difference between solid and hollow front swaybars - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 9 Old 07-04-2011, 07:11 PM Thread Starter
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difference between solid and hollow front swaybars

I want to switch my front sway bar on my 1990 mustang from stock, to a 1.125" solid bar, but i found that MM offers a 1.125" hollow bar with a .188" wall thickness.. what would be the difference between these two bars? I plan on using urethane pivot bushings and end links.

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post #2 of 9 Old 07-05-2011, 09:58 AM
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post #3 of 9 Old 07-05-2011, 01:52 PM
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Quote:
Originally Posted by miracerros
How much difference will it make if I install a slightly thicker sway bar?
A number of factors determine the stiffness of a sway bar, including the bar's thickness and length, the length of the swing arm or lever, the bar material, and the bar's attachment method. But if two sway bars differ only in their diameters, all you need to know is that their relative stiffness (torsional rigidity) is proportional to the 4th power of their diameters. A small increase in diameter results in a large increase in sway bar stiffness. You can caclulate the actual percent difference in the stiffness using the following formula:

larger bar diameter4/smaller bar diameter4 * 100 = percent change in stiffness.
What if one or both of the bars is hollow? That adds one more step to the calculations. After you have calculated the rate of the hollow bar based on its outside diameter (o.d.), you have to subtract the rate of a hypothetical bar that matches the inside diameter (i.d.) of the space within. To do that, you need to know the wall thickness of the hollow bar.

I'll illustrate the above by comparing three stock Mustang rear sway bars: a 21mm solid bar (1996 V6 Mustang), a 24mm hollow bar with wall thickness of 3.84mm (1998 GT Mustang) and a 27mm hollow bar with wall thickness of 5.4mm (1998 Cobra). Here are the calculations:

The V6's 21mm solid bar has a relative rate of (21^4) or 194,481

The GT's 24mm hollow bar has an i.d. of 24 - 3.84*2 = 16.32mm. and a relative rate of
(24^4)-(16.32^4) = 260,838

The Cobra's 27mm hollow bar has an i.d. of 27 - 5.4*2 = 16.20mm. and a relative rate of
(27^4)-(16.20^4) = 462,566
Conclusions: The 27mm Cobra bar sway bar has about 2.4 times (462,566/194,481 * 100 = 238%) the stiffness of a 21mm V6 sway bar and about 1.8 times (462,566/260,838 * 100 = 177%) the stiffness of a 24mm GT sway bar.
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post #4 of 9 Old 07-10-2011, 10:44 PM Thread Starter
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okay so...

solid bar (28.5^4) or 659,750...

hollow bar 28.5 - (4.8^2) = 23.04

(28.5^4)-(23.04^4) = 377,957

659,750/377,957*100 = 174.6

...Not sure if i did this right, but does this mean that the solid bar is about 1.7 times stiffer than the hollow bar? somebody wanna check my math?...lol.. and which bar would you recommend with 1000-1200lbs front springs,Koni sportDA struts, MM c/c plates..on a full weight 1990 gt..

Last edited by kreg90; 07-11-2011 at 11:54 AM.
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post #5 of 9 Old 07-15-2011, 12:07 AM
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Quote:
Originally Posted by kreg90 View Post
okay so...

solid bar (28.5^4) or 659,750...

hollow bar 28.5 - (4.8^2) = 23.04

(28.5^4)-(23.04^4) = 377,957

659,750/377,957*100 = 174.6

...Not sure if i did this right, but does this mean that the solid bar is about 1.7 times stiffer than the hollow bar? somebody wanna check my math?...lol.. and which bar would you recommend with 1000-1200lbs front springs,Koni sportDA struts, MM c/c plates..on a full weight 1990 gt..
First, it should be 4.8 * 2 (4.8 times 2), not 4.8^2 (4.8 squared). Either way, I'm not sure how you got 23.04 - it should be:

28.575 - (4.8 * 2) = 18.975

(28.575)^4 - (18.975)^4 = 537,086

537,086/659,750 = 80.5% (or invert it and the solid bar is ~1.24x as stiff as the hollow bar)

So you get 80% of the stiffness out of the hollow bar, while cutting a significant chunk of weight out of it.

As for what bar to go with, I would suggest something smaller. You're running quite a bit of spring, it doesn't seem to me that it should need a big ass front sway bar, too.

'89 GT convertible, N/A 357W, MM front susp w/425# coil-overs and Koni D/As, MM TA/PB rear setup (Koni Yellows, 390-430# springs), Stoptech 332mm / 12" Baers, MM 6 point rollbar, Maier 1.5" flared fenders & quarters, 18x9.5" front/18x10.5" rear Enkei RPF-1s (street), 18x10" CCW C-10s (track), etc...
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post #6 of 9 Old 07-24-2011, 06:15 PM Thread Starter
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^^^thanks for the math correction.. the whole plan is to switch to a smaller bar, I thought the 28.5mm bar was smaller than the stock 1990gt bar? I found that the 84gt and 93 cobra came with a 28.5mm bar and I was gonna grab one of those when I found that MM sells a hollow 28.5mm...I just dont want to over-do it with something too small, y'know? Right now Im using the stock bar with new rubber pivot bushings, urethane end links and MM sway bar reinforcements..
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post #7 of 9 Old 07-26-2011, 03:44 PM
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I have a 4cyl front bar on mine and have been quite happy with it. Mind you, the car hasn't seen a whole lot of driving "in anger", but going from a stock '89 GT bar to the 4cyl bar (not sure what year it came off of, took it off a car for free at a junkyard) I think made the front end more willing to turn in and take a set.
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post #8 of 9 Old 07-28-2011, 02:04 PM Thread Starter
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it was suggested to me to change to a smaller front bar for that very reason...but i dont know how small to go really, one of the MM tech guys suggested the 1.125" bar.. but never mentioned anything about it being solid or hollow...maybe hollow cause thats what they sell?.. but i was thinking solid because i thought it wouldnt be as dramatic of a change... but i dunno
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post #9 of 9 Old 07-29-2011, 12:59 PM
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Don't overthink bar changes; just slap a smaller bar on there and go drive the thing. If that doesn't work, go with a different bar or change the rear bar. I certainly wouldn't pay for a new bar unless it was a real race car and you knew exactly what size the car needed based on wheel rates, etc. Way too easy to get a hold of free swaybars.
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