Beginners Error#1 in Reconstruction 101

Topics related to collision & Trajectory analysis formerly on our 'Registrants only' area however which we get asked about frequently so believe shoud be in the open forum too
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MSI
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Beginners Error#1 in Reconstruction 101

Post by MSI » Tue Mar 30, 2010 5:18 pm

March 30, 2010: The following is a followup on the forum topic Methodology equating bone fracture tolerance to DeltaV?. Please review that topic for additional information. At the time we received that question via email, we were later provided an expert report on which we did a quick review (we were not involved in the litigation). We withheld posting anything on the report until the case was resolved.
A 3 page report prepared by what at first review seems to be by a qualified mechanical engineer and biomechanic contains fundamental errors in the analysis....please take a moment to first read the attached report. (NOTE: we have redacted the identity of the case and expert. This is not to embarrass the expert, it is to demonstrate the need to carefully evaluate ANY and ALL reports in detail!)
Now a few comments:
The author of the report calculates the speed to stop at 34 MPH based on 50 feet skid. The author assumes 0.8 g-units deceleration. The author assumes the Ford is stopped at impact? If so, where did the 16 MPH speed change come from? The trucks forward speed?
Then after his questionable way to determine impact speed change, the author assumes a locked leg (note there are articles by Viano and some others on same topic as his referenced article which I would venture to guess aren't as certain about predicting forces and deltav!! But ya never know!)
The author then makes the fundamental error when he adds the 16 MPH it to the 34 MPH to come up with 50 MPH impact speed!
Beginners error #1 in Recon 101.
  • Let’s assume there is a 16 mph impact speed change, so she’s going 16 MPH at impact.
    The 50 feet of skids were made from some unknown speed down to 16 MPH, NOT from 34 MPH down to 0 MPH!!
    Unless of course he asserts ALL the impact speed change is from the Truck? Which of course then isn’t cumulative on the Escort either?!!
    The actual Travel speed at brake application, IF the Escort is going 16 MPH @ impact, would be 38 MPH, not 50 MPH.
    (Vinitial**2 = Vimpact**2 + 30 (MU) (Distance of Skid) AND that doesn’t include any speed on the part of the truck, which would reduce the required forward speed of the Escort to produce his 16 MPH deltaV.
Also in this particular accident there was an override situation. The bumper was not engaged.
So 1st if you do a damage analysis if will be much less (see the Forum topic on Underride collision analysis which among other papers includes a 2009 SAE paper by Struble which presents evaluation of above/below bumper stiffnesses to reconstruct underride collisions. The bottom line is that whatever a CRASH damage analysis might tell you, if there was an underride, and therefore because the bumper was not engaged, the actual DeltaV will be less than what you caclulate with a CRASH type damage analysis. And therefore the damage in the subject accident may not be inconsistent with the Escort at or near stopped and the truck striking it at 12-14 mph.
But even If you even take what the other ‘expert’ opined as the DeltaV (16 MPH) and if you assume ALL of it came from the escort then you’d still only caclulate a travelling speed of 38 mph at the time of the brake brake application. And this of course is with the experts assumption of 0.8 friction coefficient. The average coefficient of friction is 0.7 with a range of 0.6-0.8 (check Baker/Northwestern & other sources).
So if you run those numbers then the approximate speed of the Escort at brake application was 35 MPH
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Steve Stockinger

Re: Beginners Error#1 in Reconstruction 101

Post by Steve Stockinger » Tue Mar 30, 2010 6:26 pm

Thanks for the reverse analysis. I found it very informative and helpful to me for the future.

The skids were down grade and on wet pavement and no way a .8g
Impact was in the center of the Escort's lane. The Semi POR was 21 feet back from impact. The claim was that the 98 Ford Escort's 16 mph impact pushed the semi (turning at 12 mph, per its driver) backward.
Escort driver testified her left leg was on the floorboard and her right leg was not stiff. She sits close enough to the wheel that this would not be possible anyway.
The foot well area was not deformed. Right foot likely slipped off brake onto floor during impact. Driver is 5'7" and weighs 165lbs.

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