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MFES: Publication Summaries |
The following are summaries of papers authored (or co-authored) by Wade Bartlett, at Mechanical Forensics. Click on the title to go to the summary. The SAE papers are copyrighted by them. MFES has have a limited number of hardcopies available free for whoever asks for them first, or they can be ordered through the SAE website at http://www.sae.org. The others are a little harder to find in the real world, but copies of the non-copyrighted ones can generally be sent via email upon request to the email contact listed below.
- Passenger Vehicle Braking Performance with a Disabled Vacuum Power Booster
- Investigation of Elastic, Plastic, and Creep Response of a Polymeric Clip Using Experimental and Numerical Techniques
- Estimating Truck Speed Based on Strap Strength
- Forensic Evaluation of Relative Sideswipe Speeds
- Driver Abilities in Closed-Course Testing
- Interpretation of Motorcycle Rear-Wheel Skidmarks
- Quantifying the Uncertainty in Various Measurement Tasks
- Accuracy of Vehicle Speed Estimates by Stationary Witnesses
- Protecting Yourself After the Accident
- Motorcycle Speed Estimates Using Conservation of Linear and Rotational Momentum
- Conducting Monte Carlo Analyses with Spreadsheet Programs
- Evaluating Uncertainty in Accident Reconstruction with Finite Differences
- Data Extraction from Sensory Data Obtained During Driving in Closed Course
- Emergency Response RWD v. FWD
- Calculation of Heavy Truck Deceleration Based on Air Pressure Rise-Time and Brake Adjustment
- Validation of the Circular Trajectory Assumption in Critical Speed
- Comparison of Drag Sled and Skidding Vehicle Drag Factors on Dry Roadways
- Calculation of Deceleration Rates for S-Cam Air-Braked Heavy Trucks Equipped with Anti-Lock Brake Systems
The brake system in modern passenger vehicles typically includes a vacuum booster. If the booster ceases to function properly, the pedal effort required to lock the wheels goes from a typical value of about 60 pounds (250 Newtons) to something closer to 200 pounds (900 N). This condition causes the brake pedal to get "high and hard," and is generally described by drivers as "brake failure." The pedal-depressing capabilities of men and women are examined, along with Federal regulations, and test results from several vehicles, in order to evaluate the possibility of estimating a person's deceleration rate during an accident involving a disabled booster.
Bartlett, Sherwood
The load-holding performance of polymeric clips exposed to elevated temperatures for 3 months was experimentally determined and compared to finite element predictions generated using two material models: one fairly simple one, and one which included an Arrhenius-type temperature term in the constitutive model. Parameters for both models were generated using least-squares curve fitting methods and standard tensile and creep test data available for ULTEM 1000. The variability of performance in the clips themselves was large enough to obviate any value in using the more complicated model in this case.
Bartlett, Hochgraf
Large rectangular loads, such as lumber, are commonly strapped in place on flat bed trailers. Following a curved path and/or roadway superelevation can generate lateral forces high enough to cause the load to shift, stretching the straps, eventually to their breaking point. This paper presents a method of estimating the speed of such a vehicle based on the strength of the straps, including a sensitivity analysis. The result of scale model testing is presented, showing excellent agreement between predicted and experimental lateral force required to break the straps. A pdf-version of this paper is available by writing to wade@mfes.com
Further research on topics discussed in this paper, including friction between loads & decks, tension in straps, and effects of cargo movement on strap tension, has been published by the CCMTA and can be downloaded for free from the US DOT Document Management Service search for Docket 2289, and then look for the CCMTA reports - Warning, some of these files are 20Meg or more.
Bartlett, Hochgraf
During a sideswipe accident a rotating wheel of one vehicle may leave marks on another's side panels. This type of contact can occur during truck-to-automobile and motorcycle-to-automobile sideswipe accidents. The ratio of the speeds of the two vehicles involved can be computed once the marking tire's axle height and the period of the marks have been determined. This paper introduces the moving-frame-of-reference analysis which forms the basis of both the direct measurement method and a graphical comparison method. The former requires direct measurement of the mark period, while the latter offers a means for determining the mark period when the mark in not sufficiently complete to allow direct measurement.
