Just to clarify, hollow bars are prefered, and, as such, are more costly to produce. Torsional rigidity comes from increasing the outer diameter, while the inner bits do little to nothing other than adding weight to the system. For this reason, we use a hollow bar.
Agreed on all counts.
The spring rate of a torsion bar varies with the fourth power of its diameter, so the outer part does a whole lot more effective work than the inner part. This makes it mechanically efficient to use a slightly larger hollow bar instead of a smaller diameter solid bar.
However a prospective buyer then can't compare the stiffness of different sway bar options without knowing both the OD, and if hollow, the ID/wall thickness (and then doing a fairly simple calculation). Or of course comparing the percentage stiffness of each vs. the stock bar, if the manufacturer publishes that info.
Just for grins I did a back-of-the-envelope calculation that suggests that a 25mm OD hollow bar with a 20mm ID (2.5mm wall thickness) would have about the same torsional stiffness as a 22mm solid bar, but the hollow version would weigh only half as much.
(Disclaimer: This is just an example. APR's bar is slightly larger than that, and I have no idea what its wall thickness might be.)
I had been looking at rear bars, but all of them I'd found are of the less desirable solid construction. APR's new hollow bar now pops immediately to the top of my list!
Neil
