Legist Tracing And Investigations

22/11/2022

23/05/2022

Today: Self-healing headlight.
When an incandescent headlight filament ages, it experiences "grain boundary etching" that thins the wire's cross-section. This weakens it to the point that it can break while driving, and if it's ON when that happens, there will usually be an arc-bead left on the loose end. The now-loose ends can wiggle and wave around while driving and might actually connect and reweld themselves together. After that, the light suddenly works again. For a little while. This joint will be a weak spot, though and the filament will fail again soon, you can be sure of it.
Peace. -W

13/05/2022

Equations we Love, Episode 7: 𝗡𝗲𝘄𝘁𝗼𝗻’𝘀 𝟯 𝗹𝗮𝘄𝘀

Sir Isaac Newton's observations of the physical world still underpin pretty much every analysis we do. His three laws should be burned into your memory as a block.

Eventually, if you do this work long enough, someone will get you under oath on the stand in front of a jury and ask, “𝑾𝒉𝒂𝒕 𝒂𝒓𝒆 𝑵𝒆𝒘𝒕𝒐𝒏’𝒔 3 𝑳𝒂𝒘𝒔?” When that happens, if you can confidently turn to the jury (remember to face them – they are the ones who matter in this setting, not the attorney) and give a good and confident answer, you can use the chance to demonstrate for the jury that you know what you're doing, and that attorney won't want to ask you any more questions, for fear you'll hit him over the head with that answer, too.

My response to the question goes something like this:

"The First Law is 'Inertia' - a body at rest stays at rest until acted upon by an external force. The Second Law is F equals m a, or Force is Mass times acceleration. Newton's Third Law states that forces between two bodies acting on each other are Equal and Opposite, sometimes recited as "For every force there is an equal and opposite force.”

If you stumble on the answer here, they will sense weakness like a pack of hyena, right? So just memorize them.

𝟭. 𝗜𝗻𝗲𝗿𝘁𝗶𝗮
There was a Nike commercial years ago that used the caption “A body in motion stays in motion” – I still remember the picture of a runner, and oddly, that helps me remember “A body at rest stays at rest”, too.

𝟮. 𝗙 𝗶𝘀 𝗺𝗮
Newton actually said that the acceleration a body experiences is proportional to the force applied and inversely proportional to the body’s own mass, more or less, but most of us learn it as force equals mass times acceleration. I like this one so much that I had a license plate in homage to Sir Isaac for several years on my Mustang.

𝟯. 𝗘𝗾𝘂𝗮𝗹 𝗮𝗻𝗱 𝗢𝗽𝗽𝗼𝘀𝗶𝘁𝗲.
The forces between two bodies acting on each other are equal and opposite. When you stand on the floor, you are exerting a downward force on the floor equal to your weight (W), while the floor is pushing upwards with exactly the same force (W). If they were out of balance, you would be accelerating - either into the air, or toward the center of the earth. The swimmer in the picture here is pushing off from the wall, but her heels are stationary on the stationary wall as her body unfolds. She creates a higher force on the wall if she pushes off harder, but the wall doesn't move, right? So it is pushing back with an exactly equal and opposite force at all times.

Just memorize them, ok?
Peace. -W

07/05/2022

Equations We Love, Episode 4: The Combined Speed Formula.

When a vehicle's path dissipates energy in different stages, either by skidding over multiple surfaces, or perhaps by having minor impacts or curb-strikes along the way, we commonly add up all the energy dissipated to find the energy at the start (because energy is neither created nor destroyed, right?). While skidding, say on pavement before getting to the shoulder, we commonly use the "minimum speed" formula [Sqrt(30*d*f)]. but rather than calling it "the energy dissipated", we should more accurately call it the "energy equivalent speed."

For instance, if a car skids from 50mph to 40mph on pavement with a drag factor of 0.76g, it will cover about 39.5 feet (go ahead, do the math, I'll wait). If you run the minimum speed equation for those values (D=39.5ft, f=0.76g), the result is 30mph. Now where was that 30mph in the original statement? It wasn't. This is tricky.

What that 30 represents is the kinetic energy carried by a car at the start of that skid if it had come to a stop in that distance. More generally, it is the ENERGY EQUIVALENT SPEED dissipated during that 39.5 foot skid.

In a reconstruction of that event, if you had some way to know that its speed at the end of that distance was 40mph (perhaps from some other parts of a complete analysis), to find the speed at the start of the 39.5-foot skid, you would use the Combined Speed Formula, which says the initial speed is the square root of the sum of the squares of all the energy equivalent speeds along the way. The math looks like this:
Sinitial = SQRT( S1^2 + S2^2 +S3^2 +...)
Or for our hypothetical situation:
Sinitial = SQRT (40^2 + 30^2) = 50mph.
Because energy is proportional to the square of the speed, the energy dissipated by slowing from 50 to 40mph (a change of only 10mph) is the same as the energy dissipated by slowing from 30mph to zero.

