Summary

The purpose of this study was to define design criteria for truck and semi-trailer wheel chocks used on loading platforms. The criteria were supposed to enable the chock to prevent a truck or semi-trailer from moving despite three destabilizing factors: unexpected starting of the truck or semi-trailer, braking of lift trucks on the semi-trailer platform and the effect of gravity. In all three cases, the semi-trailer can spontaneously or gradually move away from the loading platform until the lip falls, creating a void into which the lift truck wheels can fall and causing it to overturn. A fourth destabilizing factor, semi-trailer dock walk, causing the semi-trailer’s suspension to be compressed by the lift truck, was also considered for inclusion in the specifications. In the end, it was excluded because the wheel chock will be ineffective against this problem unless it is anchored to the ground. Although there are mechanical devices for anchoring the wheel chock or securing it to the platform, these involve practical constraints due to Québec’s wintry climate. Accordingly, our study focused on criteria that will help maximize chock resistance to surface slipping.

The scientific literature is particularly devoid of wheel chock design criteria. Current standards and some technical reports have identified primary and secondary chock functions, as well as the design constraints described in this report. We believe that adding more specific criteria or parameters would help define the actual chock design, while a number of concepts may be possible. For instance, the report establishes target values for certain parameters, among which the geometric profile of the chock surface in contact with the tire was a special focus of attention because it affects several chock functions. This profile also affects the failure modes that will be observed when the chock is in use.

Our study indicated that not all potential failure modes can be avoided by using a movable chock; specifically, unexpected starting of the truck or semi-trailer. From a safety perspective, it seems advisable to control the potential failure mode. The study determined that this mode should be forward slipping of the chock on the ground, and not movement of the tire over the chock. This choice was made because resistance will be maintained at all times even if the chock slips, increasing the chances of stalling the motor or alerting the driver to the fact that something is wrong. If the tire rolls over the chock, there will be no subsequent resistance. This happens in particular if the chocks are anchored to the ground and are low in height. It should be noted that a movable chock is a device that acts in place of the truck’s or semi-trailer’s brakes. Consequently, it cannot perform better than tires locked on the ground, unless the ground is churned up by the chock or the chock is anchored to the ground.

This study showed that chock resistance to surface slipping is closely linked to its geometric profile and stiffness. This aspect has not been discussed in the literature. To illustrate chock design considerations with respect to its geometric profile, a chock concept was proposed.  However, the concept does not take account of structural deformations and is not unique. Although, theoretically, the concept seems highly effective, experimental studies should be done to confirm the findings of theoretical models.

This report showed that in cases of semi-trailer dock walk, the chock was ineffective in restricting movement unless it was attached to the ground. Other solutions should be considered, including use of a retaining hook on the rear bar or suspension locking systems.