Are there any folks who have conducted testing regarding the critical speed formula? I am presently conducting research for a presentation and would appreciate any replies. Thanking you in advance,
Lenny Simpson
Critical Speed Formula Evaluation Testing
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actar670
- Posts: 182
- Joined: Wed Feb 15, 2012 7:43 pm
- First Name: Bruce
- Last Name: McNally
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Re: Critical Speed Formula Evaluation Testing
Lenny,
I have listed the abstracts for a few technical papers that deal with the validation of the critical speed methodology. These papers can be obtained at the SAE website for a fee. The search page for SAE technical papers can be found at the following link:
http://papers.sae.org/automotive/
2005-01-1189 Validation of the Circular Trajectory Assumption in Critical Speed
Abstract:
The critical speed model is a tool used by accident reconstructionists to determine vehicle speeds. One assumption implicit in the model is that when in a critical speed yaw, the vehicle's center of mass travels in a circular arc. The validity of this assumption was investigated by comparing the results obtained by manually measuring the tire marks, assuming them parallel to the center of mass path, and fitting a polynomial. The results indicate that the assumption of a circular path is reasonably accurate.
Author(s):
Oren Masory Florida Atlantic Univ.
Samuel Delmas IFMA - Institut Francais De Mechanique Advancee
Bill Wright University of North Florida
Wade Bartlett Mechanical Forensics Engineering Services, LLC
2004-01-1187 Yaw testing of an instrumented vehicle with and without braking
Abstract:
Two methods for calculating speed from curved tire marks were investigated. The commonly used critical speed formula and a computer simulation program were evaluated based on their ability to reproduce the results of full-scale yaw tests. The effects of vehicle braking and friction coefficient were studied. Twenty-two yaw tests were conducted at speeds between 70 and 120 km/h. For half of the tests, about 30% braking was applied. Using the measured sliding coefficient of friction, both the critical speed formula and the computer simulations under-predicted the actual speed of the vehicle. Using the measured peak coefficient of friction, both methods over-estimated the actual speed. There was less variance in the computer simulation results. Braking tended to increase the speeds calculated by the critical speed formula.
Author(s):
William E. Cliff MacInnis Engineering Associates
Jonathan M. Lawrence MacInnis Engineering Associates
Bradley E. Heinrichs MacInnis Engineering Associates
Travis R. Fricker MacInnis Engineering Associates
2009-01-0103 Variability of Yaw Calculations from Field Testing
Abstract:
This paper discusses the variability of critical speed calculations from controlled field tests. A total of 29 tests were conducted at speeds of 80 to 95 km/h, using three different vehicles, three tire types, and variable braking levels. Results of the testing are provided. Analysis of the data revealed: a chord length of 20 m measured from separation provided the most consistent results; non-ABS braking affects the results of the critical speed formula, whereas ABS braking provides similar results as non-braked vehicles; using 20 m chord measurements for the radius and the average braking coefficient of friction overestimated the measured speed at separation by 4.1% ± 6.3% (± 1 standard deviation) for all nonbraking and ABS braking tests; using center of gravity measurements along the curve for the radius and the average braking coefficient of friction underestimated the measured speed at separation by 2.0% ± 5.2% (± 1 standard deviation) for all non-braking and ABS braking tests, and there is a correlation between braking level and striation angle.
Author(s):
Gilles Amirault SAMAC Engineering Ltd.
Steve MacInnis SAMAC Engineering Ltd.
970955 Project Y.A.M. (Yaw Analysis Methodology) vehicle testing and findings, Victoria Police, Accident Investigation Section
Abstract:
A vehicle oversteered or cornering at excessive speed results in the tires of the vehicle loosing traction with the road surface. As a result, tire yaw marks may be left on the road surface. Yaw marks are common at fatal collision sites. Various methods are reported to estimate the speed of the vehicle that leaves yaw marks on a road surface. The difference in each method is how to determine the radius and whether the peak or average friction is used.Tests were conducted with four different vehicles. Variations in tire pressures, and driver inputs of acceleration, braking and steering over-correction were investigated. Yawing followed by emergency braking with and without ABS was further studied. The radar speed of the vehicle for each test was compared with speed estimates from the yaw marks.
