OK, lots of good stuff to discuss, so I will do my best to help explain in plain english as much as possible!
I think that broadening the corridor means more labs can run the tests ...
Yes, that's correct, and not even necessarily that more labs can run the tests, but even the same lab on a different day with different weight dummies and different seats needs to have some sort of "window" to shoot for. If they made that window too tight, they might have to re-test the same seat/dummy combination an unnecessary number of times to get their test pulse to fall within that window. Certainly they're doing the math and dialing in their equipment to produce the proper result, but making things match the theoretical mathematical precision in the real world is a bit more difficult.
So the parameters have to be between those boundaries for a set amount of time?
Well, the entire actual test pulse needs to remain within those boundaries from start to finish to be valid.
And if we make the boundaries wider, that makes the test more precise.
No, other way around - if the "window" were wider, that would make the test less precise, as far as comparing one set of results to another. The key again is to make the window wide enough to be achievable and still narrow enough to allow you to compare one to another.
Where/when in that diagram is the actual "impact"?
Here's where I think a lot of the confusion is coming from - the "impact" occurs at the far left of the graph, at Time = 0ms. Think of it this way: the exact instant a vehicle collision occurs is when the first piece of the front bumper hits an object, but at that exact instant, there's not going to be any force getting to the occupants, right? That first millisecond of the collision only involves, say, a piece of plastic bumper being pushed into whatever is behind it, and so on thru the entire crash event. As plastic crushes into metal, and more metal, and more and more structure of the front end of the car, the overall deceleration that the occupants "feel" increases up to a certain maximum G-force. That maximum G-force is a function of the speed that you're trying to model along with the ride-down time. Think about it this way: if we kept the speed we're trying to model (in this particular case 48km/h) constant but changed the type of vehicle we're testing (since remember this is just testing whatever "average" vehicle they want to model) we'd have a different curve. With many things in physics, it's often helpful to think thru the theory in the extreme, so if we were to model the impulse of a SmartCar, which has very little structure to crush and give us ride-down time, the slope of the graph at the beginning would be steeper, and also the maximum G-loading would be higher. If, on the other hand we imagined a car with a 20' hood that was all designed to crush evenly, the slope would be less and the maximum G would be lower.
The impact in that diagram is, if I am reading it properly, at the very end. The first up hill is an increasing acceleration (like stomping on the gas pedal), and then the flat line is even acceleration (still on the gas pedal but not burning rubber), and then the down hill is still accelerating but not as fast, as in you're nearly up to speed so you ease off on the gas. Where the down hill line meets the x-axis is when there is no more acceleration and the even speed of 48km/hr has been reached.
Nope.
So, the confusion is that you're trying to put 48km/h somewhere on the graph, but if you look at it again, you'll see that the axis are time and acceleration (G-force). If you wanted to add a "speed" axis to the graph, it would start at Time=0ms at 48km/h and end at Time=75-90ms at 0km/h. One of the other things that may be confusing is that the sled doesn't need to (and likely doesn't) ever actually go "48km/h" at any time during the test. The physics dictate that we don't care about "absolute" speed - only the accelerations applied to the occupants. This is why sometimes you'll see sled tests where the sled is at rest and then a pneumatic piston pushes it backwards to achieve the required impulse - from the standpoint of what the "occupants" "feel" there's absolutely no difference between that and getting a sled up to speed and running it into a barrier. In fact, from a technical standpoint, it's easier to control variables when the "at-speed" reference frame is at zero actual velocity.
There are a number of factors involved, but perhaps the most important when looking at a graph is the peak level and how quickly the peak level was reached. This could be roughly considered to be similar to the speed of the vehicle and how rapidly the vehicle stopped in a crash. As we all know, less ride down time means more energy transferred to the occupant. So, a broader, shorter curve would presumably be less likely to cause injury than a narrower, taller one.
Hopefully my explanations above help to make Darren's (correct) explanation also make sense?
My understanding, after some reading that might have exploded my brain, is that the crash pulse is meant to mimic what the occupants experience versus what the vehicle is experiencing, in terms of force.
Yes! :dance: Remember that different parts of the vehicle are experiencing hugely different forces - that first plastic bit of bumper that hits something is decelerating from X speed to zero nearly instantaneously, so if you were to look at its impulse curve it would be nearly infinitely narrow and infinitely tall. If you looked then at the impulse curve for say the first 1/3rd of the front end of the car, it would probably be experiencing 300 or 400G ... all we care about is the force the occupants experience, which thru experimentally gathering data from the occupant compartment of full-scale crashes can be modeled with this particular impulse curve.
So a sled crash at 35mph doesn't mean the vehicle is traveling that fast -- presumably faster, yes? -- because crumple zones and various other energy management systems within the vehicle are absorbing some of those forces.
No.
Remember, the big factor in understanding this is that the sled never actually travels across the lab floor at 30mph or 35mph ... the sled merely models the acceleration curve of what the occupants of the "average" vehicle experience in a 30 or 35mph collision. You are correct though that crumple zones and energy management features of vehicles will change the curve significantly, for instance, our SmartCar into a barrier at 35mph will impart a significantly more energetic impulse to the occupants than say a Volvo XC70 at 35mph into a barrier simply because the Volvo will have more ride-down time with its 5 feet of crumple zone in comparison with the "Smart" car's 1 foot of crumple zone.
OK, hope that helped! If not, let me know and I'll try to find some videos that will help explain it better!
