Towards an Understanding of “Beaming” in Large Trucks

“Beaming” in trucks is due to a bending vibration of the frame resulting in a fore-aft motion at the driver location. It has been found that beaming motions are relatively unaffected by the suspension type or components, and, in particular, by the shock absorbers. Since beaming is due to a distributed dynamic effect, it is believed that modal nodes exist near the suspension attachments to the frame thus rendering the suspension ineffectual at controlling beaming. Another explanation of beaming is that an overall system dynamic mode exists such that at a specific frequency the relative velocity across the suspension is near zero thus producing little damping for that mode. This concept is developed here through the use of some simple models. The sensitivity of beaming to system configuration is then demonstrated through use of a rather complete overall tractor/trailer model.