What is a sway bar?
 

I am thinking of upgrading the sway bars on my Porsche, but I see them being sold for hundreds of dollars. So I am wondering: What, exactly, is a sway bar? They look like nothing more than a bent metal rod. Is there something here I'm missing or can I just buy some steel rods (of the proper diameter) and bend them to the proper shape?

Interesting thought, and I admire your self-sufficiency. I suppose you could do exactly that, as long as you knew the type of steel to get, based on its modulus of elasticity, or whatever it's called. Just "a steel rod" wouldn't do. It might simply break after 100 flexures, it might be so stiff it wouldn't flex at all...but you're right, that's what sway bars are--steel rods bent to the correct shape. they're not tapered, hollow, made of unobtanium or in any otther sense trick.

Stephan

They are actually anti-sway bars.
They help cars handle better by keeping the wheels of the same plane (i.e. axle) down by using torsion much like the front torsion bars of the Porsche suspensions.
Most Porsches have these front bars stock, but not the early 'T' bodies.
Also, few have front and rear anti-sway bars installed but they go far to help keep the cars planted to the ground while cornering hard.
The thicker they are the better they work, but that also makes the ride harsher.

Your idea of bending a thick metal bar into shape would work as long as the material is designed to resist breaking, like cold-rolled steel, or a high-grade alloy. Then again, you must be careful not to add too much weight to the car too...

-Rick-

Actually a better term for them is anti-roll bar as they increase the roll stiffness of the car.

Any kind of steel is fine as the modulus of rigidity (the applicable characteristic) is essentially the same for all steels. As long as you don't exceed the elastic limit of the material, you're probably OK. What is really important is the geometry of the bar (diameter) and how it attaches to the car and the resulting moment arm.

Mike

And here I thought it was some type of "spring" steel. Whatever that is.....spring properties induced by the hardening process?

Wow, thanks very much for all of the quick replies. This BBS never ceases to amaze me. And thanks for calling it "self-suffiency". Others have used the more common term: lunacy. :D

So inspired by your replies I dug a little further and found that most anti-sway bar suppliers use this type of steel:

"High quality hot formed and heat treated 4140 chrome moly spring steel"

So I went to the McMaster-Carr website to see how much the raw materials would cost. And it seems they will sell 6 foot rods of 4140 steel for either $18.23 (for annealed steel) or 23.72 (for pre-hardened steel) at 3/4 inch diameter (i.e. 19mm).

Seems like a bargain. Any idea how difficult it would be shape this stuff?

Wow, thats interesting. So for the early (pre73) 911 and 914's that used the straight through the body style bar for the front, no bending would be required? Might be an easyway to go from the 15 to a higher 19mm or bigger. And the rear is only 4 90deg bends.

Down in the middle Georgia swamp one night on a week long hunting trip we experimented with bending a piece of 1" steel rod to make a hanging rack. Over a hot, open oak fire the steel gets red hot almost translucent then you cab bend it to what ever shape you want to, of course we had more than our share of alcholic spirits that night. Check out this quick tutorial on stabilizer bars at
http://auto.howstuffworks.com/question432.htm recommend you buy a good used one for a fair price. Good luck, Jerome. oh yeah don't grab the heated rod with wet gloves on.

You don't actually need (or want) spring steel. Spring steel can't be formed (bent) very easily). Like I mentioned, it is the modulus of rigidity that determines the amount of force that will be required to twist the bar (in addition to the length and the diameter). Modulus of rigidity doesn't vary much for different types of steel.

I think you'll find as you get into this that it is a little more complicated that bending a bar and attaching it to the car. Not that an enterprising do-it-yourselfer can't do this, but there are alot of things to consider (e.g. allowing enough wheel travel) that make this interesting.

To maximize your bang-for-the-buck, you might lean towards a larger diameter, hollow bar. I can give you some equations for calculating your torsional forces if you like.

Mike

Unfortunately it's not the bar itself that accounts for most of the expense, its the hardware. The SmartRacing bars I use cost about $600 an axle. To replace the bar is $175.

Stupid question, how did you know you wanted to upgrade them if you didn't even know what they were?

The anti-roll bars are really doing several things. They reduce body roll, which will hopefully keep your wheels in a more desireable geometry, and they transfer weight across the car. This weight transfer can be used to "tune" the handling. i.e. a stiffer front bar generally leads to more understeer. A stiffer rear bar generally leads to more oversteer, or less understeer.

