I didn’t say it saw compression or tension. I do understand how sway bars work. Yes a solid bar is better than a hollow bar because of the more material, I'm simply saying, the difference between a solid bar and a bar with a hole in it is minimal (obviously this depends on the hole, I'm not saying there is a 28mm bar with a 26mm hole in it.)

This is kind of long so bear with me here.

Also "ask yourself why they don’t make axles and drive-shafts hollow"

Um, drive shafts and axles are hollow. Otherwise you have a lot of rotating mass that is not doing you any good, at least not enough to make a difference.

And to ryceboi, the center of the bar has zero shear stress; it does not take most of the load. Here is some engineering for everyone.

Angle of twist (or theta in radians) = [Torque applied x length (length from point of applied torque)]/[G (modulus of rigidity for whoever was asking) x J (polar second moment of area)]

For a solid round bar J = (pi x d^4)/32

For a hollow bar J = (pi/32) x (do^4 - di^4)

Tau was mentioned earlier but just to put it with the rest up here, tau = Tr/J. When the radius is equal to zero, the shear stress is zero. No stress at the center of the bar. The greatest stress is when r = radius of the bar, meaning, the outside has the greatest shear stress.

We will use G for carbon steel which is 79.3 GPa.

So we'll test a 28mm solid bar and a 28mm solid bar with an id of 2mm. Both 1,219mm long (4 ft).

Solid bar J = (pi x 0.028m^4)/32 = 6.034371 x 10^-8

hollow bar J = (pi/32) x (0.028m^4 - 0.002m^4) = 6.034214 x 10^-8

I'm merely using more decimal places merely to show the difference in the polar second moment of area. If using significant digits, the numbers would be the same.

We will use a torque of 2000 N-m or 1475.122 lbf-ft.

Tau will not change much because J is very similar between the two, however, just for poops and giggles.

Tau at outer edge for solid bar = (2000N-m)(0.014m)/6.034371x10^-8 = 464.0085804 GPa

tau at outer edge for hollow bar = (2000N-m)(0.014m)/6.034214x10^-8 = 464.0206593 GPa

Hmm, not much more stress at the outer edge.

Now let’s look at the angle of twist from one end to the other.

Solid bar theta = (2000 N-m x 1.219m)/(79.3x10^9 Pa x 6.034371x10^-8) = .50948 rads = 29.19 degrees

hollow bar theta = (2000 N-m x 1.219m)/(79.3x10^9 Pa x 6.034214x10^-8) = .50949 rads = 29.19 degrees

Hmm, solid bar doesn’t appear to be any better than the hollow bar. So let’s try a 5mm hole.

Tau for hollow bar with a 5mm id = 464.480 MPa

Theta = 29.22 degrees

Tau for hollow bar with a 10mm id = 471.682 MPa

theta = 29.67 degrees

hmm, starting to see a difference now.

again, just for poops and giggles a 28mm bar with a 20mm id.

tau for hollow bar with a 20mm id = 627.299 Mpa

theta = 39.46 degrees. Now there’s a significant difference.

The difference in twist does not really begin to make a big difference until the hole gets to be about 11-13 mm. Then from there as the hole gets bigger, the angle increases greatly.

So, what does this mean? Is a solid bar better than a hollow bar? Technically according to the data shown, sure. Is there a noticeable difference between a 28mm solid bar and a 28mm bar with a 10mm hole? Not enough that you would notice. Now some of these numbers will change as it depends on torque applied, length of sway bar etc. But is a solid bar that much better than a hollow bar? Not unless there is a big hole in it. Why solid then? Well, much easier to manufacture a solid bar than it is a hollow bar. how much weight savings would there be between the solid bar and hollow bar? Well, here’s a quick glimpse.

Density of steel is 7.85 g/cm^3

Volume of solid bar is 121.9cm x pi() x 2.8cm^2/ = 750.60cm^3. Mass = 7.85 x 750.60= 5892 g = 12.96 lbm.

Volume of hollow bar is 121.9cm x pi() x (2.8cm^2 - 1cm^2)/4 = 310.19cm^3. Mass = 7.85 x 310.19 = 2435 g = 5.36 lbm.

So about a 7lb weight savings. So is it worth it?

The argument of whether a solid bar is better than a hollow bar should stop. Is a solid bar better, yes. Will you notice the difference between a solid and hollow bar, unless there is a big hole, no. Therefore, whether its a solid or hollow bar makes no difference (unless, its a big hole) There is basically 7 lbs of dead weight in the example just shown. Notice this was an example. If anyone can give me real world numbers for torque, length etc I would happily plug them in. This is just to show that a solid bar is not much better than a hollow bar. Just adds more weight.