Vent LATCH Manual and High School Physics

AK Dad

New member
So, being a relatively new Tech I hadn't spent much time with the 2011 LATCH Manual until someone gave me a copy about 2 months ago (likely because it's now outdated.) I still enjoyed reading it right up until I got to P.16 where it says in reference to CR sled tests:
This is a very severe test, equal to hitting a solid wall at 30 mph or two vehicles of equal size hitting each other head-on at 60 mph.

That is a huge misunderstanding of Newtonian physics!!! Not just a bit wrong, but wrong by a factor of 4 (since kinetic energy goes up as the square of velocity). It is however equivalent to two vehicles of equal mass, one stationary and the other going 60 mph, or, even simpler, two vehicles of equal mass hitting head-on at 30 mph.

Anyway, I didn't worry about it too much because I figured some physics nerd (like me) would have noticed that and let them know before they released the 2013 revision. BUT NO! I just got my new 2013 edition in the mail and the same error is right there on P.82 again!

Am I too annoyed by this? Maybe. Even the Mythbusters screwed this up a while back, but they got so much crap for it from the fans that they went back and did an entire show about it. Here's a 5 minute excerpt that beautifully illustrates this with absolutely (well, almost) no math involved.
[ame="http://www.youtube.com/watch?v=r8E5dUnLmh4"]Mythbusters - Car crash force - YouTube[/ame]

OK, so why do I care that one sentence in a 200+ page book is wrong? Well, it makes me concerned that when they give an opinion on other matters they could also be off by a factor of 4. (I'm probably making too big a deal of it though.)

Anyway, that is all ... now hopefully I can go to bed and get to sleep ...
 
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wendytthomas

Admin - CPST Instructor
Staff member
I know the people who put this out. They're very good techs and instructors. But they are not physics majors. Write a polite note, send it over, and see if they put an update on the website.

Wendy
 

Stelvis

New member
So, being a relatively new Tech I hadn't spent much time with the 2011 LATCH Manual until someone gave me a copy about 2 months ago (likely because it's now outdated.) I still enjoyed reading it right up until I got to P.16 where it says in reference to CR sled tests:

That is a huge misunderstanding of Newtonian physics!!! Not just a bit wrong, but wrong by a factor of 4 (since kinetic energy goes up as the square of velocity). It is however equivalent to two vehicles of equal mass, one stationary and the other going 60 mph, or, even simpler, two vehicles of equal mass hitting head-on at 30 mph.

Anyway, I didn't worry about it too much because I figured some physics nerd (like me) would have noticed that and let them know before they released the 2013 revision. BUT NO! I just got my new 2013 edition in the mail and the same glaring error is right there on P.82 again! :mad:

Am I too annoyed by this? Maybe. Even the Mythbusters screwed this up a while back, but they got so much crap for it from the fans that they went back and did an entire show about it. Here's a 5 minute excerpt that beautifully illustrates this with absolutely (well, almost) no math involved.
Video Link: http://www.youtube.com/watch?v=r8E5dUnLmh4

OK, so why do I care that one sentence in a 200+ page book is wrong? Well, it makes me concerned that when they give an opinion on other matters they could also be off by a factor of 4.

Anyway, that is all ... now hopefully I can go to bed and get to sleep ...

Thank you. This has bothered me every time I saw it quoted somewhere.
 

Pixelated

Moderator - CPST Instructor
I know the people who put this out. They're very good techs and instructors. But they are not physics majors. Write a polite note, send it over, and see if they put an update on the website.

Wendy

Do this. They have been very communicate and receptive every time I've contacted them with missing info or for clarification. Maybe they would enlist your help in how to clarify.
 

AK Dad

New member
Do this. They have been very communicate and receptive every time I've contacted them with missing info or for clarification. Maybe they would enlist your help in how to clarify.

