Designing sway bar - very upset!

[Knestis]

I can't for the life of me find my C. Smith "to Win" books, four years and two moves after I last saw them! This is getting VERY serious...

Can someone out there help me out with the formula for tubular anti-roll bar rates, while I decide where else I might look - or if I need to order new copies.

Thanks

K

[Quickshoe]

Kirk,

Neither Tune or Engineer to Win give a formula...perhaps Prepare (which I don't have)

For a solid bar:

Rate (Pi * G * D^4)/(16 * B^2 * L)

Where G= Modulus of elasticity of material

D= Diameter of Bar

B= Length of lever arm (measured 90 degrees to length of bar---not along the lever arm) Adjacent not Hypotenuse

L= Length of bar

4130 C34-C38 has a G of 1.2 x 10^7

I obtained this formula from a suspension engineering class I took years ago--pre FSAE type programs.

You will be guessing on the modulus of elasticity, so perhaps this will get you close enough to at least get the right OD bar...you can then tune with the lever arm. Geometry of the links and mounting bushings will also effect things, but at least it will be a good starting point.

Perhaps ADDCO has a site with a formula or a sprint car website/chat forum might be of some help.

on edit--for a hollow bar my notes say to substitute (O.D.^4 - I.D. ^4) for the "D^4" in the solid bar formula.

Using a 1" solid bar vs 1" hollow w/.125" wall gives a change of about 40% http://Forum.ImprovedTouring.com/it/eek.gif

Happy engineering!

Daryl DeArman


[This message has been edited by Quickshoe (edited May 18, 2004).]

[cherokee]

I had trouble finding the books...A couple of book stores told me that they where out of print...I even gave them ISBN numbers. I finally got a good person at Borders and they where able to find me the book.

[chuck baader]

Steve Smith autosports has them..

------------------
Chuck Baader
E30 ITA under construction
Alabama Region Divisional Registrar

[grjones1]

[quote]Originally posted by Knestis:
K,
Now why in the world would you want to know which sway bar to use. Are you not stuck with the one that came with your showroom stocker? http://Forum.ImprovedTouring.com/it/smile.gif
Really a pleasure meeting you and your wife at VIR, K.
G

[88YB1]

I am considering hollow sway bars for my car to lose weight. I have removed every legal item, and am still 108 pounds over. The problem is that I am getting conflicting information. My fabricator tells me that a hollow bar is torsionally stronger than an equal diameter solid bar. Roland de Marcellus, in his book “Handling What It Is And How To Get It” states that a hollow bar is approximately 65% less than the same size solid bar. Who’s right? The weight savings from two one inch solid bars would be substantial.

Chuck
S.E. Region
#34 ITA

[Eric Parham]

Kirk,

I don't have my books nearby, but here's an idea: Assuming Daryl's formula is correct for a solid bar, just do 2 calculations.

That is, do one calculation for a solid bar with the same OD as the proposed hollow bar, and do a second calculation for an imaginary solid bar that would fit inside the hollow bar (OD of imaginary bar is ID of hollow bar). Then subtract the two numbers to get your resulting stiffness...

And, I might add, please make it MUCH stiffer before taking any more cornering pictures or I might get seasick http://Forum.ImprovedTouring.com/it/wink.gif

[Eric Parham]

Chuck,

It all depends on WHAT they're made of. Assuming exactly the same materials (unlikely), a hollow bar of exactly the same OD will be a little softer than a solid bar. But, A hollow bar can be made with a slightly larger OD to have the same stiffness as the solid bar while saving weight.

[D. Shine]

You already know what the lightest,stiffest bar is,so what is the issue? Do you think you can make a lighter effective bar?

Dick Shine

[jsgprod]

Substitute (D^4 - d^4) for D^4 in your formula above and it will work.

D = outside Dia.
D = inside Dia.

I believe I got this from an old Steve Smith book. I just happen to keep that formula in a little notebook I always seem to have handy http://Forum.ImprovedTouring.com/it/biggrin.gif

[MarkL]

As far as the index of elasticity, I think you will find that there is such a small difference in the indices that the difference between mild steel and chrome moly will be so small that they will be inconsequential. It is the method and degree of heat treat that will make the difference.

