Beginners Error#1 in Crash Reconstruction 101
Posted: 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.
- Here is the 'expert' report.
the important section is:- 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!
- 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!
- He adds the 16 MPH it to the 34 MPH to come up with 50 MPH pre-impact travel speed!
- Beginners error #1 in Crash Reconstruction 101.
- Let’s assume there is a 16 mph impact speed change, and so we assume she’s going 16 MPH at impact (if into a fixed object).
- ->The 50 feet of skids were made from some unknown speed down to 16 MPH, NOT from 34 MPH down to 0 MPH!!
- Which of course then means the 16 MPH isn’t cumulative on the Escort either?!!
- in that case the escort is going 34 MPH!
would be 38 MPH, not 50 MPH.- Vinitial**2 = Vimpact**2 + 30 (MU) (Distance)
- Distance of Skid using Speed from skid equation
- What is the proper MU (the drag factor) in the particular case? (wet, dry, new, old, etc.?)
- Also this doesn’t include any speed on the part of the truck,
- Speed on the truck would reduce the required forward speed of the Escort at IMPACT to produce his opined 16 MPH deltaV.
- So 1st if you do a damage analysis it 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,
- if there was an underride,
- 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.
- And this of course is with the experts assumption of 0.8 friction coefficient.
- However it may have been higher (ABS?) but a range of coefficients of surface friction should be considered!
See our forum topic Specs, Accel, Braking, Crash Tests. Motorcycles which includes many topics on coefficient of friction for various surfaces and conditions
- Let’s assume there is a 16 mph impact speed change, and so we assume she’s going 16 MPH at impact (if into a fixed object).