Will do! Also, didn't mean to be too caustic about it, so apologies there - really did just need to write it down before I could get to sleep last night! I'm sure I have some sort of diagnosable psychological condition! Physics OCD, maybe? :D
 

SafeDad

CPSDarren - Admin
Staff member
I reviewed an edition of the LATCH manual many years ago. It was probably in that edition too, and I no doubt overlooked it like everyone else, including a number of engineers and other top experts on crash testing who work in the field daily. So, there have certainly been people who know this stuff inside and out that have reviewed the LATCH manual. Mistakes happen sometimes and even the best proofreaders may not catch all of them, unfortunately. Most likely, for new editions, material that was not revised may not have been re-checked, as is often the case with publications. I imagine they meant to say "60 mph closing speed" or something to that effect. Even that may not be technically correct, since the sled is trying to reproduce the change in velocity realized by the occupants, not necessarily the vehicles. That's where CR made their mistake in their 2007 side impact testing for child restraints.

For the typical audience reading the manual, "equal size" and "equal mass" seem reasonably synonymous, though it probably would have been easier just to say "two identical vehicles".

A lot of confusion in these discussions also comes depending on whether or not the barrier used in such analogies or testing is completely rigid or deformable. Lots of semantics can be important when someone says a stopped vehicle, fixed vehicle, brick wall, etc.

I think what most advocates can take away from all this is that two identical vehicles traveling at the same speed and strike each other head-on is roughly equivalent to one vehicle traveling at the same speed (not twice the speed) and striking a completely rigid brick wall. This is essentially the principle behind the NHTSA's full-width frontal crash tests for vehicles used for the 5-star ratings program (where the speed is 35mph).

When this type of crash is simulated for a child restraint on a sled, the sled speed tends to be lower than the speed of the vehicle, as the vehicle and occupant protection systems tend to absorb energy. In addition, the sleds stop in a very controlled manner, or "pulse", that is designed to mimic what the occupant might experience in an actual vehicle crash. (Other sleds actually have the necessary pulse as they launch, rather than when they stop, though the effect is the same). There are many such pulses used, as vehicles and other factors vary considerably.
 

Pixelated

Moderator - CPST Instructor
I didn't take physics in high school. My biology program in university 'strongly recommended' I take first year physics, so stupidly I did. I got 54% in that class and quite possibly burned my text book in effigy at the end of my final exam :whistle:

I understood the above post, despite my physics ineptness. So thank you Darren :eek:
 

AK Dad

New member
I didn't take physics in high school. My biology program in university 'strongly recommended' I take first year physics, so stupidly I did. I got 54% in that class and quite possibly burned my text book in effigy at the end of my final exam :whistle:

I understood the above post, despite my physics ineptness. So thank you Darren :eek:

See, I'm much more hapless in biology myself! Physics is pretty basic in relation to bio if you ask me!
 

AK Dad

New member
I imagine they meant to say "60 mph closing speed" or something to that effect.

Yep, that would have been correct!

I guess my biggest beef is that when common misconceptions like this make their way into print in respected publications they tend to propagate more rapidly, thus more and more people have the fundamental misunderstanding further ingrained.
 

Brigala

CPST Instructor
Don't feel bad, AK. I feel the same way about the misuse of apostrophes in official publications.

I managed to get all the way through high school and college without taking a single physics class. I might have done OK in physics (algebra and chemistry come pretty easy to me; I like patterns) but I was horrible at biology.

I majored in English, though, so physics or advanced mathematics were not necessary. As my academic advisor put it when I was trying to decide whether to take Algebra (which I'd taken in high school, so it was a refresher for me) or Introductory Physics (after a 15 year gap since the last time I'd taken a math class of any kind), all while trying to maintain a 4.0 GPA, "Taking Physics right now would not be good stewardship of your intellectual resources." LOL.

Maybe I need to go take Physics at the community college now. Hmm? Because Darren's whole explanation was totally meaningless to me and this was not an error I ever would have noticed and I'm not sure I understand it now even after it's been explained.
 

