 |
|
|
1994-2002 [Subscribe to Daily Digest] |
From a stress engineer's perspective, I'd concur with Dean on the hollow tube. EdG, your life is in good hands. Trust in the engineering, my friend! :-) Hollow tubes are used all the time in aerospace where weight is a concern. If it makes you feel better, I work on hollow struts that transfer thrust from the Space Shuttle main engines to the fuselage. Those loads are kinda big ;-)
You can achieve approximately the same polar moment of inertia with a tube of 1" OD and 13/16" ID as with a solid bar of 7/8" OD. Or you can make it a bit larger if you go with 3/4" ID on the tube. If you skip all the imperial units nonsense and use metric, as you should anyway on a proper Swedish car, make your OD 13mm and your ID 10mm and you're good to go.
Equal Polar Moment of Inertia means equal maximum torsional stress. You also have the same Moment of Inertia for bending, so that means bending stresses are the same. And you achieve all of this while simultaneously cutting the weight of your torsion bar in half, which is good. The downsides of the hollow tube include manufacturability concerns, cost, and a reduced capability in axial loading (since you reduced your cross
sectional area). I don't think axial loading is a concern here, but someone please correct me if I'm wrong. Someone also mentioned increased corrosion concern on a tube. This is true and would have to be addressed. Dean, you suggested capping the ends?
I don't have a good sense for the geometry at the ends of the bar without looking at it, so I can't really comment on Dean's analysis with respect to the bending there.
As far as the titanium suggestion, I'm not so sure. Yes, titanium has better strength, but it also has a much lower elastic modulii. It's softer than steel, so it's gonna deflect more under load. Under torsion, the angle of twist will be greater for a titanium bar than for an otherwise identical steel one. On a sway bar, more angle of twist is bad. That said, it's hard to beat titanium on resistance to corrosion. But it's also a lot more expensive. I'd say you're better off to corrosion protect
your steel bar.
Speaking of life, another potential issue with a hollow bar could be fatigue. You've got twice as much area for crack initiation, and a sway bar certainly is gonna see lots of cyclical loading. Without a good idea of loads and material processing though, it's hard to say for sure. The part would probably be fatigue critical at the attach lugs before in the tube itself anyhow. Thoughts?
From the sounds of the talk here, we've probably got enough engineering, drafting, and manufacturing resources to design and build an improved bar if we really wanted to. I'll volunteer for the stress analysis if someone else will back me up (Dean? now that we know you have a stress design background :-) What we really need is a loads/dynamics guy though. I'm not ready to tackle suspension dynamics yet, though I'd love to learn.
Fun discussion. JimBlake, where are you?!
Cheers,
'Roo
'01 9-3SE
'89 900S
posted by 12.13.238...
, Mon, 9 Dec 2002 10:00:54
, Thu, 5 Dec 2002 15:51:14
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
