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Old 07-05-2004, 11:20 PM   #1
Back Road Runner
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Default Swaybar rates - diameter to equivelant spring rates

I'm curious about getting swaybars but dont' know what rates they are. You have various diameters as well as hollow and non-hollow, but how do those values translate to equivalent spring rates?
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Old 07-05-2004, 11:29 PM   #2
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Default You need

to specify what sway bars you're looking at. And you need to do a search (Google is your friend). I did a search and this was at the top of the list. "Brand X" are Cusco bars I believe. IMHO, the front "Cobb" (Hotchkis) is too stiff (unless you want to induce more understeer). The rear Cusco information is wrong. The Cusco rear is 20/21/22/23/24mm capable.

San

Front
COBB (25mm) +90%
Brand X (21mm) +36%

Rear
Position 1 2 3
COBB (25mm) +48% +92% +160%
Brand X (22,23,24mm) +35% +48% +85%
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Old 07-06-2004, 12:43 AM   #3
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Ok, so what are the percentages? What do they refer to and how do they translate to spring rates?

My concern is not what's available. It's how I figure out how stiff they are, i.e. equivalent spring rate, lbs/in or N/m.
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Old 07-06-2004, 01:04 AM   #4
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Default The

percentages are "above" stock. As to spring rates, I don't think sway bars apply. Sway bars "flatten" the steering, by stiffening the rate at which one side is independent of the other. Everyone, please jump in and correct me, as I'm a "Specialist", but don't know everything. Scotty?

San
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Old 07-06-2004, 01:20 AM   #5
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Sway bars act as springs but only sideways and due to the car's roll. It adds sideways roll resistance. Along with front and rear springs, sway bars can change understeer/oversteer characteristics. You can also add sway bar to add roll resistance or even equal the front/rear roll resistance(squat/dive) if desired which can lead to a very stable and controllable car even in slides and accelerating/braking. I know what I want to do with them. There's just no info saying what stiffness they create besides those percentages, not too useful for me.

A sway bar is simply a bar that twists with two lever arms. The length of the bar wheel to wheel and the lever arm from the cross bar to the mounting point on the suspension determines the stiffness or spring rate which can be measured in force per distance like lbs/in. It's kind of goofy they don't rate the sway bars like this. It would seem proper, at least to me, to measure them as a spring rate.
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Old 07-06-2004, 01:24 AM   #6
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Default I have

the STI 6-piece rear bits, Cusco front and rear sway bars (rear set to 22mm), and Tein Flexes. The sway bars and rear bits really "planted" the rear end. Not sure what kind of data you're looking for, if the percentage numbers don't help.

San
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Old 07-06-2004, 01:33 AM   #7
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I'd like to find the twist rates of the sway bars. Basically, I want spring rate. To twist the bar one inch(at the mounting points), you'd need to apply a certain force, say 200 lbs of force.

To picture it, think if you were using a torque wrench to tighten a bolt but instead of a bolt, you have a long rod you're twisting. 200lbs of force created say 5 degrees rotation with one side fixed. This would result in a certain distance traveled by the torque wrench. If the mouning point was at say 6 inches up on the wrench, you would have moved an inch up from those 5 degrees of rotation.

Now the distance isn't linear, but it can be approximated for small angles or an average could be given for normal operating ranges.

Translating back to car talk, it would be the same as if the car leaned 2.5 degrees in a corner. 2.5 degree car lean means 5 degrees sway bar rotation or an added force of 200lbs on the arms, 100lbs each side. It's like going from 200 lbs/in springs to 300 lbs/in springs when cornering.
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Old 07-06-2004, 01:40 AM   #8
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Default I'm reaching,

but if you had the numbers you're looking for, you'd know which sway bar to buy? Numbers look good on paper, but my experience is that real world knowledge is all that counts.

San
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Old 07-06-2004, 02:02 AM   #9
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True, for the numbers to be usable, you'd have to know how they were derived and if you needed to make some additional compensations. Still, if done in a normal, accurate, or universally accepted way, it would get you in the ballpark you were looking for.

I personally find it hard to just pick a set of springs and swaybars because so and so liked them or that guy had really fast lap times with them. I'm a numbers person, so I want data.

I like stuff on paper, but I also want to know what makes the paper values different from real world values. For example stock mounts and upgraded, stiff mounts would provide different results.
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Old 07-06-2004, 02:13 AM   #10
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Default I don't

think I can help you in your quest. Any numbers I could provide would probably not help you. If I had it to do over, I'd think about the Hotchkis bars, but I think the front bar is way stiff. The only reason I'd go with the Hotchkis is I'm a weight weenie (I own two titanium mountain bikes). I'm not bashing you, you are doing your research. But at some point, you're going to need to listen to someone for a reference point (IMHO). Good luck!

San
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Old 07-06-2004, 09:20 AM   #11
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Here is a table of relative stiffness for solid sway bars - http://www.northursalia.com/t...sway/sway.html
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Old 07-06-2004, 09:47 AM   #12
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And here's another problem. Spring rates (for coil springs) are quoted for the spring itself, not at the hub. On Subarus the spring is an inch or two inboard of the hub, which reduces the effective rate. On other cars the springs are even farther away.

So, even if you can get actual swaybar effective spring rates at, say, the droplink, you can't directly add them to the coil spring rate with any accuracy.

That said, I'd like to know, too. It would be enough to get a good estimate of relative order of magnitude: If a coil spring is 250lb/in, does the swaybar add 50lb/in? 100lb/in? maybe even double the spring rate in larger sizes?

Read this thread: http://forums.nasioc.com/forums/show...hreadid=176031
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Old 07-06-2004, 10:56 AM   #13
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Fred Puhn's book has a formula to determine stiffness, but he even mentions that it's very complicated and that the best way is real world application. Try a bar (or a notch on a adj bar) and "measure" the results. If not what your looking for, try another bar or setting. I think the only usefulness would be in comparing various makes of bars, but again those #'s might not be all there cracked up to be either.

