Skidmarks of a Different Sort


by Sgt. C. Dean Ose

Originally published in the September, 1989 issue of LAW and ORDER Magazine

Routine but very careful examination of the speedometers of vehicles involved in heavy collisions can be a simple and valid method of speed determination.

Examination of instrument readings in aircraft crashes is routinely done. In automobile collisions it has usually been relegated to only those very familiar with it, such as the investigating officer’s Crime Bureau or Laboratory. As the workload of most crime bureaus is excessive, the turn around-time for such an examination is usually long. Most investigating officers simply do not have the time to wait and therefore it is not widely done.

However, most investigators are comfortable and competent with head or tail light filament examination. Much of speedometer examination is very similar; that is, the care and handling are the same. Therefore, this is to illustrate that field officers can make the preliminary determination if indeed the speedometer is collision marked.

In a severe collision with a fixed object the deceleration of the vehicle occurs in a very short period of time. The speedometer needle will usually strike the face of the odometer with great force, marking it at collision speed. This is also true for the striking vehicle in a Head-on or T-bone collision.

An accident reconstruction of several years ago is a good example of how examining a speedometer can provide excellent evidence in determining speed.

It was a rare underride collision; the high performance Chevelle went under the semi trailer, shearing off its roof . It made a U turn and came to a stop against a sign post. The semi was turning left onto the four lane undivided roadway from a stop sign.

The truck driver stated the car driver appeared to be accelerating before collision. He explained that the car was “fishtailing” on the wet road. The accident was at first written off as simply another high speed or “too fast to stop” accident.

The Chevelle had little damage except for its missing roof. Unfortunately, the young driver did not have sufficient room to survive. As we were intrigued with speedometer examination this was one of the first items we addressed.

The car had spent all its life in North Dakota, where vehicles collect a lot of fine dirt in the instrument panels. Upon removing the speedometer and the plastic cover it was apparent that the needle had marked at zero miles per hour. We were puzzled, especially when further examination showed that the needle appeared to have “skidded” up the face to about 20 miles per hour.

Instead of reinstalling the unit and checking to see if it worked, we did a standard mechanical inspection of the vehicle. The brakes were first on our list of concern. As the vehicle had been properly secured as evidence we were confident it was as the officers had found it. Upon raising it on the lift we noticed the right front brake assembly was wet with brake fluid and found the right front brake hose had burst.

The pieces came together. The young man was not accelerating-he was standing on the brakes with the rears locked up, trying to steer it with no front brakes. He went under the semi trailer at an angle which accounted for the “skidding” of the needle upward.

The tragic part of it was the speed was not really so fast. Yet he was unable to stop the Chevelle on the wet road with only rear brakes. What appeared to be high speed was simply mechanical failure; which was found by speedometer examination. In another example, a drunken driver going against traffic in the wrong lane of the four lane roadway was being paced by a frantic driver in the proper lane. As the drunk pulled out to pass, the driver looked at his speedometer. It was a multiple fatal with an oncoming pickup. The speedometer of the drunk’s sedan marked nearly precisely at the speed the eye witness had stated.

In another reconstruction, of a fatal accident on an ice covered surface, the speedometer of one of the vehicles was solidly marked at zero mph. The brake pedal was bent to the floor, and the pattern of the brake pedal pad was plainly embossed into the drivers right shoe sole. While this did not clue us to the impact speed, it did establish a time/distance/reaction for the driver.

While a speedometer examination cannot replace established investigation/reconstruction methods, it can be a valuable aid. It usually can speak for itself with simple photography documentation. And, it will be an independent finding.

While minimum speed, momentum calculations, etc. are now common, they are still difficult to impress upon a judge or jury. Since the simplest method is usually the easiest to get across, a speedometer examination may be it. And it will help you get to the determination of cause.

The driving method for a speedometer needle is still usually cable, but some manufacturers now use an electrical system. However, different materials, colors and compositions of materials will always be needed for a clear distinct difference for the needle and its background.

What does it take to conduct a speedometer examination? A little patience, a sharp eye and the proper equipment. A laboratory might use exotic magnification such as a stereo microscope with built-in 35mm camera. Short and long wave ultraviolet portable lamps are used occasionally, and laser light and computer read-out analysis will be common in the near future.

An examination is not difficult. In a violent collision the needle strikes the face with nearly hammer force. In older vehicles, the needle will mark on the dirt on the face. For newer vehicles, the needle will mark on the manufacturing “debris” or material from the needle will transfer to the face.

Microscopic examination shows much plastic debris on the face of most domestic car speedometers: they are seldom assembled in dust-free environments. This is not the case for many foreign manufactured cars, however. Conversely, the material of the needle then transfers to a “clean” surface.

The cover of the speedometer must be removed, which necessitates the removal of the unit from the vehicle in most cases. A simple magnification glass can be used. Many automotive tool sellers have a glass with a light that shines on the object to be examined. This is the most basic equipment. You will be able to see an obvious impact marking, but if you don’t, it does not mean there isn’t one.

You will need a UV light; long wave is preferred as short wave is dangerous to use. Many police supply houses sell these for low cost. The UV light must be used in total darkness. If there is a severe impact the iridescent material of the needle will usually transfer to the face of the speedometer.

Okay, it looked like the car was going about 80 mph when it hit the bridge and disintegrated. The speedometer was intact and plainly showed a mark. Now what? Take a picture of it!

As with any evidence the usual procedures must be followed.

Take a shot before removing.
Take a shot after the faceplate is removed.
Use a macro lens or auto bellows (with a tripod) to give a 2-10 power magnification.
You may want to remove the needle and faceplate of the unit to get close enough. Pull the needle straight off, it is only a friction fit.
Really ambitious? Use a microscope. (I used my local hospital’s.) Most colleges have a stereo microscope and are willing to let it be used for police business.
Didn’t see anything, except under UV? Take a timed photograph-about 10-30 seconds.
Film to use? Really exotic is infrared, but it takes two weeks to get back and the reason we are doing this ourselves instead of the Crime Bureau is time. The 400 ISO film works well, but for a really plain mark use 100-200 ISO. Always use color!

How does a speedometer work? Talk to your local speedometer repair center or mechanic, but in most cars the same principle is used today as in the Model A Ford. The standard revolutions of the cable are: 100 rpm equals 60 mph. The cable turns a magnet inside a drum which holds the speedometer needle. The faster the magnet turns, the more the drum moves. Through precision manufacturing, modern speedometers are highly accurate.

Something to keep in mind; these methods will also tell you how many RPM’s a motorcycle engine was turning, and how much fuel was on board.

All instruments mark. L&O Left, a microscopic shot of a needle after an accident.
Right, where the needle “bit” through the paint of the speedometer face.

cartruk2Car position at point of entry under semi trailer.

Sgt. Dean Ose is an experienced accident reconstruction specialist formerly with the Minnesota State Patrol. (Now Retired)

DAR Documented Accident Reconstruction
N. 13420 Stephanie
Mead, Washington 99021

Home educational use only Copyright Dean Ose, All Rights Reserved.