[dyoungre]

You are right on, Mark; The STIFFNESS of chromoly is virtually identical to mild steel. What makes it useful is that it can deflect a great deal more before it plastically deforms. Essentially - it makes for a better spring. Remember though - it loses it's properties when welded - in fact, it becomes brittle if you don't anneal it, at which point, it is virtually identical (in the heat effected region) as mild steel. (assumming you don't then reheat treat the bar). Bottom line - only bother using chromoly if you are bolting together a swaybar.

[Sandro]

back up from the dead !!!


when calculating a sway bar like this(not my pic)
http://i114.photobucket.com/albums/n274/winglb/SRS%20bar/P1010007.jpg

what do you use for the lever arm length?

is it from the center of tube to the mounting holes? what about it being mounted on both sides or is that negligible?

thanks

[Quickshoe]

The lever arm length will be the distance from the center of the bar diameter TO the end of the arm that is providing the twisting force on it (trailing arm?).

[Knestis]

...and depending on what car it's for, you'll come back and want to do the math again for the additional rear bar. :)

The answer to Shine's question above was, "The issue is it doesn't give us enough rear roll stiffness."

K

[lateapex911]

Man, Dick had to wait a looong time for THAT answer, LOL!

[Sandro]

so would it basically be from the center of the bar to the center of the stub axle? or the actual end of the trailing arm?

pic of the type of beam it will be going on(not my pic)
http://www.frommydyinglips.com/car/mk1/beam.JPG

[Quickshoe]

Yes, in a straight shot from the center of the stub axle to the center of the bar, not following the curvature of the trailing arm.

[Sandro]

thanks, now I am getting numbers that look reasonable :024:

[Greg Amy]

Yes, in a straight shot from the center of the stub axle to the center of the bar, not following the curvature of the trailing arm.[/b]
Well, not exactly.

The problem with the rear suspension design of the V-dubs is that it *is* a rear swaybar all by itself. Adding Shine's bar to that design creates two "problems":

One, the change in torsion rate is not nearly as large as one might think. The "real" change in torsion is not a function of lever arm vs. torsion rate, as it is for most cars, it's a function of percentage change in torsion of the whole system, which is a lot less than one might imagine.

Two, that supplemental bar does not act on the same axis as the main beam (no co-axial). Therefore, the force is a function of the lateral relative deviation of the stubby "legs" it's mounted on, rather than in torsion. That bar is not being torqued about its axis, it's acting in tension, resisting the arms moving via stretching.

It's more of a complex function than you might think, but to break it down into its simplest form, you *might* be able to calculate the tension value of the torsion bar itself over a lever arm of the linear distance from the bar's axis to the beam's pivot axis (short). But it would only be an approximation.

Bottom line, it's not really doin' much at all, except maybe stiffening up the main beam a bit, and it's possible that it may very well be creating some minimal toe changes as the front of those "stubbies" are pulled closer together during deviation. You'd get about the same action as welding in a piece of tubing to those triangular stiffening tabs. To increase torsion you're better off "boxing" in the beam itself like we used to do on the old Rabbits...

[Quickshoe]

Greg,

So that "plate" section on its' edge is the only factory roll resistance at the rear? This blue bar bolts to that plate and adds to its' rigidity? Are there clearance issues that would prevent a more common bar design? Why not add "lever arms" on this bar that attach directly to each trailing arm?

[Greg Amy]

So that "plate" section on its' edge is the only factory roll resistance at the rear?[/b]
Do you mean the vertical part of the beam? If so, yes. Those "plates" that attach the two trailing arms together are both structural items to keep the arms in place *and* torsion bars to resist twist. Anything you can do to increase the "torsion" of those plates will increase roll resistance in the same manner as an "anti-swaybar".


This blue bar bolts to that plate and adds to its' rigidity?[/b]
Don't get me wrong: yes it does. But, since it's not acting co-axially to the axis of bending of the trailing arm assembly, it's acting more in tension than in torsion (though, you MEs will argue with me that a torsion bar does, too. I hear ya).


Are there clearance issues that would prevent a more common bar design? Why not add "lever arms" on this bar that attach directly to each trailing arm?[/b]
Haven't been under the Professor's car in a while - most of my experience was in Rabbits - so I'll look next time I'm there. What I was basically trying to indicate is not that this bar is ineffective - it is - but it's not *really* doing the same thing that a torsion-based anti-swaybar does. This bar is acting more as a stiffener.

