Drag Sled and Drag Factors

Article excerpt

Over a decade ago, I wrote an article entitled "What's All This Fuss About Drag Factors?" (Law & Order, April 1988). The crux of the article was that since the drag factor, AKA skid number or coefficient of friction, usually ends up beneath a square root sign, the precision of the number itself doesn't make a great deal of difference. Also in the equation is a distance of some sort. It might be the distance a vehicle skidded prior to impact or the distance it traveled afterwards. Those distances are just as difficult to measure with precision as is any given surface's roughness or smoothness.

Measure the distances from the center of mass of a vehicle at one location, such as at its maximum engagement in a crash, with something else to the center of mass of the same vehicle at a second location, such as its post-collision final resting place. Knowing where to spot the exact center of mass is hard enough and defining the absolute, actual distance is impractical at best, impossible at worst.

Another rather non-problem with drag factors, but one with which a lot of lawyers like to argue, is the difference various drag factors make in a particular situation. For instance, assume you have a skid distance of 100 feet. Assume the roadway is plain, ordinary, everyday asphalt. You can use one of those drag factor charts that's been around since dirt was new and assume the surface to be someplace between "traffic polished" and "traveled." However, investigators disagree on what's "traveled." Does it really matter?

Let's conjure up a few numbers. How about 0.68 to 0.82? Assuming the 100 feet of skid was to a stop, the range of speeds is 45 to 49 miles an hour. Big deal. If the accident car hit a pedestrian in a 20-mph school zone, you have something to hang your hat on because you know the car was going at least 25 miles an hour over the speed limit. If the speed limit had been, say, 45 mph, then you still have something on which to hang your hat. You know the vehicle was going at or pretty near the speed limit. Moreover, any reconstructionist worth his salt will probably admit that his estimate could still be plus or minus five miles an hour.

Another situation we must deal with is how we determined what that drag factor was. Did we use a chart? How about an electronic accelerometer? What if we used a bumper gun? Perhaps we did a skid test with the actual vehicle. Or maybe we utilized an interesting little homemade device called a drag sled.

Drag sleds come in all shapes, sizes, weights and designs. I have two of them. One weighs precisely ten pounds and is made of stained plywood and brass fixtures that I use for demonstration purposes. My workhorse is an old 30 pound wooden ammo box with bricks inside and with rubber tire material on the bottom. The designs of such devices come mostly from the spirited, fertile imaginations of police personnel. Officers usually find that the material they have on hand dictates the design of their sleds. In my case it was a wooden ammo box. Some people may have an old section of steel I-beam lying around the basement. Or an empty metal fishing tackle box, a section of railroad track, a metal ammo box, a concrete block or an old car tire. Lots of guys use old car tires. They fill sections of tires with everything from lead to concrete.

Years ago, an enterprising entrepreneur produced a fine, professional-- looking device containing intricate instrumentation that gave the user a digital display of the drag factor as he pushed the contraption along a surface. Yes, they pushed the drag sled. They accomplished that by attaching the sled to a two-wheeled hand push-cart or dolly. When the person pushed the dolly, it dragged the sled behind it. Alas, this particular implement was fairly light and bounced around too much, causing the operator difficulty in obtaining a relatively steady number.

Some lawyers want to make a big deal out of drag factors and grouse about how the investigator arrived at whatever numbers he used in the computations. …