32mm hollow bars = what in solid bars?

[cornernfool]

Just bought a 1977 that was a divorce sale project. Guy told me had done alot of stuff but the car was still a project. Some of the work I questioned some, so I'm tearing it back down to make sure it's right. Did the rear suspension today. Well surprise he didn't lie about the parts. I'm building the car to autocross, run the canyons, and take Honey to dinner in. Are these 32mm hollow bars going to be to stiff? What would they equate to in a solid bar? It also had Neatrix bushings, any thoughts on those? Thanks, Mike.

[Flieger]

I am still waiting on some 32mm hollow bars from Sway-A-Way ordered through Pelican. They are an effective rate of 31mm. Should be good with anything up to 22mm fronts. If you don't have sway bars I would suggest 21 or 22.

That said, they do make hollow bars with an effective 32mm rate. So unless you measure the OD and ID you can't tell for sure what the rate is.

[cornernfool]

Max, I measured the outside and came up with 32.5mm, did not measure the inside. These bars are brand new/installed but never driven. I think they are going to be to stiff for my purpose. Got any 27/28mm to trade?

[SilberUrS6]

If I am not mistaken, whether or not the bars are hollow or not makes no difference as to their torsional spring rate. Only the material used and the outer diameter make a difference.

[remembering materials science from the '80s]

[Flieger]

Originally Posted by SilberUrS6

If I am not mistaken, whether or not the bars are hollow or not makes no difference as to their torsional spring rate. Only the material used and the outer diameter make a difference.

[remembering materials science from the '80s]

Incorrect. The material on the outside does the lion's share of the work, but a 32mm solid bar will have a stiffer rate than a 32mm hollow bar.

In fact, Sway-A-Way says their 32mm actual OD hollow bar is an effective 31mm solid rate.

[1QuickS]

The torsional spring rate has the relationship of (OD^4-ID^4) so you can compare easily; ID of a solid bar is zero. All other components of the equation are constant to the comparison.

So a 32mm bar would have a "rate" of 32^4 or 1,048,576

A 32mm x 19mm ID bar would have a "rate" of 1,048,576 minus 19^4 or 1,048,576 - 130,321 or 918,255 which is 88% that of a solid 32mm bar.

If you take the 918,255 and square root it twice you get the equivalent rate for a solid bar or 30.96mm.

Also, within very small differences of metallurgy all steel exhibits the same modulus of elasticity so a heat treated 4340 rod and junk steel will behave similarly for elasticity/spring rate...the difference being the junk steel will yield/fatigue much more quickly than the heat treated 4340.

[ClickClickBoom]

Originally Posted by 1QuickS

The torsional spring rate has the relationship of (OD^4-ID^4) so you can compare easily; ID of a solid bar is zero. All other components of the equation are constant to the comparison.

So a 32mm bar would have a "rate" of 32^4 or 1,048,576

A 32mm x 19mm ID bar would have a "rate" of 1,048,576 minus 19^4 or 1,048,576 - 130,321 or 918,255 which is 88% that of a solid 32mm bar.

If you take the 918,255 and square root it twice you get the equivalent rate for a solid bar or 30.96mm.

Also, within very small differences of metallurgy all steel exhibits the same modulus of elasticity so a heat treated 4340 rod and junk steel will behave similarly for elasticity/spring rate...the difference being the junk steel will yield/fatigue much more quickly than the heat treated 4340.

Owwww! You made the inside of my head hurt!

[Rich Gas]

Eric... Reminded me of this...

Dr Venkman: Ray, pretend for a moment that I know nothing about metallurgy, engineering, or physics, and just tell me what the hell is going on.
Dr Ray Stanz: You never studied.

[SilberUrS6]

Originally Posted by Rich Gas

Eric... Reminded me of this...

Well, gosh, it WAS thirty years ago. I have a good memory, but...

Hey, what were we talking about, again?

[Chuck Moreland]

No need to bust out the calculator.

Hollow bars are marked and sold with the solid equivalent rate, not actual diameter. Hence a hollow bar marked 32mm has the same rate as a solid 32mm bar.

But if you actually measure the 32mm hollow OD it will be larger than 32mm.