Author(s):
Peter Bellion Victoria Police
I have listed the abstracts for a few technical papers that deal with the validation of the critical speed methodology. These papers can be obtained at the SAE website for a fee. The search page for SAE technical papers can be found at the following link:
http://papers.sae.org/automotive/
2005-01-1189 Validation of the Circular Trajectory Assumption in Critical Speed
Abstract:
The critical speed model is a tool used by accident reconstructionists to determine vehicle speeds. One assumption implicit in the model is that when in a critical speed yaw, the vehicle's center of mass travels in a circular arc. The validity of this assumption was investigated by comparing the results obtained by manually measuring the tire marks, assuming them parallel to the center of mass path, and fitting a polynomial. The results indicate that the assumption of a circular path is reasonably accurate.
Author(s):
Oren Masory Florida Atlantic Univ.
Samuel Delmas IFMA - Institut Francais De Mechanique Advancee
Bill Wright University of North Florida
Wade Bartlett Mechanical Forensics Engineering Services, LLC
2004-01-1187 Yaw testing of an instrumented vehicle with and without braking
Abstract:
Two methods for calculating speed from curved tire marks were investigated. The commonly used critical speed formula and a computer simulation program were evaluated based on their ability to reproduce the results of full-scale yaw tests. The effects of vehicle braking and friction coefficient were studied. Twenty-two yaw tests were conducted at speeds between 70 and 120 km/h. For half of the tests, about 30% braking was applied. Using the measured sliding coefficient of friction, both the critical speed formula and the computer simulations under-predicted the actual speed of the vehicle. Using the measured peak coefficient of friction, both methods over-estimated the actual speed. There was less variance in the computer simulation results. Braking tended to increase the speeds calculated by the critical speed formula.
Author(s):
William E. Cliff MacInnis Engineering Associates
Jonathan M. Lawrence MacInnis Engineering Associates
Bradley E. Heinrichs MacInnis Engineering Associates
Travis R. Fricker MacInnis Engineering Associates
2009-01-0103 Variability of Yaw Calculations from Field Testing
Abstract:
This paper discusses the variability of critical speed calculations from controlled field tests. A total of 29 tests were conducted at speeds of 80 to 95 km/h, using three different vehicles, three tire types, and variable braking levels. Results of the testing are provided. Analysis of the data revealed: a chord length of 20 m measured from separation provided the most consistent results; non-ABS braking affects the results of the critical speed formula, whereas ABS braking provides similar results as non-braked vehicles; using 20 m chord measurements for the radius and the average braking coefficient of friction overestimated the measured speed at separation by 4.1% ± 6.3% (± 1 standard deviation) for all nonbraking and ABS braking tests; using center of gravity measurements along the curve for the radius and the average braking coefficient of friction underestimated the measured speed at separation by 2.0% ± 5.2% (± 1 standard deviation) for all non-braking and ABS braking tests, and there is a correlation between braking level and striation angle.
Author(s):
Gilles Amirault SAMAC Engineering Ltd.
Steve MacInnis SAMAC Engineering Ltd.
970955 Project Y.A.M. (Yaw Analysis Methodology) vehicle testing and findings, Victoria Police, Accident Investigation Section
Abstract:
A vehicle oversteered or cornering at excessive speed results in the tires of the vehicle loosing traction with the road surface. As a result, tire yaw marks may be left on the road surface. Yaw marks are common at fatal collision sites. Various methods are reported to estimate the speed of the vehicle that leaves yaw marks on a road surface. The difference in each method is how to determine the radius and whether the peak or average friction is used.Tests were conducted with four different vehicles. Variations in tire pressures, and driver inputs of acceleration, braking and steering over-correction were investigated. Yawing followed by emergency braking with and without ABS was further studied. The radar speed of the vehicle for each test was compared with speed estimates from the yaw marks.
