“The wall” in reference to the trucking industry, is the rate of speed where fuel usage suddenly increases in transport trucks.

Explanation of the cause:
All vehicle suspensions have a natural rebound frequency, or up and down motion as the vehicle travels down the road. Truck suspensions have a natural rebound frequency of 10Hz, (600 times per minute). Hertz are a measurement of waves, or the up and down motion in times per second. 10 times per second is called 10 Hz, or the up and down hop of the suspension in this case.

An out of balance wheel or heavy spot (heavier side of the wheel) pull up and down on the suspension at every revolution. These revolutions are measured in revolutions per minute (RPM)

The average commercial truck size tire travelling at 65 mph has a revolution of 10 times per second, equalling 600 Revolutions Per Minute (RPM). So at 65mph, the out of balance portion of the wheel assembly (the complete axle end) is pulling up and down on the suspension at the same frequency as the natural rebound of the suspensions.

This resonate coupling of these two frequencies, meaning when the heavy spot of the wheel’s up and down motion meets the same up and down motion as the rebound frequency, multiplies the out of balance portion of the wheel assembly over 1000 times, enough to bounce the axle of a fully loaded truck and trailer up and down sending vibrations through the complete vehicle.

The consequences are a substantial reduction of tire life, increased suspension, wheel and shock absorber failure and increased fuel consumption. These resonating vibrations cause a rough ride overall, damaging the vehicle and it’s cargo and have also been shown to negatively effect the vehicle’s handling.

A STRONG VIBRATION AFTER HITTING A BUMP is caused by the out of balance wheel assembly, from the bump compressing the axle spring at the rebound resonating frequency of the suspension and RPM of the tire.