A previously presented vector-based method, founded on the principle of instant-centers-of-rotation, is also reviewed. This method can be used in cases where no mark period can be determined. A pdf-version of this paper is available by writing to wade@mfes.com
Bartlett, Masory, Wright
The most frequently cited papers on driver abilities are somewhat dated. This paper reports on the abilities of a large sample of drivers as they negotiated a closed cone-marked course using modern vehicles. The steering wheel position, brake line pressure, and throttle application were monitored, along with vehicle chassis accelerations.
The objective of this paper is to report on the physical inputs utilized by operators, and compare gender-specific and vehicle-specific results. Willingness limits and g-g diagram results are presented. Results of this testing show that typical steering wheel rates in a modern vehicle are higher than earlier reported values, brake pedal forces utilized are significantly lower than those of which people are physically capable, only approximately half of all drivers utilized sufficient pedal force to lock the wheels, a significant portion of drivers never utilized wide-open-throttle, and the willingness limits for the two vehicles were virtually identical, despite variations in vehicle capabilities.
There is a commonly held perception among law-enforcement drivers that traditional rear-wheel-drive cars are best suited to their performance-driving needs. However, the average and standard deviation of the front-wheel-drive group's elapsed time through the course were smaller than those of the rear-wheel-drive group, suggesting that traditional rear-wheel-drive vehicles require more skill to operate in near-limit conditions. This paper can be purchased at SAE's website, www.sae.org
When a motorcycle is involved in an accident, it is common to find a single skidmark leading up to the area of impact. It is sometimes averred that a wavy skidmark indicates the rear wheel was locked with no front brake use, while a straight skidmark indicates that the front brake was in use while the rear wheel was locked. This paper will discuss the interpretation of such skidmarks, and present the results of testing conducted with several motorcycles. It was determined that the skidmarks often became wavy at low speeds, but that the marks were often essentially straight at speeds above approximately 20 km/hr. A means of estimating the deceleration for any specific motorcycle during rear-wheel-only skidding is presented. The analysis is based on estimation or measurement of the vehicle's center of gravity and includes dynamic load transfer. Test results are compared to published data. A pdf-version of this paper is available by writing to wade@mfes.com
Bartlett, Wright, Brach, Baxter, Schmidt, Masory, Stanard
When performing calculations pertaining to the analysis of motor vehicle accidents, investigators often must select values for a number of parameters, such as drag factor, distances along roadways, crush depths, skid lengths, and yaw marks. The uncertainty of the final answers is a function of the variations of each parameter involved in the calculation. Though it has long been recognized that variations exist in all measured data, including those related to accident analysis, very little published information exists to assist the investigator in assigning appropriate values to the input parameter variations.
This paper presents the results of recent tests conducted to obtain sample distributions of some common parameters, including measurements made with tapes, measurements made with roller-wheels, skidmark measurements, yawmark measurements, estimation of crush damage from photographs, measurement of crush damage, and measurement of drag factors, all of which may be used for uncertainty analyses. The paper also reviews the distributions of pertinent data reported by other researchers in the accident reconstruction field. Prudent application of proper distributions and ranges can lead to more accurate accident reconstructions, regardless of which range-evaluation method is selected. This paper can be purchased at SAE's website, www.sae.org
Bartlett, Wright, Brach, Baxter, Schmidt, Masory
The ability of stationary witnesses to estimate the speed of a passing vehicle was tested with a large group of participants. During one run, participants were not warned that this experiment was going to be conducted. During two others, they had foreknowledge of the experiment. The results indicate that stationary witnesses individually are not capable of accurate speed estimates, but the average estimate for groups of 25 or more may be very close to the true speed.
Information to help riders involved in accidents protect their own interests after an accident, including preserving scene and vehicle evidence, recording witness names, and taking useful photographs.
McNally, Bartlett
This paper discusses the conflicting published information on the use of conservation of linear momentum in motorcycle/automobile collisions. The proper methodology for both linear and angular momentum analyses in motorcycle collisions is reviewed and two case studies are included as examples of successful use of these techniques. The use of linear and angular momentum in collisions where significant weight disparities exist between the vehicles should always include a sensitivity analysis that evaluates the level of confidence of the speed estimates. Use of the sensitivity analysis will allow the reconstructionist to determine if the techniques should be applied to the given analysis or be abandoned in favor of other methods of speed analysis.
Monte Carlo analysis has been shown to be a powerful tool for evaluating confidence limits and probability distributions for values calculated during the analysis of vehicle accidents. Using this tool has generally required specialized software. This paper presents a method of using the tools provided with most simple spreadsheet programs to conduct Monte Carlo analysis with both evenly distributed and normally distributed variables for cases where the equations can be expressed in closed form. The accuracy one can expect given a particular number of trials is discussed. Example analyses using both even-probability and normal-probability variables are shown.
Bartlett, Fonda
Various methods of assessing the overall uncertainty in a set of calculations based on the uncertainty of each variable have been promulgated. For most real-world accident analyses the available methods are simplistic, mathematically intractable, or highly computation-intensive. This paper presents a numeric approach to the partial differentiation technique that requires no high-level mathematical ability to apply, uses very little computation time, provides good results, and can be used with analysis packages of any complexity. Additionally, the method inherently incorporates a sensitivity analysis allowing evaluation of the effects of each variable. Three examples of uncertainty evaluation in accident analyses are presented.
Masory, Benoit, Wright, Bartlett
Sorry, no abstract available at this time for this paper.
Bartlett, Wright
A comparison of timed results through a cone-course obtained using a FWD cruiser and a RWD cruiser. Though the RWD car had a horsepower advantage that theoretically should make it faster, getting the most out of it required more skill than most drivers displayed. The FWD car was more forgiving, and despite a power deficiency generated overall times that were slightly lower than the RWD unit. Driver-selected limitations on cornering appear to be about the same for both vehicles.
It has been shown that one can calculate the braking deceleration capabilities of an air-braked heavy truck given a modest amount of information about the components in the brake system and their adjustment level. The error introduced by ignoring the transient air pressure effects early in the event has been found to be negligible during stops from normal road speeds, but during stops from low speeds, the actual decelerations achieved can be expected to be lower than this overall average value. This paper presents an extension of the Heusser analysis technique to include the air pressure rise-time.
Masory, Delmas, Wright, Bartlett
The path carved out by a car as it creates a critical speed tire suff was found to be well modelled by a simple circle, as compared to several other more complicated trajectories.
Bartlett, Baxter, Livesay, Schmidt, Stanard, Wright
Lightweight "drag sleds" have long been used by crash investigators to determine the "drag factor" at a crash scene. Despite this long history, no published work has ever shown a correlation between drag sled results and the skidding performance of vehicles on multiple "uncalibrated" surfaces. Indeed, some researchers have noted that their testing appeared to show a poor correlation between the two. It has become clear in recent years that the interaction between braking or skidding tires and pavement does not fit the simple weight- and speed-independent friction model that has been assumed, leaving the accuracy of drag sleds in doubt. This paper presents the results of several comparison tests at different locations, involving multiple skid-test vehicles, dozens of drag sleds of various designs, and more than a hundred "pullers," and attempts to correlate the results of the two methods. Drag sleds tended to read higher than even the peak value generated with skidding cars, though occasionally this was not the case, and no relationship between individual drag sled results and skidding vehicle results could be identified which applied across multiple surfaces.
Evaluating the deceleration rate of air-braked heavy trucks based on brake components and their adjustment has been well documented for vehicles not equipped with anti-lock brake systems (ABS). Though there are many non-ABS trucks still on the road, ABS is becoming more common, having been required by NHTSA on all heavy trucks manufactured since March 1998. It has been found that the stopping-capabilities of ABS-equipped trucks are often not well modeled by the traditional Heusser technique. This paper will present an expansion of that method to accommodate ABS-equipped trucks, a means of characterizing air brake chambers to obviate the need for lookup tables, and discuss some sources of uncertainty in the analysis. A comparison of predicted stopping characteristics to test results for one truck is presented.
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This page last modified 23-MAR-2007