TRAILER MOMENT OF INERTIA

The general formula for approximating the moment of inertia of a vehicle based on the formula for a Rectangular Parallelepiped of uniform density was demonstrated in an 1970 IME paper (Reference 25, p 88) to correlate well with motor vehicles (see Figure 10).

To approximate the moment of inertia of a trailer, the following are some published values compared with the Rectangular Parallelepiped method:

NOTE: Iz = k2M, Where Iz=Moment of Inertia, k=radius of gyration, M=vehicle Mass

From Reference 22 (pg. 60)

Trailer WB= 357 (fwd Bogey Position), Empty kingpin static Load= 2815 lbs., Rear Suspension Weight= 8650 lbs.

Tot Wgt = 11465 lbs.

Iz= 769718 in-lb-sec2 = k2M, therefore Trailer k = 161.1 inches

Rectangular Parallelepiped method: (OL=447, OW=80), Iz=509,874, k=131 (19% low)

From Reference 20 (pg. 44)

Trailer WB= 383, Empty kingpin static Load= 2815 lbs., Rear Suspension Weight= 8650 lbs., Tot Wgt= 11465 lbs.

Iz= 644483 in-lb-sec2 = k2M, therefore Trailer k = 161 inches

Rectangular Parallelepiped method: (OL=473, OW=80), Iz=569,019, k=138 (14% low)

From Reference 21 (pg. 60)

Trailer WB= 432, Empty kingpin static Load= 4500 lbs., Rear Suspension Weight= 7500 lbs., Tot Wgt= 12000 lbs.

Iz= 750000 in-lb-sec2 = k2M, therefore Trailer k = 155.4 inches

Rectangular Parallelepiped method: (OL=522, OW=80), Iz=721,749, k=152 (2% low)

These comparisons appear to demonstrate that the Moment of Inertia in Yaw of an empty trailer may be somewhat independent of the trailer size and therefore the Rectangular Parallelepiped method should not be used. This is probably due to the offset of the trailer dual/tandem tires and suspension from the CG of the trailer.

The general approximation of the empty trailer moment of inertia for a typical 40 trailer should be based on use of a radius of gyration of approximately 160.


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TRUCK/TRAILER TIRE PROPERTIES