AK Dad

New member
Don't feel bad, AK. I feel the same way about the misuse of apostrophes in official publications.

Me too! I actually had a commander one time before I left active duty who habitually mixed-up possessive vs. plural in his emails and I finally went into his office to have a little talk! I think he was actually appreciative because he'd somehow gotten it wrong a long time back and nobody ever bothered to correct him.

On the other hand, I can't spell (without a computer) to save my life!

Maybe I need to go take Physics at the community college now. Hmm? Because Darren's whole explanation was totally meaningless to me and this was not an error I ever would have noticed and I'm not sure I understand it now even after it's been explained.

Well, since all techs are really practicing "applied physics" on a daily basis, I think that would be a great thing to do! Only caution I'd add is that you'd have to see what level of math they'll go into, because that would actually be difficult for me at this stage since I haven't done any of the calculus or differential equations math myself since '99. Fortunately, the math isn't absolutely essential to understanding the concepts.

So, let me take a shot at explaining what we're talking about without the math portion ...

When we're talking about forces acting on a car (and its occupants) in a crash, all we're concerned with is how fast the car is going and how quickly it stops. Those two factors are what create the "impulse" force that Darren was mentioning in his post. The method by which the car stops in a certain period of time is immaterial to the force applied to the occupants.
Here's a good thought experiment to do - take a look at this still of a crash test into a barrier:
image.ashx

Now, take your hand and cover the actual barrier and just look at the car itself. Can you tell now that it has run into an immovable barrier, or could it have run into a car of equal size and weight? The answer is of course that you can't tell which one made the car stop that quickly, and neither can it (or its occupants). The "impulse" they feel is identical either way.

Did that help, or just make it more confusing?
 

tiggercat

New member
I didn't take physics in high school. My biology program in university 'strongly recommended' I take first year physics, so stupidly I did. I got 54% in that class and quite possibly burned my text book in effigy at the end of my final exam :whistle:

I understood the above post, despite my physics ineptness. So thank you Darren :eek:

I switched majors to avoid more physics (just from biochem to biology) ;)

Sent from my iPod touch using Car-Seat.Org
 

AK Dad

New member

Sweet!

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AK Dad

New member
I got a very nice response from the SRN folks who were indeed happy to have the error brought to their attention. Actually included some interesting information, so I wrote back and asked if I could copy and paste it here, so waiting to hear back on that.
 

AK Dad

New member
For those who are interested, here was the nice reply I got from SRN:

Thanks so much for your careful read of the LATCH Manual and for notifying me of this error. Though I rewrote and revamped the majority of the text for this year's edition, I am sorry to see that this mistake slipped past -- apparently not for the first time. I haven't gone back to look in older editions, but this may have escaped notice for quite awhile. (At some point, if I ever have time, I may trace it back, out of curiosity.)

You are, of course, correct. Force reflects the change of velocity, and so a 30 mph sled test is like hitting an immoveable object at 30 mph—but that could also be equated to roughly the force created by two cars of equal size hitting head on when each are traveling at 30 mph -- not 60. A very common misconception that got into the book at some point, and that I should have caught this year.

I do have to disagree about the 30-mph test not being "very severe," though. Yes, it is far less severe than NCAP tests (which are more severe than 99% of all real-life crashes), but NHTSA has determined that it replicates a force that is equal to or greater than 95% of all real-life crashes. Of course, "very severe" is a subjective descriptor -- so we may agree to disagree!

Luckily, whether we think it is severe enough or not, these days many of the major manufacturers have their own test facilities and do voluntarily run lots of additional sled tests, including those at NCAP speeds. (The engineers I've talked to say they're very uncomfortable with running sled tests at speeds greater than that because the force becomes so extreme that it starts to risk damage to their expensive equipment and crash test dummies.)