The Delta-V recorded by an EDR is different: it is the ACTUAL delta-v (including restitution), so you can't use it in the combined speed equation except under some really special circumstances. But that's a discussion for another day. Most recon texts books will have this derivation somewhere, though sometimes it's called the "combined energy equation".
Peace. -W

23/04/2022

Equations we Love, Episode 3: Motorcycle Cornering and CSY

Having discussed Critical Speed Yaw in the last episode, it seems timely to mention that the exact same equation governs the relationship between lateral traction usage, motorcycle lean angle, and path radius. The derivation is shown below but can be found in most general recon texts. An SAE paper a couple years ago confirmed that the theory is pretty good...there is some play as a result of operator positioning and tire width, but I'll let you read that for yourself, here:https://static1.squarespace.com/static/5dc9c7180417d3127fdab3bc/t/5dd05b7bfbae9d7217844e61/1573936005047/2015-01-1422.pdf

and here:https://static1.squarespace.com/static/5dc9c7180417d3127fdab3bc/t/5dd04e656d59eb44ead6c9c2/1573932708371/2018-01-0529.pdf

Peace. -W

11/04/2022

Equations We Love (and sometimes hate), Episode 2: 𝘾𝙧𝙞𝙩𝙞𝙘𝙖𝙡 𝙎𝙥𝙚𝙚𝙙 𝙔𝙖𝙬 (CSY)

This is a great and useful tool that gets misapplied more than any other in my experience. When a car is turning, it uses lateral traction. When a car uses all the lateral traction available, it will begin to rotate more than its path would require, in other words, the back end starts to "step out." As it starts to slide to the outside, the tires will mark the pavement. The two outside tires (usually the best-marking tires) leave diverging marks. If we carefully measure the marks, we can assess the car's path radius.

The total force of friction pushing towards the center of car's path is the product of weight and friction (w*f = mgf), this is the “center-seeking” or centripetal force acting on the car. The equal and opposite inertial force acting to pull the car away from path center is called the centrifugal, or “center-fleeing” force is equal to [(mv²)/r]. Since these are equal in magnitude, we can say that ( mgf = mv²/r ). The mass cancels out – so the weight doesn’t matter – and we’re left with v²=rgf. This defines the velocity - radius - friction relationship.

Converting to speed (mph), radius (feet), and drag factor (g's), and doing a little algebraic rearranging, we get: 𝐒=𝟑.𝟖𝟔*𝐒𝐐𝐑𝐓(𝐫𝐟). This relies on a bunch of simplifications, such as all four tires are generating maximum lateral friction (so there is officially no braking, among other things), the path is circular, all four tires are pointed straight ahead, the lateral forces are acting directly toward the center of the path (in other words, the car's slip angle is zero), and more.

These simplifications provide lots of directions for people who don't like this technique to attack it as unreliable. However, extensive testing over many many years has shown empirically that the technique, when applied correctly, gives good results that tend to be a little conservative. If ABS or stability control are active, it gets even more conservative.

The most common problem I see is people applying it to cases where there's only one mark and insufficient scene data to confirm slip angle at least roughly, meaning some of the underlying foundations for the equation are not supported, and it shouldn't be used. The other common misapplication is measuring a curved mark after the car has turned too far and has high sideslip, so it's no longer CSY but approaching a broadslide. This gets a calculated speed much too high because the lateral forces are mostly acting to slow the car, not turn it.

Misapplication of the equation is easy to do, and can sometimes get very wrong results.

Some additional reading on this topic can be found at NAPARS member John Daily's website:http://www.jhscientific.com/downloads/CSY_handout.pdf, Bellion's SAE paper https://www.sae.org/publications/technical-papers/content/970955/ , and also in the Reference Library ARJ archive:
ARJ 5(6) Nov 1995 for a good summary article by Al Baxter;
ARJ 5(3) May 1993 pg 48;
ARJ 7(1) Jan 1995 pg 43;
ARJ 18(3) May 2008 pg 28, and more.

Peace. -W

05/04/2022

Today's Scene Observation: Not all fresh skids are part of your wreck.

When I arrived, the officer on scene was happy to show me the skidmark from the crash. On closer examination, the skid on the passenger side, which disappears under the torn up tire, is the wrong shape for a side-sliding tire. As a bonus, it didn't fit the rest of the dynamics of the event (there was a pole on the opposite side of the street involved). This mark was just a red herring.

Peace. -W

05/04/2022

Today's Tool Tip: Measuring Crush.

A few years ago, an investigator for NHTSA came to an inspection of this car which experienced a serious frontal impact. He brought a pair of folding music stands, and set them at a common distance from the undamaged rear axle to serve as a reference baseline. He tied a string at bumper height and used tape on the string to flag the locations for his 6 crush measurements (equidistant on the original face, now distorted on the car). Low tech, but also low cost, easily transportable on a plane, highly adjustable, easy to explain, and repeatable enough for the task of measuring crush. He let me take a picture of it. Thanks, NHTSA-guy!

Peace. -Wade

15/03/2022

Today's Action Shot of someone's worst day of their life: Yep. I can feel that gouge, but I can only see the scratches next to it. That's how I choose which word to use in my reports. Peace. -W