Hi everyone and welcome to the new forum. My question is with crush points. The energy result from crush points is this Kinetic Energy, and if so then the impact speed is calculated from this number? I hope I am right in what I am saying.

V=sqr(2*J/W) J= joules W= weight

Please let me know if I am right with this, thanks.

If you are using the 2- 4- or 6-point crush formulas the answer comes out in inch-pounds. You must convert to foot-pounds (or jules, if you prefer) to use as the Kinetic Energy value in other formulas.

Be careful, though. It will only be appropriate for single vehicle crashes into an immovable fixed barrier (no damage to barrier).

For vehicle-to-vehicle crashes it is much more complicated than that.

Thanks Jim, I did not know that it came out in inch pounds. So that makes a big difference to my answer. Yes I know that you must use a simultaneous equation and a quadratic equation to solve for vehicle to vehicle collisions. But the info you have given will help a lot in solving the question. One other thing I do not understand is why is it that V=sqr((2gKE)/W) in imperial, but when you use metric V=sgr((2KE)/W) there is no multiplication by gravity in metric?? Thanks again.

I'm not sure where you got the formula from but typically when you are working with the metric version (joules) you are working with mass instead of weight. So, I would suspect the formula is expecting units of mass instead of weight and that would remove gravity from the equation.

Thanks Eric Paul, the formula's I have are from a publication called the third edition of the international guide book for traffic accident reconstruction. It is a good publication but not all formula's are worked in both metric and imperial, so you have to experiment with them. The formula for crush points 2-4-6 are only worked in imperial. Here they give an example of two cars colliding head on, one with 2 crush points and the other with 4. The one with two crush points is then propelled backwards. It then goes on to show how to work out the impact speed from the crush points and post velocity.

Probie, this may already be obvious to you but if you are using the crush formulas in imperial units you might want to stay in imperial units through the end. You will end up in feet/per/second or miles/per/hour and then you can convert to metric units of kilometers/per/hour or meters/per/second.

If you really prefer to work in metric you can convert your ft-lbs over to joules and then work with the kph or mps formulas. The speed formula using joules is S=sqr((26*KE)/m). The answer will be in kilometers/per/hour. Notice the "m" under the divide symbol rather than a "w". You have to convert the weight (kilograms of course) of the vehicle into mass or put gravity above the divide symbol to keep weight.

Thanks Eric Paul, I have already decided to converted everything over too imperial, even though I prefer metric. Although your speed formula equation S=sqr((26*KE)/m) is a life saver, I will definitly try it in metric once I finish with the imperial version. I find some of this stuff confusing, but I am sure with time and practice it will all become clear.

Just have one more question on this subject. I know this may sound like a stupid question. If you have two vehicles hitting head on with no pre impact skidding (so no way to calculate pre impact speed), can you combine the post impact speed with the impact speed to arrive at the pre impact speed, or can we take the impact KE and add it to the post impact KE to arrive at the KE before the collision then covert to mph? Thanks ~ Probie

To answer your very first question, the energy that is calculated by any method of crush or damage analysis is, by definition, work energy. It cannot be kinetic energy because there is no element of motion and it is not a function of anything's ability to do work. However, WE and KE are oftentimes interchangable and one can usually be evaluated in terms of the other, as in the "slide to stop" formula we are so familiar with. I will likely get some objections to this gross oversimplification by any physicists out there, but for our purposes this generalization will usually do.

Your discussion about the confusion that results when publications omit important parts of formulas is a particular peev of mine, and one of my strongest (and only) problems with the way IPTM teaches accident reconstruction to their students. When a publication "over-derives" a formula and does not show physical constants, it makes it difficult for people to work in other units of measure unless they already know the formula and/or where it came from.

For example, IPTM uses formulas like: t = (.249)(sqrt(d/f))

rather than: t = sqrt(2d/gf) like Northwestern University would teach.

(Formula for the time elapsed during acceleration from a stop.)

