The OE sway bar is 34mm and is hollow. Most replacement aftermarket sway bars are 35mm and are solid. My understanding was that outer diameters being equal, hollow sway bars are actually more rigid than solid. So are the aftermrket replacement bars actually equivalent in torsional resistance or maybe a little "softer" than the stock bar they are replacing? How does one compare the resistance to torsion of a stock sized hollow sway bar to a solid aftermarket bar of a different diameter?

Thanks,
Blair

 

The formula (a greatly simplified one is all you need to get a ratio of stiffness between two different bars) is:

[RelativeStiffness] = [OutsideDiameter]^4 - [InsideDiameter]^4

This also works for a solid bar, you just use zero for [InsideDiameter].


So no, a hollow bar is not stiffer than a solid bar of the same diameter. A hollow bar is more efficient in terms of its stiffness compared to its weight, but that's not quite the same question.


Edited to add - the two bars being compared must be bent the same amounts in the same directions in the same places or else the simplified comparison is not valid.


Norm

 

So for a stock hollow GT bar at 34mm OD^4 - 29mm ID^4 = 629,055 units

An aftermarket solid GT bar at 35mm OD^4 - 0mm ID^4 = 1,500,625 units

So the aftermarket bar is twice as stiff (relatively speaking) than the stock bar?!?!

Really!?!

Blair

 

If those were the numbers, yes.

But you have two wall thicknesses to subtract, and with a wall thickness (15% of OD) equal to 5.1mm that leaves the ID at 23.8mm. Now you've got the OE front bar at 1,015,182 units.

About 50% more stiffness is still quite a bit, and you should clearly notice the difference.


Just for another perspective, the OE 34 OD x 5.1 wall bar is equal to a solid bar of 31.7 mm (which would be literally about twice as heavy!).


Norm

 

Thanks for clearing that up for me.

Blair

 

Careful, too stiff and the other tire will be
forced up off the road surface by the loaded tire.
There are trade-offs with sway bar rates.
Bigger is not always better....
Otherwise we would all have 100MM sway bars all round... LOL JK

 

I have a Roush stage 2 suspension on my car and it came with springs, dampers, shorter bumpstops, and springs. The Roush sway bar is 35mm and is solid. The rear sway bar is 24mm and is also solid. I called Roush and initially they wouldn't give me any specifics about the springs, but I kept pushing them and the guy on the phone told me that the springs were about 35% stiffer than stock.

I was getting a little too much understeer with the Roush front sway bar, just a bit nothing unbearable. So I switched to back to the softer stock bar expecting that it would reduce some of the understeer, but to my surprise corner entry understeer is noticeably worse. Feels like the front end washes out on corner entry. I don't get it. The softer bar should reduce this. It definitely feels like the front end is softer and more compliant. Only thing I can think of is that the front springs are too soft, the large front sway bar was put on to balance this out, and the front springs/struts are now bottoming out causing the understeer. Any insight is appreciated...

 

Blair check with steeda their great with suspension may have some bind on the sway bar not sure but when you lowered the car you also may have changed the sway bar rate because of position may need to try diff. links to get it right, Blair give a call I get Fran to give a ride in his 427 r and see if you like that, then will put in the air and see what different than what you have??

 

Hi John:

I figured out the problem with the stock bar. I am only able to get 1 degree of negative camber up front. I don't have plates or camber adjustment via the strut bolts. I borrowed a tire pyrometer and my tire temps were really high on the outside so the lack of stiffness in the stock bar was causing tire rollover and understeer. I went back to the Roush front bar and the stiffer front bar is keeping the tire from rolling over so much. I'm going to stick with the Roush until I can get camber plates.

I'd like to check out the 427r though, I'll give you a call or drop by the shop...

 

Hi Blair,

Actually most of the major S197 suspension manufacturers front 35mm anti-roll bars are hollow due to weight and cost. Hollow bars are stiffer by weight not in absolute terms. Norm's explanation is close enough for our purposes here and is an excellent way to figure out the relationships between different bars if you can get the bar diam. and thickness.

Cheers/Chip