I think the title for this thread should be:
Good, Bad, and Uncertain about Blue Ox Sway ProIMO, there is not enough information or experience to say what is good and what is bad, but I sure have my doubts as to how effective the hitch will be at controlling sway.
In particular, I see no theoretical basis for believing the rearward tilt of the ball mount will make any significant contribution to sway control.
With due respect to Andy Thompson, I do not see any connection between the effects of
bicycle head-tube angle on bicycle stability and the effects of WDH tilt on sway control.
The combination of bicycle head-tube angle and rake result in some amount of caster for the bicycle's front wheel. However, I see nothing remotely similar to a caster effect resulting from the rearward tilt of a WDH.
Andy goes into a much more detailed discussion of his views on the effects of ball mount tilt in an article titled, " Why is your hitch crooked?", in the Summer 2010 edition of
AIRSTREAM LIFE magazine.
He correctly describes how ball mount tilt will cause the height at the tip of an unloaded WD bar to change as the bar changes direction with respect to the TV.
He also describes how the horizontal angle between WD bar and TV centerline will affect how load is applied to the TV. Specifically, when a WD bar is non-parallel to the TV's longitudinal centerline, there will be a component of bar-induced torque which tends to rotate the TV about its roll axis, in addition to the pitch-axiz torque which causes load on the front axle to increase and load on the rear axle to decrease.
Andy explains how the roll-axis torque will affect the TV's stability. However, his discussion pertains to TV/TT articulation angles of 22 degrees and greater. He mentions an emergency maneuver such as a sudden lane change. Andy's company,
Can-Am RV Centre, conducts slalom-course testing with a variety of TV/TT combinations (see the "Towing Videos") on their web page.
At the end of the
AIRSTREAM LIFE article, Andy states:
Beyond making the tow vehicle much easier to control in an emergency maneuver, the ball mount angle assists with directional stability going straight down the road at highway speeds.---
I don't know if Andy has test data to support the second part of this statment, or if he is using the following reasoning for support.
He goes on to state:
---Even in small degrees of direction change, the torsion bar on the outside of the turn gains tension much faster than the bar on the inside loses tension. This is because as a torsion bar bends it requires progressively more effort to bend the same distance.---
Again, with due respect, Andy got it wrong here. It is correct that the curvature of the outside bar is increasing, although relatively slowly. It is not correct that as a bar bends it requires progressively more effort to bend the same distance. There is no "progressive" spring rate for a WD bar. For all practical purposes, the spring rate remains constant. If one could measure accurately enough, the spring rate (incremental load divided by incremental deflection) actually would decrease with deflection because that's a fundamental characteristic of steel as it approaches its elastic limit. For a WD bar to be "progressive", it would have to be built like a leaf spring or have a "helper" spring attached.
The bar tip follows a circular path in an inclined plane which is perpendicular to the trunnion axis. When the trailer is articulated a few degrees, the tip of the outside bar moves closer to the low point of its orbit. Since the bar already is quite close to the low point, there is relatively little increase in the load on the bar.
The tip of the inside bar also is close to its low point, but is moving away from its low point and is losing load at small rate.
But, as they say on the infomercials: Wait, there's more. As stated before, the WD bars not only generate pitch-axis torque on the TV, they also generate roll-axis torque. Since the outside bar is moving toward being parallel to the TV's centerline, its roll-axis torque moment arm is decreasing toward zero. That means its CCW (when viewed from rear) roll torque is decreasing towards zero.
OTOH, the inside bar is swinging away from the TV's centerline, so its roll-axis moment arm is increasing. Although the load on the bar is decreasing, the moment arm is increasing at a faster rate, and the inside bar's CW roll-axis torque continues to increase.
Roll-axis torque is an important consideration because it generates increasing load on the TV's outside tires. This results in a phenomenon similar to roll steer,
scroll down to "Roll Steer". Roll steer can make the TV steer to the right when the TT swings left, thereby decreasing the tendency to sway. A friction-based sway control also has a steering effect which results from yaw-axis torque.
HOWEVER, the foregoing is not an arguement in favor of the benefit of a large amount of ball mount tilt versus zero tilt. At small articulation angles which might be associated with "sway", the WDH-induced roll torque is virtually independent of ball mount tilt. In fact, the torque decreases as rearward tilt increases. So, it appears there is no basis for the claim that a relatively large rearward tilt of the ball mount gives improved sway control.
Getting back to Andy's
AIRSTREAM LIFE article -- he concludes by stating:
Another way to think about it is to picture the forks of a bicycle or motorcycle. They always angle backward at the top. This is why you can ride a bike with no hands. If the forks on a bike were completely vertical, you would not be able to ride it with "no hands" -- it would be too unstable. The angle on a ball mount provides a similar function as forks on a bicycle.
Okay, let's assume that what Andy says about the effect of head-tube angle on the stability of a bicycle is true. Let's assume, when riding "no hands", you can control the stability of a bicycle by shifting your weight side to side.
A table in Andy's article shows that, for 22 degree articulation, a vertical ball mount generates more side to side load shift than does a rearward ball mount angle.
If the "similar function" between bicycle forks and ball mount angle is to cause a side-to-side load shift, the vertical ball mount did a better job for the 22 degree test case.
For small articulation angles of the magnitude that might be associated with the onset of sway, there is virtually no difference between vertical ball mount and 15 degrees of rearward tilt.
So, IMO, there is no basis for the claim that increased rearward tilt will lead to better sway control.
As far as I know, there is no published test data to support the claim that ball mount tilt or trunnion axis tilt can eliminate or reduce the likelihood of sway.
I think there will continue to be much uncertainty about such claims.
Ron