For now, the thing for me to do is to get this correction posted to the LATCH Updates page at www.saferidenews.com. I hope you've noted that this page exists -- though we certainly try, mistakes do happen to the best of us, and updates to printed material are, naturally, ongoing. We certainly appreciate the support of readers like you who help us keep on track! In response to your concern that this casts doubt on other parts of the manual, I don't think that there are many other physics-type comments in the text, but please do let me know if you see other points that concern you.

And here's my reply back:
Thanks for the quick reply! I know it's not a terribly pressing issue, but I do appreciate you taking the critique so well. That right there gives me more confidence than ever!

As for the "is it severe enough" question, that's some very interesting info about higher speeds damaging the test equipment! Wouldn't have thought of that! My reasoning is that I'd like someone to start doing some "pushing of the envelope" on seat testing like IIHS has and continues to do. I realize that we're still working as a CPS community to correct the ridiculous misuse rate we still see, but I think we can fight that battle and push the technology forward simultaneously.

What I'd really love to see would be IIHS putting some random samples of seats into the cars they test, much like EuroNCAP does. I'm not sure what they'd find, but it always seems to me that they're missing a lot of potential data by only putting a dummy in the driver's seat.

I bet the actual G-loading from an IIHS 40% offset frontal test at 40 mph ends up being relatively similar to the NCAP full-width 35 mph test. The IIHS one certainly tests the car structurally more, but may or may not be putting more force on the passengers.

And the reply from SRN:
I agree that a lot of opportunity is lost when cars that are crashed into barriers aren't loaded up with child dummy occupants. These are very expensive tests, so it would be nice to get as much info as possible each time. I suppose it is possible that the IIHS has, in fact, done some tests with child dummies in the back, but nothing has been formalized. I'm sure they are curious, too. I do know that this was being done for a few years by NHTSA during NCAP tests. Unfortunately, I heard that there was some concern that these other occupants could influence the tests primary purpose, the NCAP rating, so this was largely discontinued about 4 years ago.

As far as the other types of tests go, it is hard to anticipate all the scenarios that might injure an occupant. At the Assoc. for the Advancement of Automotive Medicine conference in October, joint research from GW University and a university in China was presented that analyzed testing of yet another type of crash -- "between-the-rails" crashes, in which a car hits a narrower object like a tree, pole, or smaller car head on. The researchers found that these crashes, though they occur less frequently, were more likely to cause serious injury (based on higher AIS scale ratings) than the other types that are already being tested by NHTSA and IIHS. So there is always more to look at. And, of course, all of these tests are conducted primarily to predict the outcome to the driver. So far, I don't know of any crash testing that is designed to focus on the situations most likely to cause injury to children in the rear.

Anyway, it was a very nice exchange, and I feel like a jerk for not having just written to them directly in the first place. So if I could I'd go back and edit the thread title icon from :mad: to :eek:
 

Ozella21

New member
The whole physicis part is a little past where I went in school (I'm a history major, no math required), but I understand the visuals and have actually been in enough car crashes to understand the great amount of force at low speeds thing.


At this rate I would almost like to see Mythbusters putting random car seats, in a car and crashing them. (with proper and improper use if they could) Maybe that would help with the misuse (or maybe I just want to see a crash test with car seats and dummy's in an actual you know car). For some reason people would seem to swallow information easier either from techs, or celebrities (or shows that are entertaining).

Who knows I may be way off board here, the kids are having a mythbusters-athon at the house so they are on my mind.
 

wendytthomas

Admin - CPST Instructor
Staff member
Mythbusters is why I stopped saying that a head on 60 mph crash from both cars equaled 120 mph. :)

I can tell by the writing of those emails who that was. She is as awesome as she sounds. And she knows her stuff and takes criticism very very well.

Wendy
 

featherhead

Well-known member
AK dad, have you seen the Transport Canadian crash tests? There are a whole bunch of videos and data from car seats being crashed in vehicles. I don't have the link, but
 

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