Ive noticed that many compainies selling aftermarket swaybars offer both hollow and solid swaybars.
My question is, is there any difference between the two? Is one better than the other, or is it just a marketing gimmick?
solid will have a higher buckleing(sp?) strength than hollow. In a vehicle it the properties will not be that noticeable, you will need a testing machine to find actual strenght differences. If you do buckle a sway bar in you car I think that will be the least of your worries
torsional stiffness typically is very linear with both solid and hollow cross sections, i can’t see much difference in performance of the spring. However different rates could be achieved by altering the x-section, but that can be achieved by changing outside diameter on a solid bar… so no real advantage there… the only thing i can think, and i don’t feel like opening a text to verify, but by moving more material far from the neutral axis, you may notice increase in fatigue resistance, because you may decrease the shear in the theta direction further from the plastic limit… this is all speculation, however…
haha, yeah… buckling a swaybar would definitely load it in a way it was not designed for…
what did just cross my mind was the portion of the bar that is in almost pure bending. (the part that runs longitudinally) given two bars with the same torsional stiffness, one hollow & one solid, the hollow bar would have a much greater Ix than the solid one. But again, i’d be surprised if a swaybar ever failed in that mode.
no, there is a difference…but you need more of a comparison that just “hollow” versus “solid”… like size.
A solid shaft is stronger than a hollow shaft of the same diameter but may be weaker than a hollow shaft of the same weight (same amount of steel for a given length). There is only one design for a solid shaft but the inside and outside diameters and thickness of a hollow shaft can be optimized to transmit maximum torque
if your “gimmick” is referring to price… it generally costs more to form thick wall tubing than solid tubing.
you sure about that… torsional stiffness is typically a constant. 0,0 intercept… So you can make a solid bar behave in exactly the same manner as a hollow bar… it’s just a matter of cross section design… But you aren’t going to get it to behave as anything other than a linear spring… however, like i said before, this is strictly for torsion… bending is another story. A hollow bar will be “stronger”, but from a performance standpoint, they are more or less alike…
i mentioned weight as being the constant.
EDIT:
the weight of a solid shaft …you can’t change the diameter. for a hollow tube, you can design larger diameter with a thinner wall or a smaller diameter with a thick wall and everywhere in between.
EDIT: EDIT:
If the wall thickness is too thin to be stable, the tubing would collapse prematurely. obvious.
OK, agreed. if weight is constrained, then i agree with what you have said…
EDIT: i like this thread.
the ones i was looking at were both the exact same diameter, even the same price, its just the you had the option of hollow or solid.
hollow would be “softer” and “more forgiving” in a street sense…
Do they say it’s the same material too? If so… ^ what he said.
Given the same available stiffness, I’d go hollow just for the weight savings over the solid bar, and adjustable over non-adjustable.
but the stiffness is different.
www.matweb.com it will be your friend, than go to www.engineeringtoolbox.com and use the bedning moments
How is a sway bar normally loaded? Axial or torsion? Yes I do have James Gere Mechanics of Materials 5th edition out looking for conclusive info to add to the thread. 
EDIT: I probably don’t have time to relearn that stuff so I’ll just summarize what’s already been said:
If the 2 bars are made of the same material and are the same diameter, then the solid bar will be stiffer and heavier.
a sway bar is loaded torsionally across an axis transverse to the vehicle and loaded in bending in an axis that exists in the longitudinal plane.
I’d go solid and the main reason being… I had a swaybar (hollow) corrode on me a few years back, on the inside, and taking a turn a little to hard snapped it. I’d think that it would of lasted longer had it been solid (then again, that may be a non issue on a summer only car)
edit: I also dont think there is any advantage to weight in either application… having handled hollow and solid, there wasnt really much of a difference (maybe a lb or two if that)