[ksfrogman]

I've searched the forum, but have come up short of finding the specs for the stock anti-roll bars on the e90.

Apparently Eibach and H&R offer anti-roll bars with different specs. Here are my questions:

1) Does anyone know what the diameters of the stock front and rear anti-roll bars are? If not, would someone with a micrometer mind measuring this?

2) Are the stock anti-roll bars solid or tubular (i.e., hollow)?

3) Most difficult question: Does anyone have the formula that shows the mathematical relationship between bar diameter and stiffness? If memory serves me right, a slight increase in diameter gives an exponential increase in stiffness.

[ksfrogman]

Well, I found something that essentially is something like this. Correct me if I am wrong.

Let Q = stiffness in lbs/inch
C = Constant
D = Roll bar diameter
L = Length of bar
A = Length of lever arm

Q = C * D^4/[L*A^2]

So stiffness is directly proportional to diameter ^4

[HRC]

So far, this is what I dug up...
Front 26.5mm

Rear 13mm


I would guess the bars are hollow, but I don't know.

EDIT: Whoops, only the diagrams showed up...oh well, go to realoem and see for yourself...

Danny

[ksfrogman]

Thank you. This is real interesting. This is what MarkE90M3 posted for the aftermarket diameters:

Anti-roll bars:

Eibach: 28mm / 15mm (part # 2085.320)
H&R: 27mm / 20mm (part # 33211-1)

[HRC]

Oh my! The H&R's will ROCK! The e90 must have a strong frame! With only a slight increase on the front, and a big one in the rear, the car will turn on a dime! Now I want a set of H&R's!

The Eibach's are sooo conservative, but that's whey we have a choice!

Danny

Originally Posted by ksfrogman

Thank you. This is real interesting. This is what MarkE90M3 posted for the aftermarket diameters:

Anti-roll bars:

Eibach: 28mm / 15mm (part # 2085.320)
H&R: 27mm / 20mm (part # 33211-1)

[ward]

if I remember my schoolin correctly

stifness goes up to the to the third power as diameter of the bar changes

a small diameter change can be a HUGE stiffness change

[ksfrogman]

I just ordered a set of H&R Sport Springs along with the anti-roll bars. I plan to have them installed next Tuesday.

I didn't think I'd be "mod-ing" much of anything on this nice car, but here we go. I must control myself!

[HRC]

Originally Posted by ksfrogman

I just ordered a set of H&R Sport Springs along with the anti-roll bars. I plan to have them installed next Tuesday.

I didn't think I'd be "mod-ing" much of anything on this nice car, but here we go. I must control myself!

Why don't you compete the upgrade with some shocks? Seems logical to me!

Danny

[ksfrogman]

Originally Posted by HRC

Why don't you compete the upgrade with some shocks? Seems logical to me!

Danny

This is a consideration. However, every car I added performance shocks to became very stiff riding. I'm not going for broke on the performance handling if it significantly compromises ride quality. I don't want my e90 to ride like a go-cart.

[longtran]

Originally Posted by ksfrogman

This is a consideration. However, every car I added performance shocks to became very stiff riding. I'm not going for broke on the performance handling if it significantly compromises ride quality. I don't want my e90 to ride like a go-cart.

get the adjustable shock. like koni. let me know if you want them i can get them for $590 from my friend. the install of course would be free. since you are installing your springs they can change them out for you. just give me a call....

[HRC]

Originally Posted by longtran

get the adjustable shock. like koni. let me know if you want them i can get them for $590 from my friend. the install of course would be free. since you are installing your springs they can change them out for you. just give me a call....

Yea, get Koni's! I've had the sports cranked out to max and all my cars felt good, a little stiff, but good (even on my preludes)! Turn them down to halfway and it's like riding stock, but better!

Danny

[ksfrogman]

Originally Posted by longtran

get the adjustable shock. like koni. let me know if you want them i can get them for $590 from my friend. the install of course would be free. since you are installing your springs they can change them out for you. just give me a call....

What are you doing to me? I'm turning into a mod-man freakazoid.

Let me sleep on the idea of the shocks.

[Thavee]

How much are those Front/Rear anti-roll bars cost?

Thanks

[ksfrogman]

Originally Posted by Thavee

How much are those Front/Rear anti-roll bars cost?

Thanks

$500 front+rear installed, but the install will be done the same time the springs are put in, otherwise i understand the labor cost would be more expensive.

[Ajoasika]

Try Koni Special Active shocks from Larkspeed here in the UK. I have them on a standard E90, and they are simply awesome. Control without the M-Sport jiggle.
Looking for an M3 front anti-roll bar now. The E93 /M3 one is said to be slightly stiffer.

[DurdenJames]

Originally Posted by ward

if I remember my schoolin correctly

stifness goes up to the to the third power as diameter of the bar changes

a small diameter change can be a HUGE stiffness change https://spartan-agency.com or https://royal-game-of-ur.com/

Yes, you are correct. The stiffness of a bar or rod is directly proportional to the diameter to the third power. This means that if you double the diameter of the bar, the stiffness will increase by a factor of 8. This is because the cross-sectional area of the bar increases with the diameter to the second power, and the stress on the bar is proportional to the cross-sectional area. So if you increase the diameter by a small amount, the increase in cross-sectional area will be much larger, and the increase in stiffness will be even larger.