Motorcycle Accident Reconstruction Continued

Motorcycle Accident Reconstruction Continued

crashed motorcycle in Las Vegas

It all starts with the triangle of reconstruction. If you remember Pythagorean theorem from your 10th grade geometry class, you know that if you know two sides of a triangle, you can calculate the third side. The triangle as it relates to riding is TIME-SPEED-DISTANCE. You are already familiar with the idea. For instance, if you are going 60 mph for 30 minutes (speed and time), you can easily calculate distance. Hired reconstruction experts use the formulas to try to convince juries of their view of the case. The basic relationship formula is, for ease of calculation, 1.5 feet per second per mph. (It’s actually slightly less, but 1.5 is easier to play with). So at 60 mph, you are traveling 90 feet per second. (actually 88 feet, but for this discussion, 90 feet is easier). So if you know someone traveled 90′ in one second, you can do the math and calculate the speed to be 60 mph. Reconstruction experts play with every variable from this point forward. Some of the bigger pieces of data they use are:

1. Skid or slide marks;
2. Braking factors; and,
3. Coefficients of Friction for both the tires and road.

Most of the time, we find all kinds of assumptions and guess work in these numbers. As to skids, every rider knows that you can’t leave more a few feet of locked front wheel skid. Yet I’ve seen experts described “two wheel locked skid” of 80 feet or more. I’ve seen experts testify that they can tell if a skid is front or rear wheel by the width of the skid. I’ve seen experts create “pre braking” or “impending brake” marks in order to get distances they need. Meaning a “shadow” mark on the road way that only they can see that indicates pre-lock deceleration. I’ve seen experts use imaginary “slide to stop” formulas that are untethered to any scientific standard, or worse, on brake marks that “stop” into the side of a car, so that even if there is a standard, it doesn’t apply because the slide to stop was interrupted by a crash. Juries don’t understand these issues, they bring their car driving experience to the table and assume a bike works the same way. A non-riding attorney thinks in terms of cars too, and fails as far as cross examining the expert on these issues.

As to braking factors, motorcycles do not brake in the same rate as a car. Weight transfer is much more dramatic on a bike, and a skilled rider can lift the rear wheel off the ground in a braking situation. Yet I see experts apply .4 g braking factors to a bike, when a knowledgeable expert will agree that a bike can exceed 1.0 g of braking. This will dramatically alter the calculations as far as stopping distances. A high performance car might achieve .7, while an average bike should achieve a .7. So another area of manipulation for experts is using made up braking factors in order to manipulate their speed and distance calculations.

Coefficients of friction (COF) are rarely tested by experts. Using a high performance sport bike tire will result in different COF, and therefore, different braking distances. A cold hard slippery road with a low COF will require a longer stopping distance than a warm, clean fresh road surface. For the sake of the calculation, experts will use “assumptions” for COF’s. Changing the COF changes the speed calculations. So another area to target these guys is their assumptions about COF and why they picked what the they picked and the effect on their calculations if they had picked another number out of thin air.

There is a lot more to a reconstruction, but we have seen a rash of failures by experts lately using programs that don’t even have motorcycle parameters programmed in. So they pick a light weight car in the place of the bike to let the computer run the simulation. A light weight car that weighs 3,000 pounds, and that doesn’t stop the same as a bike, and doesn’t steer the same as a bike, will generate much different numbers for speed calculations, to the delight of a defense lawyer. These are traps for the unwary.

Another area that has evolved over time, is the idea of perception and reaction (PNR) time. The “oh shit” moment it takes your mind to perceive and start your body to reaction. It used to be said that bikers were faster than car drivers, and that they had 1.25 to 1.5 second PNR times. The new studies show its probably more like 2 to 3 seconds. That difference dramatically increases total stop time, and has dramatic effects on speed calculations. By manipulating the time and distance with assumption upon assumption, the expert can then force a higher speed number in his calculations. If you don’t know where to look, you lose this issue to the expert and watch the expert tell the jury how your biker client was going 25-30 mph over the speed limit, feeding into the jury’s preconceived notions that we all have a death wish.

The only legitimate way to calculate speed is the vault of the rider from the point of impact to point of rest. Assumptions can be made to try to fit any pre-impact physical evidence into the reconstruction, such as skid marks, but the absence of a vault takes away the ability of an expert, or the cops, to make speed calculations. Crush damage does not work with bikes or bike v. car collisions either.

Obviously their is much more to these work ups. In one case, I spent 16 hours deconstructing, piece by piece, formula by formula, a defense expert’s opinions. He made fun of me every step of the way, but at the end of the day, the judge agreed there was too much speculation and guess work, and excluded his testimony from the jury, which resulted in a significant outcome for our client, in a case in which zero had been offered prior to trial.
If anyone ever wants to see how this sort of thing works, come by the office and I will let you read a transcript of a highly paid, well respected “motorcycle expert” be taken apart piece by piece for two days. And I’m always happy to answer questions about these issues!


Sheila Linehan
July 31, 2019 at 5:56 am

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Edgar Bowman
February 6, 2020 at 6:09 am

The article is so helpful and informative. I get some important information to read this article.

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