Big Sky
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Old 07-06-2004, 12:26 PM   #14
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swaybar rates are not that complicated. the formula is:

---
modulous of elasticity of the material used

x

bar diamater^4 (outer D minus inner d if hollow)

divided by

(the bar's width) x (the bar's lever arm length^2)
---

the greatest effect is to change the bar's diameter, smaller changes can be made by altering the lever arm length.


With regard to coil spring rates, you want to find out your motion ratio, which is the difference in the length of the pivot to the coil spring mount compared to the hub mounting surface. A lot of strut cars are in the 60% motion ratio range. A lot of people refer to their "wheel rate" which is the rate of the spring at the wheel, taking into account the motion ratio.

Chris H.
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Old 07-06-2004, 01:32 PM   #15
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Quote:
Originally posted by subrew2

With regard to coil spring rates, you want to find out your motion ratio, which is the difference in the length of the pivot to the coil spring mount compared to the hub mounting surface. A lot of strut cars are in the 60% motion ratio range. A lot of people refer to their "wheel rate" which is the rate of the spring at the wheel, taking into account the motion ratio.

Chris H.
The sway bar also has a motion ratio. The effect at the wheel will not be the same as the effect at the bar.
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Old 07-06-2004, 03:14 PM   #16
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True, but you could still still find out the effective wheel rate by doing a couple more statics equations employing motion ratios. Just a few more steps with a tape measure.

In the meanwhile, what modulus of elasticity are you using for the swaybar rate calculation?
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Old 07-07-2004, 01:18 AM   #17
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Yep, E (modulus of elasticity, think of it as a measure of how easy it is to stretch, twist, or bend) and a tape measure is really all you'd need to figure out everything your self. I'm an engineering major, so I know all that fun stuff, lol. To me, this stuff isn't hard, to a lot of other people, it might be a pain. It's all ratios due to distance as far as effectiveness, and I'm well aware of the need to take that into account for both the spring and sway bar figures.

I can see I'm not the only engineer here, lol, subrew2.

That comparison chart for the relative ratios is useful for a quick guide if any one of them is known.

All in all, I'd actually be happy with needed torque per degree. I could figure out the rest from there. Say if they rated a 20mm solid sway bar at 50 ft*lbs per degree, measured in the lab with one end link fixed and the other rotated. That would provide accurate and even results between all sway bars and would allow anybody to derive the equivalent sway bar rate per specific car setup.
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Old 07-07-2004, 01:24 AM   #18
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Hmm wonder what sway bars are made from...spring steel? I've heard some are a plastic or something too, the lighter ones...I think. If we knew the material used, E would be pretty easy to find, i.e. Google to search, lol. The rest is just measuring lengths. Maybe some sway bar owners could get out their tape measures and get a list of lengths for various brands. Then a data table could be made up for the various brands and sizes. You could even get suspension lengths and have a built in adjuster for your particular car. That way the differences in lengths of where the forces are applied are accounted for and taken care of for you. ...would be nice, lol.
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Old 07-10-2004, 10:48 PM   #19
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More?
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Old 07-11-2004, 12:41 AM   #20
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Quote:
Originally posted by Back Road Runner
Yep, E (modulus of elasticity, think of it as a measure of how easy it is to stretch, twist, or bend)
No - Modulus of elasticity (Young's) is ONLY how much it takes to stretch without permantly deforming it. If you Bend it, you are now entering the plastic range of the metal and a very different modulus. Also, the torsional modulus will frequently be less than a half of the basic youngs modulus.
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Old 07-11-2004, 02:46 AM   #21
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Good luck !!

944 turbo guy




Nick , you're right,..my bad,....(it is amazing though )

Last edited by RaceComp Engineering; 07-11-2004 at 03:18 AM.
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Old 07-11-2004, 03:02 AM   #22
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come on myles, this IS a technical forum. If you want to stifle technical knowledge, that's your choice, but I wish you wouldn't.

Truth is, if you know the torsional modulus of the bar and you can take a few measurements with a tape, it's very easy to calculate effective spring rate of a sway bar.
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Old 07-11-2004, 03:18 AM   #23
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Yeah Nick I know , but man,..sometimes it amazes me. I am glad that you reminded me ,..really.

944 turbo guy
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Old 07-11-2004, 03:20 AM   #24
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Default No offense,

but for you it might easy to determine the effective spring rate of a sway bar with certain data, but for me it's modulus schmodulus. I still feel that numbers don't always tell the full story, and personal experience has it's place.

San
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Old 07-11-2004, 03:26 AM   #25
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Quote:
Originally posted by oldmansan
numbers don't always tell the full story
This is 100% true!

To get "the full story" you really need the numbers AND observations from the driver seat - no doubt about it.

Although it is a relatively trvial task to calculate the effective spring rate of a swaybar, it is somewhat non-applicable, because as you know, a swaybar ONLY resist rolling, unlike springs, which resist rolling AND bump.

The only reason you'd ever need to know effective spring rate of a swaybar is if you're trying to calculate swaybar/spring combos that have the same roll stiffness but different bump stiffness. And again, the only practical reason you'd ever have to worry about this kind of thing is if you already KNOW you have the optimal roll stiffness but need more or less bump resistance.

No amount of numbers on paper will be able to tell you the perfect spring rates or swaybar stiffness to make your car handle best - only a skidpad (or slalom, track, autocross) can do that.

If this kind of thing is your interest, then you ought to invest the time on your own to do the calculations. As I mentioned, the calculation is trivial, but the measurements take time.

Last edited by nhluhr; 07-11-2004 at 03:41 AM.
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