Originally Posted by TyCreek
I know that travel arc can be a small factor in long travel suspension like that of a pickup. In the context here, on heavy laden short spring travel vehicle that only varies its carried weight duties a few percent, I'd be curious to hear about a disadvantage.
First, I would suggest that even if a motorhome doesn't change in overall weight very much (which I agree), it doesn't mean the suspension won't go through almost as much travel. Spring stiffness is set at factory in proportion to maximum expected axle load so that the ride isn't too stiff. Pickup trucks when empty do ride higher and often too stiff at back, but full suspension travel isn't that different for many other reasons. On large bumps a motorhome's rear suspension could momentarily unload a bunch, allowing the track bar to pull axle to one side or other (depending on initial angle). Another unintended consequence is body roll which changes travel arc and thus apply lateral force on axle. However, if that were the main issue you could go with a better set up than a track bar that eliminates lateral movement; but we digress.
I think the real reason track bars are not normally used with solid axle leaf spring suspensions is that if you do the math, a 3-inch-wide steel leaf just under 5-feet long can't flex laterally enough to worry about. By comparison, for example, tires would flex much more.
My personal concern is that a track bar could load the leaf springs laterally due to the travel arc and the stiff leaf springs not being able to move as much laterally.
Let's not forget that there is also an inherent travel arc of the axle and leaf springs versus frame, so that when the vehicle rolls the springs can split the difference between the two. When a track bar is added and the vehicle body rolls, one spring will likely get loaded less at the expense of the other getting loaded more. And that's not a good thing.
If track bars were needed and there were not some disadvantages when combined with leaf springs, I think OEMs would include them from factory because their weight and cost are relatively low.