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Old 06-05-2006, 12:45 PM   #1
jpr
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Swaybar Trivia

Here's some consolidated swaybar info I've collected, the sizes are from either RealOEM or the manufacturer's websites. The front/rear stiffness ratio is calculated as F^4/R^4 and while it gives a rough indication of the balance, but should NOT be confused with the actual balance as it neglects moment arm length, especially in the case of adjustable bars. Also, since the M3 and convertible have some significant chassis differences in comparison to the rest of the models, comparing the ratios isn't really apples to apples. But nevertheless, the ratio is somewhat useful as a relative range indication of each bar set. For adjustable bars, figure the ratio can probably be adjusted by roughly 15% to 20% (about 0.3 to 0.5 for most bars). Additionally, figure another 0.1 to 0.2 as rounding and over-simplification error. In other words, ratios of 2.7 and 2.6 should for all intents and purposes be considered more or less the same.

Make Front Rear F/R ratio
'99 & '00 Std 21.5 15 4.2
2001+ sport 23.5 18 2.9
H&R 27 21 2.7
Eibach 27 21 2.7
2001+ Std 23 18 2.7
'99 & '00 Sport 24 19 2.6
UUC M3 30.7 24.7 2.4
M3 26 21.5 2.1
H-Sport 30.2 25.4 2.0
Convertible 23.5 20 1.9
RDSports 27 24 1.6
Dinan 24 22 1.4
UUC 25.4 23.8 1.3
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Old 06-06-2006, 11:27 AM   #2
hummer
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How does 27/24=1.6? My calculator says 1.125.
27/21=1.28. UUC 25.4/23.8=1.06 etc. HUH
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Old 06-06-2006, 11:36 AM   #3
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Originally Posted by hummer
How does 27/24=1.6? My calculator says 1.125.
27/21=1.28. UUC 25.4/23.8=1.06 etc. HUH
Because stiffness increases by the ratio of fourth power - so the proper relationship is not 27/24 but rather (27x27x27x27)/(24x24x24x24), more commonly written as (27^4)/(24^4)
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Old 06-06-2006, 04:47 PM   #4
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Interesting info.
Curious, but in your thread. what's with the "***8217" and the "***8211" stuff all about? Did something get censored?
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Old 06-06-2006, 05:45 PM   #5
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Originally Posted by Alex323Ci
Interesting info.
Curious, but in your thread. what's with the "***8217" and the "***8211" stuff all about? Did something get censored?
It's an artifact from cutting and pasting the post from MS Word - I didn't notice it while I still had a chance to edit... for some reason an apostrophe shows up as ***8217 and a dash as ***8211
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Old 06-06-2006, 05:57 PM   #6
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i just went with the TMS sways since they seem to be thicker than most
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Old 06-06-2006, 06:28 PM   #7
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Originally Posted by Devious1
i just went with the TMS sways since they seem to be thicker than most
TMS bars are 27/21, same as the H&R and Eibach - they may even be rebranded H&R bars.
As mentioned on some other threads - by increasing overall roll stiffness, swaybars can improve transient response By changing relative roll stiffness (the ratio) you can change the understeer/oversteer balance of the car. With both properties it is possible to overdo it.
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Old 06-06-2006, 07:28 PM   #8
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Quote:
Originally Posted by jpr
TMS bars are 27/21, same as the H&R and Eibach - they may even be rebranded H&R bars.
As mentioned on some other threads - by increasing overall roll stiffness, swaybars can improve transient response By changing relative roll stiffness (the ratio) you can change the understeer/oversteer balance of the car. With both properties it is possible to overdo it.
yeah, the TMS sways come with H&R packaging and i think they might also say H&R on them. either way, i still need to install them to find out what all the fuss is about.
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Old 06-07-2006, 09:37 AM   #9
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Anybody know if the H&R bars use conventional rubber versus urethane mounts and if they bolt to the std. E-46 sway bar ends?
If the case, they maybe a good alternative to UUC without the urethane serviceability issue and clearance issue of UUC specific sway bar ends.
Thanks,
George
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Old 06-07-2006, 10:22 AM   #10
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i dont know about the mounts, but turner replied and said that using the stock endlinks is fine.
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Old 06-07-2006, 11:09 AM   #11
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Quote:
Originally Posted by jpr
Because stiffness increases by the ratio of fourth power - so the proper relationship is not 27/24 but rather (27x27x27x27)/(24x24x24x24), more commonly written as (27^4)/(24^4)
I see. However,
If I use your method to compare RDS 27/24 to my 24/21 it's 1.6 to 1.7 as opposed to my method 1.06 to 1.12. These ratios are essentially the same .94 at 2 decimal places! If whats done the the numerator is also done the the denominator = same answer, same ratio.
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Old 06-07-2006, 11:35 AM   #12
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Quote:
Originally Posted by jpr
It's an artifact from cutting and pasting the post from MS Word - I didn't notice it while I still had a chance to edit... for some reason an apostrophe shows up as ***8217 and a dash as ***8211
fixed
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Old 06-07-2006, 11:44 AM   #13
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Quote:
Originally Posted by hummer
I see. However,
If I use your method to compare RDS 27/24 to my 24/21 it's 1.6 to 1.7 as opposed to my method 1.06 to 1.12. These ratios are essentially the same .94 at 2 decimal places! If whats done the the numerator is also done the the denominator = same answer, same ratio.
...sorry there Hummer but raising the numerator and demoninator to the 4th power respectively does not yield the same linear ratio...only if the numerator is equal to the denominator which isn't the case here. They are close and why there is not much of a departure but there is a difference. And for those wondering why the 4th power...beam stiffness is proportional to moment of inerita of a round cross-section of a sway bar.
HTH,
George
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Old 06-07-2006, 11:53 AM   #14
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Quote:
Originally Posted by Devious1
i still need to install them to find out what all the fuss is about.
After my SSK, its the best mod I've ever done.
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Old 06-07-2006, 12:29 PM   #15
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Quote:
Originally Posted by hummer
I see. However,
If I use your method to compare RDS 27/24 to my 24/21 it's 1.6 to 1.7 as opposed to my method 1.06 to 1.12. These ratios are essentially the same .94 at 2 decimal places! If whats done the the numerator is also done the the denominator = same answer, same ratio.
Almost - the relative order of the ratios will remain the same, but not the magnitude of their differences. The further you get from baseline, the magnitude error will be compounded exponentially.
As I mentioned earlier, the front/ratio is a somewhat simplistic approach to start with, so the calculation error may be somewhat irrelevant. However, if you want to compare the relative stiffness of front or rear bars across manufacturers, it is very relevant. A 27mm bar is roughly 60% stiffer than a 24mm bar even though it is only 12.5% larger.