Regards,

Jerry Kroeger

Slant930: ...laughing...The title of the thread probably should have been "What are sway bars made of?" I actually know what they do (but thanks anyway to the guys who offered the explanations). I should have been more specific in the title of thread. Note to self: Don't post until AFTER the first cup of coffee.

IROC - McMaster also sells tubular steel. So I'd be curious to know how it translates. But keep in mind I am a programmer and not an engineer. So the one to one relationship between 19mm solid bars and 19mm solid bars seemed just about right for my minimal understanding. Anything more complicated may require novice-level explanation.

Also, if super duper spring steel isn't required (or desirable), are the anti-sway bar suppliers just blowing smoke when they advertise their hyper-heat treated hardened aircraft grade retro rocket space mil-spec e-ramified :eek: chromo moly steel sway bars? Perhaps a bit of marketing magic is at work here to add more sizzle to an otherwise common steak?

And I thought this thread was going to be about a bar in Key West or San Francisco........

Actually, while both of those expressions seem to be equally common, neither one is particularly accurate. A better descriptive name would be "tunable weight transfer devices", but it just doesn't have that same ring when bench racing. :rolleyes:

Jerry seemed to have the best description:

But I'd like to split hairs a bit.

1) While sway bars (or whatever you call them!) do tend to reduce roll, that isn't really their primary beneft. If that is your objective you'd most likely do better to increase the spring rate or change the suspension geometry. The first is easy but the second isn't.

2) The big value of the bars is Jerry's second item. To be more specific, sway bars transfer weight to the other end of the car. So in a hypothetical car that's cornering without sway bars, let's say that at 1G 95% of each axle's weight is transferred to the outside wheel. Assuming the car has a 50/50 weight distribution and the same tire sizes, that would result in the weight percentages when cornering being....

Inside - Outside
2.5% --- 47.5% Front
2.5% --- 47.5% Rear

Let's just say that for whatever reason the car is loose because it's got a lot of HP going through the rear wheels. This means that the outside rear is loosing traction which is limiting the car's performance. If you were to add a front sway bar (and your chassis is structure is suitably stiff) you might get this result in the same situation:

Inside - Outside
1.5% --- 58.5% Front
2.5% --- 37.5% Rear

Note that the addition of the front sway bar has transfered weight from the rear axle to the front outside wheel. This might be what's needed to reduce the stress on the outside rear tire and thus allow it to accept car's HP without losing traction. On the other hand, if without bars the car had been perfectly neutral, by adding a front bar you would be loading up the outside front tire more so then the outside rear tire which would result in the outside front tire losing grip earlier then the rear. In a word understeer.

The neat thing is that it is comparatively easy to mount or adjust a sway bar, some of them can even be adjusted by the driver while under way.

Did that help?

Not getting into the details of suspension theory...

To give you an example of the effectiveness of hollow vs solid bars - all other things being equal, the "stiffness" of the bar in torsion is a function of the polar area moment of intertia (J) of the bar. Using a simple equation, it can be seen that a 19mm solid bar has the same torsional spring rate as a 21mm hollow bar with a 5mm wall thickness. The beauty is that assuming each bar was 1 meter long, the 21mm OD bar weighs about 2.5 lbs where the 19mm solid bar would wiegh around 4.9 lbs.

Anyway, just some somewhat useless information.

Mike

I think those are "sashay bars"

LOL
 

I only read this because I am *certain* that we will be getting a "how not to bend a thick metal rod and have it snap back in your fa....." kind of article. ;)

It would be great to actually know the origin of our "entertaining enlightenment" :D
Looking forward to the "safely-made sway bar" article on your site, Janus. SmileWavy

edit: the DIY will also be educational, I'm sure

Ha !! Yes, well actually the helpful members of this BBS have prevented many of those "How Not To..." articles from being written. For instance, Chris Bennet warned me that the engine and tranny are bolted together and aren't mounted seprately ("How Not To Have Your Motor Fall On Your Head While Removing Your Tranny"), Jim Sims and others warned me not to try building a paint booth without proper ventilation and "sprarkless" fans ("How Not To Blow Up Your House With Automotive Paint Fumes"), and Warren explained how to properly reassemble CV joints so they won't bind ("How Not To Replace Your CV's and Have The Wheels Suddenly Sieze At 90MPH"). The list goes on and on...:D

And don't even ask about the time the Pelicanheads advised me against trying to weld a socket to my stuck tranny drain plug ("How Not To Permanently Weld Your Transmission Shut So It Can Never Be Drained Again").

*clink* "BRILLIANT" :D