It is correct that the friction bar will resist allowing the TV/TT articulation to return to normal.
However, I believe many people over estimate the magnitude of the effect.
The most commonly sold friction bar SC (FSC) is factory-set to produce a yaw-axis torque of about 500 lb-ft.
A 25' TT will have a distance of about 18' from ball to axles.
This means the FSC torque would equate to a lateral force of about 28# at the TT's tires.
On dry pavement, if a 6,000# TT swings to an angle of 1 degree, its tires will develop a lateral restoring force of about 500#.
The TT would need to have an "off tracking" angle of only about 0.06 degree to balance the FSC torque.
On snow or ice, the angle might increase to about 0.2 degree.
For comparison, an angle of 0.06 degree would result in the rear end of a 25' TT being out of line by about 0.35".
Some people also over estimate the magnitude of added steering force required to overcome the torque generated by the FSC.
If the 500 lb-ft of torque is applied to a TV with a 120" (10') wheelbase, the front tires must generate a lateral force of about 50# to counteract the torque.
If the front axle is carrying 3000#, the tires must be turned about 0.15 degrees on dry pavement to generate a lateral force of 50#.
Required increases in steering angle for other surface contitions might be: wet pavement = 0.2 degrees, packed snow and dry ice = 0.3 degrees, glare ice = 0.55 degrees.
So, yes, a friction bar SC can induce some "understeer".
However, the added steering angle required to overcome the "understeer" is relatively small compared to the maximum tire slip-angle of 5-6 degrees at which the lateral force begins to plateau.
The required added angle might be so small as to be undetected by the driver.
The "stiffening" effect of the FSC might be noticed when coming out of a curve.
When not towing, the camber and toe-in of the TV's front suspension will tend to straighten the steering tires.
When towing with a friction-based SC, the FSC torque will tend to oppose the camber and toe-in effects and reduce the TV's self-straightening.
It might or might not be possible for the TV to self straighten.
However, as long as the driver is aware that it might be necessary to provide some assistance in the straightening process, the FSC torque should not cause problems.
FSC manufacturers typically advise to loosen the friction bar when towing in low-traction conditions.
Since the FSC bar can be loosened without affecting weight distribution, you still have the WDH effect.
Manufacturers of integrated WDH/SC devices (which generate considerably more yaw-resting torque than do FSC devices) do not specify any reduction in sway-control force during low-traction.
Reducing the yaw-resisting torque also would result in less load transfer.
I'm guessing these manufacturers do not believe there is any significant risk in using their devices in low traction.
Ron
