G. Wayne Phillips, Private Investigator

10/28/2024

03/29/2024

Post 2 of 3 in this week's series.
Yesterday, we left off with a question regarding the colliding cars with PDOFs pointed off to one side of each vehicle's CG: "What does that mean for post impact travel?" The answer, of course, is that this offset (measured as "h" here) induces post-impact rotation. Maybe a lot of rotation.

The engineer-ese term for this is "a moment arm", but most normal people might think of it as a lever. The red arrow (representing the force) pushes to the left on the red van, behind the CoM. The force acts along the dashed red line, and through a perpendicular moment arm "h-van" (the grey arrow). This torque twists the van Clockwise (or to the right, for those who don't "analog").

The orange force acting thru the Taxi's damage centroid along the orange dashed line pushes rearward and to the taxi's right, acting through a shorter moment arm, "h-taxi." The forces are equal and opposite (Newton's Third Law) but since the lever is shorter (h-taxi < h-van) there is less torque, and so lower propensity to rotate. The inertias of the vehicles will come into play here, though, so it's not a slam dunk which one will actually rotate more. I'd expect the Van to spin a little more, most of the time.

I didn't start this discussion to talk about h, though. I really wanted to talk about the change in speed recorded by the vehicles' event data recorders. Any time we have impact-induced post impact rotation, the PDOF is not pointed toward the CoM. By extension, since most EDRs are mounted near the Center of Mass, and the accelerometers used to calculate the change in speed are in the EDR case, the PDOF doesn't pass through the accelerometers and they will experience a reduced acceleration. This means they will underreport the change in speed. How much they are off depends on h. We'll talk about that tomorrow.
Peace. -W

03/15/2024

A Motorcycle Vault and Fall.
Boys will be boys, right? Based on a review of his instagram account, flight1915 spends plenty of time on airborne 2-wheel machines, so one wonders what went wrong here as he jumped an '87 Harley Davidson Big Twin FXR. Maybe someone pushed the landing ramp closer when he wasn't looking? Maybe at the last minute he had the gut feeling he was going too slow at 19mph and gave it a little extra at the end? The safety director, seen jamming a cooler under the takeoff ramp, certainly seemed to have things under control, and Tyler's other flights documented online seem much closer to being right.

Looking at this one video, I estimate the takeoff angle to be about 25° and the "distance of the fall" to be 3 feet (from top of takeoff ramp to pavement level), with a horizontal distance to landing of around 35 feet. A scene evaluation could get these values more accurately...assuming that his friends didn't take the ramp and bike and leave before you showed up to the scene. I've had that happen, too. Anyway, we can plug and chug those numbers in the standard vault equation (remember - this one is TO FIRST TOUCH, not total distance, there's other formulae for just total travel.) This gets a takeoff speed of just 24mph.

Since he doesn't lose much speed in the air, his slide distance should be commensurate, but he scrubs speed on the landing, and his post impact slide looks more in line with 20mph.

The whole video is online here: https://www.youtube.com/watch?v=Aa6cfdCK9Ns
Don't try the stunt at home, but calculator button poking is encouraged!
Peace. -W

03/06/2024

Longitudinal friction (braking / accelerating) develops primarily as a function of tire slip at the roadway interface. At one extreme - the tire is rolling at the same speed as the car is going, so slip=zero, and there's essentially no forces acting on the tire. At the other extreme, the tire speed is zero (locked) but the car is still moving forward or backward and the slip is 1, or 100%. The highest forces are developed somewhere in between, typically between 15 and 20% slip.

This issue of tire slippage matters to recons for a few reasons. But the biggest that comes to mind is because EDRs record "Vehicle speed, indicated" by law - EDRs do not record the vehicle speed over the ground. So under hard braking, the speedo will read too low. Not a lot, but not nothing, either. This is the sort of weird nuance that makes it easy for untrained persons to misunderstand the "plain English" CDR-reports, and it's some of what one learns about in an EDR analyst class.

Here's a couple videos on the topic of tire slip:
https://www.youtube.com/watch?v=HUwN3rmk8Lg
https://www.youtube.com/watch?v=J3Kt30yW7LU

Peace. -W

p.s. There are also some other equations that might be used to think about tire slip, but this is the one that works for me.

04/07/2022

Today: Murphy's Law.

Around 1949, Col. J.P. Stapp was conducting his famous experiments on human tolerance. The team needed to accurately measure accelerations during this work, and in one test with a newly installed array of strain gages, got no data. Capt. Ed Murphy, referring to the assistant who installed them, said, "If there is any way to do it wrong, he will." Col. Stapp referred to this in a press conference some weeks later as "Murphy's Law," and put a more positive spin on it as being a guide to planning for obstacles and overcoming them. Turns out there's more to the story, maybe, and though the name may not have existed prior to that event, the sentiment did. According to wikipedia, an 1877 text observed that, "It is found that anything that can go wrong at sea generally does go wrong sooner or later." I have a copy of the 1977 Bloch booklet cited online, which conveys only one slice of that story (See the attached photo). I am continually amazed at what's online these days. https://en.wikipedia.org/wiki/Murphy%27s_law

May all your known unknowns and unknown unknowns cancel each other out in the most pleasant way possible.

