(Not to hijack)
Gary... But will too much tension on the WD bars do the same too? Curious???
Trucks, and most cars, are normally are set up with an nose-heavy under
steer configuration. In understeer, the vehicle will tend to resist a turn and try to maintain or resume a straight line course. The vehicle is said to "plow" a turn. This is the most controllable/stable steering configuration. Over
steer, on the other hand, is the condition in which the vehicle tends to move easily and quickly into a turn, tending to turn tighter as it continues in the turn. The vehicle will "dive" into a turn and evince a tendency to get "sideways" in turning. Steering with be nervous and unstable require constant correction with the steering wheel.
Wikipedia's article on the subject: When an understeer vehicle is taken to frictional limits where it is no longer possible to increase lateral acceleration, the vehicle will follow a path with a radius larger than intended. Although the vehicle cannot increase lateral acceleration, it is dynamically stable. When an oversteer vehicle is taken to frictional limits, it becomes dynamically unstable with a tendency to spin out. Although the vehicle is unstable in open-loop control, a skilled driver can maintain control a little past the point of instability with counter-steering. However, at some limit in lateral acceleration, it is not physically possible for even the most skilled driver to maintain a steady state and spinout will occur.
Over-tightening WD spring bars will transfer an excess of trailer tongue weight forward onto the truck's front axle. It will act to slightly stiffen the trucks steering and increase the understeer condition. It will increase the resistance to turning. Not all that great, but not a major concern.
The opposite, under-tightening the bars, will shift the tongue weight back onto the rear axle and increase the oversteer tendency. This will significantly increase the instability of the tow. That is a really major concern. In towing a trailer, oversteer is not our friend.