Another way you could re-create this action - as a "for example" - would be to use flat plate and make those triangular tabs one big plate across. You could then see that as the beam is twisted the plate would be "stretched". Then you could add a vertical angle on the front edge to increase its torsion, and you'd be effectively doing the same thing.

The Professor and I have discussed this, but given the whole rear suspension is one big anti-swaybar design, and rear swaybar are free, I think there's a LOT of clever things you can do with this that are well within the letter of the rules. However, I don't see the rear suspension design of this car as the limiting factor; you'd get a lot more performance out of trying to make the Mac front end work better...

[Knestis]

We added the Shine bar, in addition to the stock "GTI" bar that I think the base Golfs didn't have, and THEN added a separate more conventional bar that hangs under the beam. The latter is made of Speedway Eng. NASCAR parts, with a 1.25 OD x.25 wall tube and splined arms.

Cameron Conover has what he calls the "Happy Bar" modification that does what Greg describes, plating some portion of the open side of the transverse beam. He's experimented with MkII's, varying the length of the plate to tailor the "size" of the virtual rear bar that creates. I don't know but I think that approach is more efficient, since torsional rigidity is so influenced by diameter.

Left to start over, I would indeed invoke the "sway bars are free" clause and modify the stock beam heavily, trying to add less weight than our ERBB* has caused. I've even sketched on napkins what I might do if I could build one from scratch.

K

Do your part to help fight Embarrassing Rear Bar Build-up.

[Grumpy]

I am considering hollow sway bars for my car to lose weight. I have removed every legal item, and am still 108 pounds over. ... The weight savings from two one inch solid bars would be substantial.

Chuck
S.E. Region
#34 ITA
[/b]

If I read this correct, you have done everything legal to reduce you weight, and now you are looking for "other opportunites?

Just askin'

[RacerBowie]

If I read this correct, you have done everything legal to reduce you weight, and now you are looking for "other opportunites?

Just askin'
[/b]

It sounded more to me like he has unbolted every legal item to lose weight, and now is starting to think about replacing components with lighter but still legal components. Perfectly legal and WELL within the spirit of IT!

[Grumpy]

Thank you. :023:

[Quickshoe]

Thanks Greg, I am now up to speed.

Effectively this Shine bar isn't twisting along its' centerline but trying to push one end forward as it twists (co-axially) and resisting an increase in length (tension).

Do the racers that use this bar have issues with the fasteners bending/breaking?

I like Kirk's supplemental tubular bar idea.

How "FREE" are swaybars in IT? Can they be of the blade type lever arms common in formula cars and cockpit adjustable?

[lateapex911]

Thanks Greg, I am now up to speed.

Effectively this Sine bar isn't twisting along its' centerline but trying to push one end forward as it twists (co-axially) and resisting an increase in length (tension).

Do the racers that use this bar have issues with the fasteners bending/breaking?

I like Kirk's supplemental tubular bar idea.

How "FREE" are swaybars in IT? Can they be of the blade type lever arms common in formula cars...[/b]

yes, they can....


and cockpit adjustable? [/b]

but no, not that.

[Conover]

I can't for the life of me find my C. Smith "to Win" books, four years and two moves after I last saw them! This is getting VERY serious...

Can someone out there help me out with the formula for tubular anti-roll bar rates, while I decide where else I might look - or if I need to order new copies.

Thanks

K
[/b]


Dood, they are all on the shelf at my shop, haha, I reminded you that they were here last time you were down here and you said something to the effect of "Good, you'll probably use them more than I will."

You want me to ship them to you?

[Greg Amy]

Dood, they are all on the shelf at my shop, haha, I reminded you that they were here last time you were down...[/b]

DOOD, welcome to 2008...! (check the date of Kirk's post in your quote above...exhuming old dead threads has its own unique dangers...)

;)

[Conover]

Haha, I knew it was sorta old, but I didn't realize it was THAT old. I just thought that Kirk was forgetting his:
http://www.goodnessdirect.co.uk/detail/823592b.jpg

[Knestis]

I found them long after that post and then dropped them at CMS World HQ...

K

[timo944]

Gents,

Another related question: What is the guideline for the ratio of spring rate to sway bar rate? I am undergoing the same exercise and don't eally know what spring rate to design the sway bar to.

And I guess as spring ate goes up, the ratio goes down???

[Fastfred92]

kirk,,

i found it.......

FWD=(rear bar as big as you can get it)/more rear spring