the center of the earth as the fixed point and dkel is moving at 1000mph while i’m moving at 1030mph or
-define that squirrel over there as the fixed point and we’re both moving highly erratically in a weirdly choreographed fashion that somehow results in us coming steadily closer to each other at 30mph.

The physics are the same. We still smash into each other at 30mph. Not 15mph. Not 1000mph. Not whatever the heck that squirrel is doing. 30mph, the total combined speed at which we’re approaching each other.

[Author]

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