Peace. -W

04/03/2022

A tale of Two New Tires.

TLDR: PUT THE BEST TIRES ON THE REAR.

When someone gets two new tires, where should they go? Historically, they were put on the rear. But then front-wheel-drive cars arrived on the scene and people started to put them on the front, because "That's the drive axle". The general consensus now is that is WRONG. The greatest hazard to a driver on the street is loss of control leading to going sideways into something. The best way to prevent that is to keep the rear axle behind the front, and the best way to do THAT is to have best tires on the rear. Always. If you lose traction, you're better off losing the front and sliding straight into something (airbags and seatbelts are pretty awesome in this configuration) instead of losing the rear and going sideways into something (side airbags try hard, but they just don't have the space required to protect as well as fronts). I see a few crashes and deaths each year locally from people who have made poor choices on their tire location. Sometimes based on bad advice from tire shops they should be able to trust.
https://www.tirereview.com/so-where-do-new-tires-go/

The photo here shows a car I spotted in the wild - the rear tires are former front tires, showing severe wear from poor alignment. They should be replaced ANYWAY, but they should never have been put on the rear when good tires were put on the front. Don't be that person - you and your family's safety may depend on making good choices when you get only two tires. Best tires go on the back always.

Peace. -W

03/22/2022

https://newstalkkzrg.com/2020/08/14/covid-19-related-wrap-scam-reported-in-springfield/ Please read if you have been contacted. This is a SCAM!!!!!

The Better Business Bureau says new COVID-19-related scam has been reported in Springfield. A man was contacted online and told he'd be paid $500 a month if he

12/07/2021

For cyclist.........Bi or Motor!!!!!!!!

Today's tidbit: Motorcycle Counter-steering.

All single-track vehicles, whether they are motorcycles, scooters, or bicycles, turn by leaning to the side they want to turn towards. The most effective way to initiate a lean is to turn the handlebars _slightly_ the wrong way. You might hear this coached as "push right, go right," meaning push the right handlebar grip away from you to go to the right.

Think about that for a moment. Imagine you're on a bicycle or motorcycle holding the handlebars in front of you, and push your right hand away...which way does that turn the front wheel? To the left. Not intuitive at all that we turn left to go right, but it's a fact.

Some instructional outlets attribute the efficacy of counter-steering to "gyroscopic precession." While the gyro effect is real, it is small. Researchers in the past decade or so have shown that the primary reason bikes lean right when you steer the front to the left slightly is that the front tire is steering out from under us, and the bike begins to fall over. Gravity pulls the bike towards the earth once the CG is no longer over the line formed by the front and rear contact patches.

This "turning the bars the wrong way" to initiate lean sounds wrong on the face of it, right? The movement is so slight that many motorcycle riders and almost all bicycle riders never notice it - they incorrectly attribute the lean to body language.

built a "No Body Steering Motorcycle" aka "The No BS Bike", with a set of rigid bars bolted to the chassis. If a rider holds them instead of the real handlebars, his motion doesn't affect the front wheel's direction, and he can climb all over the bike like a monkey, but only barely affect the path of travel. The lean comes from turning the front wheel turning to the opposite direction and the bike beginning to fall over, and almost not at all by leaning one's body.

Whenever a rider claims there was a problem with the steering, you should spend some time to inquire about which way they pushed when they wanted to turn, and what they did to try to recover.

Peace. -W

11/15/2021

Friction, day 4, but a break from numbers.

Pretty commonly people use the term "Drag Factor" instead of friction. When I started in recon (in the late 1980s), coming from an engineering background, I kinda hated that term. In classic Dunning-Kruger fashion, I was SURE it should be called FRICTION. But eventually, I came around. The distinction is that "friction" applies to the interaction of two surfaces sliding past one another (sometimes with some defined amount of slip), whereas "drag factor" is a measurement of how rapidly a body slows, including aerodynamics, brake inefficiencies, grade, and more.

Sometimes friction (mu) and drag factor (f) are are the same, for instance when a car stops with all four tires locked. The acceleration a is equal to mu*g and f*g, so f=mu.

But a car slowing on good pavement (mu=-0.76) under engine braking at rate of 3mph/s (a drag factor of -0.15g) is only utilizing a fraction of the available friction or traction. [Side Note: Try saying "a fraction of the friction or traction" 3 times fast. It makes me crack up.] I still sometimes use one when I really mean the other, but both terms really do have a place in our lexicon, IMO.

Peace. -W

10/02/2021

Someone sent a message earlier wanting me to call them; however, the telephone number was incomplete. My number is 912-293-2916. Please feel free to call me.

08/14/2021

Today, and every day: Newton's Laws.
Pretty much everything we do in recon has some connection to Newton's 3 laws. Just memorize them, ok? Trust me one this.

For me it's INERTIA, F is MA, Equal & Opposite.

Some day a smarty-pants attorney will ask you under oath without warning, "What are Newton's Laws?" If you stumble, he'll smell blood in the water. If you immediately say, "Inertia, F is ma, and Equal and Opposite" and go on to recount the one-sentence descriptions of each, he will stop asking questions, afraid that you'll know the answers to those "gotcha" questions, too, only raising the jury's estimation of your opinions.

Peace. -W