Author(s):
Peter Bellion Victoria Police
Bruce McNally
McNally & Associates ARS, LLC
41 Champlin Ridge Road
Rochester, NH 03867
Office (603) 516-4560
Web http://mcnallyassociates.com
McNally & Associates ARS, LLC
41 Champlin Ridge Road
Rochester, NH 03867
Office (603) 516-4560
Web http://mcnallyassociates.com
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Jhowell567
- Posts: 1
- Joined: Mon Jan 16, 2012 8:31 am
- First Name: John
- Last Name: Howell
- Location: Las Vegas, Nevada
Re: Critical Speed Formula Evaluation Testing
Myself and an associate in my firm are currently working on a project studying differences in measuring and determination of radius of curves from yaw marks. Results from this study will be presented at the 2012 ARC/CSI Conference in Las Vegas. It will also be published in Collision Magazine. It's specific focus is on the measurement of the yaws from at scene and a determination at a later time from a scale diagram. If we can be of any assistance feel free to contact us.
John Howell
ACTAR 1543
JH&A LLC
3864 Comb Court
Las Vegas, NV 89104
(702) 256-6470
ACTAR 1543
JH&A LLC
3864 Comb Court
Las Vegas, NV 89104
(702) 256-6470
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Bob Anderson
- Posts: 196
- Joined: Fri May 27, 2011 8:13 am
- First Name: Bob
- Last Name: Anderson
- Location: Tempe, AZ
- Contact:
Re: Critical Speed Formula Evaluation Testing
Other resources that may be of interest:
Dickerson et al. "Evaluation of Vehicle Velocity Predictions Using the Critical Speed Formula," SAE Paper 950137
Dr. Limpert addresses directional stability in Chapter 26 of his Motor Vehicle Accident Reconstruction and Cause Analysis
Fricke also addresses velocity to side-slip in Traffic Accident Reconstruction.
Bob Anderson
Tempe, AZ
Dickerson et al. "Evaluation of Vehicle Velocity Predictions Using the Critical Speed Formula," SAE Paper 950137
Dr. Limpert addresses directional stability in Chapter 26 of his Motor Vehicle Accident Reconstruction and Cause Analysis
Fricke also addresses velocity to side-slip in Traffic Accident Reconstruction.
Bob Anderson
Tempe, AZ
Bob Anderson
Tempe, Arizona
Tempe, Arizona
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Bob Anderson
- Posts: 196
- Joined: Fri May 27, 2011 8:13 am
- First Name: Bob
- Last Name: Anderson
- Location: Tempe, AZ
- Contact:
Re: Critical Speed Formula Evaluation Testing
What vehicle(s) are you planning on using for your demonstration?
Bob Anderson
Tempe, Arizona
Tempe, Arizona
Re: Critical Speed Formula Evaluation Testing
Bruce, John & Bob,
Thanks for your recommendations and cites concernning this matter. My approach is to develop a course project to demonstrate the differences in various measuring techniques of a critical yaw. It is also my intention to highlight discrepancies and attempt to determine why these differences exist. By presenting the identified “pit falls” the course attendees would be cognizant of their future measuring methods.
Again, thank you Gentlemen for your assistance!
Lenny Simpson
Collision Analysis & Reconstruction LLC
Olney, Maryland
Thanks for your recommendations and cites concernning this matter. My approach is to develop a course project to demonstrate the differences in various measuring techniques of a critical yaw. It is also my intention to highlight discrepancies and attempt to determine why these differences exist. By presenting the identified “pit falls” the course attendees would be cognizant of their future measuring methods.
Again, thank you Gentlemen for your assistance!
Lenny Simpson
Collision Analysis & Reconstruction LLC
Olney, Maryland