My issue is that when one is only familiar with the former equation, how can a person reasonably extract enough information from it to use metric units if they so desire? Not only that, but I've used the basic motion equations to derive other equations by inserting them into one another until I come up with what I am looking for. That cannot be done (or it is greatly complicated) if one does not know that the IPTM version includes gravity and that the constant is really 2 and not .249. And a third reason I object to "over-derived" formulas is that a skillful attorney can shred a reconstructionist on the stand or in deposition who is not familiar with the true forms of the equations, and what the equations' constants represent.

That is just a particular peev of mine (like nails on a chalkboard ) but your conversation illustrated the issue so well, I had to offer my 2 cents.

__________________

David

"The only reason for time is so that everything doesn't happen at once." -- Albert Einstein

I was interested to read Crashxprt's comments on IPTM versus Northwestern regarding the equations each teach. Thanks for the insight. So for an engineer new to the accident reconstruction filed, what are great intro thru advanced training organizations ? I just purchased the Traffic Accid Recon by L.Fricke but it's copyright 1990 so a couple of workshops seem like a good idea. thanks

partridge512 wrote:So for an engineer new to the accident reconstruction filed, what are great intro thru advanced training organizations ?

I just purchased the Traffic Accid Recon by L.Fricke but it's copyright 1990 so a couple of workshops seem like a good idea. thanks

I've attended both N'western University and IPTM courses and, in my experience, I think both institutions have something to offer. IPTM classes tend to be more "police-based", which is actually some benefit to an engineer who is unfamiliar with police methods. From attending IPTM classes, I have gained some understanding of how police officers are trained to approach accident analysis without attending a police academy. This has been valuable beyond measure, but in an indirect way. This is not to say that IPTM classes are not based upon sound engineering, but that there is a practicality in method that IPTM does a very good job of presenting (we engineers have a tendency to over-think stuff and work up calc's like we are building pianos or something). I have had some very good instructors at IPTM, and some that were not so great, but apart from my criticism in the previous post, overall I think the quality of their instruction is high.

The perspective that IPTM generally does not present is one that seeks to identify problems with the design, maintenance, or operation of highways as a causative factor in traffic accidents. Since they are police oriented, they tend to look for people to find at fault rather than things to find fault with. As a result, civil litigation tends to be less emphasized than criminal prosecution. This is only a tendency and varies greatly with the particular instructor.

I attended N'western TAR 2 a long time ago, so it might have changed considerably since then. I noticed that they now have an online version of TAR1 that one can take at their own pace, so that seems pretty simple (except for that $1000.00 tuition!). Again, their clients are mostly police officers so they tend toward that perspective, but, IMO, not as much as IPTM. They also offer courses in highway design and traffic engineering so they have staff knowledgable about design defects and related subjects.

If you are already familiar with calculus, mechanics, and vector dynamics, then the basic courses will be very tedious to some extent, and I would suggest looking into N'western's online version of TAR1. The mathematics review portions were especially rough for me to sit through because they have to make sure that their students are familiar with some very basic trig and algebra. The basic at-scene course will also have some review info if you have any surveying experience, but you will need it for tiremark identification and other invaluable skills.

There are some other organizations that offer courses, but I have not attended any of them. I know that N'western and IPTM are the most well-known as far as accident reconstruction training goes. I think either one will present a solid foundation, but there really is no substitute for good critical thinking skills and hands-on experience in working with crashes.

It really comes down to which is more convenient for you to attend. I hope this helps, and remember that my perspective is only one among many others who have attended both institutions. You might want to get opinions from other members before making a final decision.

Good Luck.

P.S. Fricke's book is still an accident reconstructionist's best friend, even though some of it is not considered state-of-the-art. Even so, physics is still physics and falling objects still fall, even since 1990. I still refer to the blue book probably once a week and have for many years. I wish they'd publish an update...

__________________

David

"The only reason for time is so that everything doesn't happen at once." -- Albert Einstein