The Wheel Chock Experiment

Every extrication textbook and program covers stabilization and its importance in setting the stage for successful vehicle rescue operations. One of the initial steps in stabilizing a wheel-resting vehicle is chocking the wheel(s) and preventing further horizontal movement. The questions I pose to you and your department: How seriously do you take this step? Do you require chocking the wheels on any vehicle involved in a collision, just those that require extrication, or none? At extrication incidents, do you stipulate to chock the wheels but rarely accomplish it in light of other tasks?

Charleston’s New Policy
The City of Charleston (S.C.) Fire Department recently adopted a motor vehicle collision (MVC) policy that addresses the use of wheel chocks at all collisions, regardless of the amount of damage. And while it’s true that fire departments respond to a much larger number of collisions that do not require extrication than those that do, the potential for a vehicle to move horizontally always exists–perhaps more so if the vehicles are minimally damaged and can easily roll. In fact, due to responders being less aware at minor incidents, there may be even more potential for the movement to occur.

Our policy dictates that the first-arriving engine crew is tasked with chocking the wheels on wheel-resting vehicles. Requiring the first-arriving engine to carry out this task ensures that it is completed as early in the incident as possible and, therefore, before any considerable work is done in and around the involved vehicle(s). This not only eliminates the potential for movement of wheel-resting vehicles, but it also makes this step a habit, which is especially important for those situations when the step could be overlooked due to the damage of the vehicles and/or the injuries to the patients.

At our department, two sets of wheel chocks were constructed and placed on each apparatus. These chocks are specifically used for vehicle collisions and are different than the wheel chocks used for our apparatus.

Key Points for Chocking Wheels
As the popularity of hybrid vehicles increases, so too does the importance of chocking the wheel(s). One major hybrid vehicle hazard is the lack of an outward sign indicating that a vehicle’s ignition is engaged when it is in the “Ready Mode,” meaning it has the potential to move at any given time. It may actually be a more prevalent hazard than the electrical system itself. But regardless of whether the vehicle is a hybrid, there are many key points to consider:

  • Approach the vehicle from the sides, avoiding the potential path of travel, and chock the wheels before other tasks. Never assume the vehicle will not move.
  • Remember that horizontal movement is possible with any wheel-resting vehicle, regardless of whether tires are inflated or deflated.
  • Use manufactured wheel chocks or wood blocks at least four inches in height. Do not use wedges or 2 x 4s because they don’t create enough resistance for vehicles with larger tires.
  • Choose the most appropriate wheel based on vehicle position and possible extrication tactics. At a minimum, chock one wheel on the front and rear of the tire.
  • Pick a wheel. If all four wheels are contacting the ground, it doesn’t matter which wheel is chocked; however, it may be preferable to chock more than one tire based on elevation and vehicle position.
  • Select a wheel away from the area being lifted for operations where one side or end of a wheel-resting vehicle is lifted.

Experiment Makes the Case
With the assistance of my department’s current recruit class, we conducted an informal test to determine how much force it would take to move a four-door passenger vehicle against a variety of wooden chock configurations. Although this test does not account for all conditions (vehicle weight, ground conditions, vehicle damage, selected wheel, chock material, etc.), it should show the value of chocking the wheels.

The test uses what should be considered the worst-case scenario for responders as far as potential movement is concerned: an undamaged vehicle on level, smooth ground. We chocked the same wheel each time: the front driver’s side wheel. We placed a dynamometer in line with a tow strap between the test vehicle and a pull vehicle. It’s safe to assume a quicker jerk may result in less force needed, but for safety purposes, we pulled with a gradual, steady force. Here are the results:

There are several key points that can be taken from this informal test.

  1. First responders should always attempt to put the vehicle in park after entry is made.
  2. It takes a significantly higher force to overcome a vehicle that’s chocked and placed in park vs. a vehicle that’s in either neutral or drive.
  3. The 4 x 4 chock was more effective regardless of the transmission gear.

In Sum
Properly placed wheel chocks limit any further horizontal movement of the involved vehicles and increase the overall safety of the scene. It should be the first step of stabilization, completed prior to additional tasks in and around the vehicle. The Charleston Fire Department was able to outfit all of our apparatus with two sets of wheel chocks with a few hours of labor and less than $200. This simple and cost-effective action has an immeasurable value. A special thanks to the City of Charleston Recruit Class 11-02, “Strength in Numbers.”

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