BTW - something seems to be wrong with your calculator. 27/24 = 1.125 not 1.06 and 24/21 = 1.143 not 1.12.

BTW2 - the actual entire formula is
twist = (2 x torque X length)/(Pi x d^4 x material modulus)
which is why even though changes in moment arm length affect the stiffness, the dominant factor is changes in the diameter

BTW3 - good article on swaybars - http://www.teamscr.com/sway.htm
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Old 06-07-2006, 01:12 PM   #16
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Everything posted here seems to be assuming solid bars. Aren't some of these hollow?
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Old 06-07-2006, 01:40 PM   #17
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Quote:
Originally Posted by TxZHP04
Everything posted here seems to be assuming solid bars. Aren't some of these hollow?
True - to calculate the equivalent stiffness of a hollow bar you would use (OD^4 - ID^4). The UUC M3 bars are hollow and it is the "equivalent" diameter listed above. The H-Sport bars are also hollow, but unfortunately no wall thickness info is provided on their website. But if they are similar to the UUC bars, this would mean they have an equivalent diameter of about 3% to 5% less than cited, roughly 29/24.5 vice 30.2/25.4.
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Old 06-08-2006, 11:31 AM   #18
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Ok, I goofed it's really .94 vs .98. We're still splitting hairs here though.
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Old 06-08-2006, 02:06 PM   #19
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Originally Posted by hummer
Ok, I goofed it's really .94 vs .98. We're still splitting hairs here though.
Most everything in life is splitting hairs if you stand far enough way from it and squint...
You are certainly free to use or not use whatever methodology you see fit, but be aware that the farther the simple ratio is from 1:1, the greater error there will be.
In you particular case with your particular bars, ther error is only about 4%, which given the several assumptions already going in the the calculation is not exactly earth shaking. But if you were to compare the RDsports setup with the H&R setup, the error would be close to 30%.
But regardless of the magnitude of error, the real thing to remember is that unless you are trying to do all these calcs in your head, it is unnecessary to have it in the first place. So long as you've splurged $1.99 on a calculator, it's easy to do it the right way.
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Old 11-27-2006, 09:03 PM   #20
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Quote:
Originally Posted by jpr View Post
Here's some consolidated swaybar info I've collected, the sizes are from either RealOEM or the manufacturer's websites. The front/rear stiffness ratio is calculated as F^4/R^4 and while it gives a rough indication of the balance, but should NOT be confused with the actual balance as it neglects moment arm length, especially in the case of adjustable bars. Also, since the M3 and convertible have some significant chassis differences in comparison to the rest of the models, comparing the ratios isn't really apples to apples. But nevertheless, the ratio is somewhat useful as a relative range indication of each bar set. For adjustable bars, figure the ratio can probably be adjusted by roughly 15% to 20% (about 0.3 to 0.5 for most bars). Additionally, figure another 0.1 to 0.2 as rounding and over-simplification error. In other words, ratios of 2.7 and 2.6 should for all intents and purposes be considered more or less the same.

Make Front Rear F/R ratio
'99 & '00 Std 21.5 15 4.2
2001+ sport 23.5 18 2.9
H&R 27 21 2.7
Eibach 27 21 2.7
2001+ Std 23 18 2.7
'99 & '00 Sport 24 19 2.6
UUC M3 30.7 24.7 2.4
M3 26 21.5 2.1
H-Sport 30.2 25.4 2.0
Convertible 23.5 20 1.9
RDSports 27 24 1.6
Dinan 24 22 1.4
UUC 25.4 23.8 1.3
Hey JPR, found one more today for your list reading an older Roundel (Feb '04).
It seems the E46 M3 CSL has 30.8mm front and 22.5mm rear.
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