The aerodynamic shape of most vehicles is designed to direct air flow from the front of the car to the sides and up over the hood and windshield in a streamline fashion. Cargo items placed on a vehicle roof are affected by front-end air flow when they overlap the upward streamline. Larger cargo is affected by both the direct force of air as the vehicle moves forward and the updraft. As cargo larger than the vehicle roof length is subjected to upward air flow, the force has a natural tendency to try and push the extending cargo upward, causing an imbalance on the roof rack. Horizontal airflow as the vehicle moves forward will want to push the cargo toward the back of the vehicle. If either force becomes too strong, cargo may be pushed completely vertical or directly off the roof.
Air Flow Diagrams: Click for a larger view
To prevent damage to the vehicle and/or cargo, as well as overcome updraft concerns, it is absolutely imperative to tie-down the cargo extending past the vehicle windshield and rear window. A bow tie-down strap is designed to properly secure cargo to the front vehicle bumper undercarriage. The front strap counteracts air updraft forces preventing cargo from flying upward or off the vehicle during travel. To balance the roof cargo load a stern tie-down strap is also required which connects the cargo to the rear bumper undercarriage.
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Important Safety Precautions: